Generalized structural description of calcium-sodium aluminosilicate hydrate gels: the cross-linked substituted tobermorite model

Co-treatment of municipal solid waste incineration fly ash and alumina-/silica-containing waste: A critical review

Municipal solid waste incineration fly ash (MSWI-FA) is a hazardous by-product of the incineration process, characterized by elevated levels of heavy metals, chlorides, and dioxins. With a composition high in calcium but low in silicon/aluminum, MSWI-FA exhibits a poor immobilization effect, high energy demands, and limited pozzolanic activity when it is disposed of or reutilized alone. Conversely, alumina-/silica-containing waste (ASW) presents a chemical composition rich in SiO2 and/or Al2O3, offering an opportunity for synergistic treatment with MSWI-FA to facilitate its harmless disposal and resource recovery. Despite the growing interest in co-treatment of MSWI-FA and ASW in recent years, a comprehensive evaluation of ASW's roles in this process remains absent from the existing literature. Therefore, this study endeavors to examine the advancement in the co-treatment of MSWI-FA and ASW, with the focus on three key aspects, i.e., elucidating the immobilization mechanisms by which ASW improves the solidification/stabilization of MSWI-FA, exploring the synergies between MSWI-FA and ASW in various thermal and mechanochemical treatments, and highlighting the benefits of incorporating ASW in the production of MSWI-FA-based building materials. Additionally, in the pursuit of sustainable solid waste management, this review identifies research gaps and delineates future prospects for the co-treatment of MSWI-FA and ASW.

Preparation and characterisation of alkali-activated blast furnace slag and Na-jarosite catalysts for catalytic wet peroxide oxidation of bisphenol A

In this study, cost-effective alkali-activated materials made from industrial side streams (blast furnace slag and Na-jarosite) were developed for catalytic applications. The catalytic activity of the prepared materials was examined in catalytic wet peroxide oxidation reactions of a bisphenol A in an aqueous solution. All materials prepared revealed porous structure and characterisation expressed the incorporation of iron to the material via ion exchange in the preparation step. Furthermore, the materials prepared exhibited high specific surface areas (over 200 m2/g) and were mainly mesoporous. Moderate bisphenol A removal percentages (35%-37%) were achieved with the prepared materials during 3 h of oxidation at pH 7-8 and 50°C. Moreover, the activity of catalysts remained after four consecutive cycles (between the cycles the catalysts were regenerated) and the specific surface areas decreased only slightly and no changes in the phase structures were observed. Thus, the prepared blast furnace slag and Na-jarosite-based catalysts exhibited high mechanical stability and showed good potential in the removal of bisphenol A from wastewater through catalytic wet peroxide oxidation.

Seawater Mixed with One Part Alkali Activated Material: An Environmental and Cost Evaluation

Concrete production is associated with extensive energy consumption and significant CO2 emissions. In addition, tremendous amounts of freshwater are used as a mixing agent. Urgency is increasing to develop sustainable cementitious materials and promote freshwater-saving strategies. An environmentally friendly alternative binder, seawater mixed with one part alkali activated material, is studied. In this work, a cradle-to-gate life cycle assessment was applied to study the equivalent CO2 emission and cost properties of the clinker-free binder. The seawater mixed mortar possesses comparable mechanical properties to Portland cement, with 3 d flexural and compressive strengths of 5.3 MPa and 25.2 MPa. In addition, the mortar developed in this work is of similar cost as commercial cement, but reduces CO2 emissions by 44.8%.

Design of Alkali-Activated Materials and Geopolymer for Deep Soilmixing: Interactions with Model Soils

This study focuses on the use of alkali-activated materials and geopolymer grouts in deep soilmixing. Three types of grouts, incorporating metakaolin and/or slag and activated with sodium silicate solution, were characterized at different scales to understand the development of their local structure and macroscopic properties. The performance of the soilmix was assessed by using combinations of the grouts and model soils with different clay contents. Feret's approach was used to understand the development of compressive strength at different water-to-solid ratios ranging from 0.65 to 1. The results suggested that incorporating calcium reduced the water sensitivity of the materials, which is crucial in soilmixing. Adding soils to grouts resulted in improved mechanical properties, due to the influence of the granular skeleton. Based on strength results, binary soilmix mixtures containing 75% of metakaolin and 25% of slag, with H2O/Na2O ratios ranging from 28 to 42 demonstrated potential use for soilmixing due to the synergistic reactivity of metakaolin and slag. The optimization of compositions is necessary for achieving the desired properties of soil mixtures with higher H2O/Na2O ratios.

A Performance-Based Test for Mitigating the Risk of Geopolymer Concrete Surface Efflorescence Due to Alkali Leaching

Geopolymer concretes are considered to be a potential sustainable, low-embodied carbon alternative for Ordinary Portland Cement (OPC) concrete. Alkali leaching is considered to be a major esthetic concern for Na-silicate-based geopolymers as it can lead to the formation of efflorescence products on the surfaces of concrete members exposed to humidity. In this context, this research aims to investigate the effect of the alkali content and the FA/GGBS mass ratio on the alkali leaching and formation of the efflorescence products. Paste cylinders were fabricated and cured in ambient conditions. Samples were submerged in deionized water and the concentration of the leached-out ions was measured. Efflorescence potential was also investigated by partial immersion of the samples in deionized water. The results highlight the complexity of the interacting parameters governing the formation of efflorescence products in geopolymer materials. Establishing relationships between the concrete mix variables and the risk of efflorescence seems unfeasible particularly because of the wide range of possible precursors and activators available to design geopolymer concrete mixes. To overcome this barrier, a practical performance-based testing method is developed. For the first time, by testing a wide range of geopolymer materials, performance-based requirements associated with the risk of efflorescence for geopolymer concrete surfaces exposed to humidity are calibrated. Four categories of risk are proposed and typical suitable exposure conditions for geopolymer concrete surfaces are suggested for each risk category.

Improvements in Hydrolytic Stability of Alkali-Activated Mine Tailings via Addition of Sodium Silicate Activator

Over 14 billion tons of mine tailings are produced throughout the world each year, and this type of waste is generally stored onsite indefinitely. Alkali activation is a promising strategy for the reuse of mine tailings to produce construction materials, converting this waste stream into a value-added product. One major problem with alkali-activated mine tailings is their low durability in water (i.e., low hydrolytic stability). In this article, the influence of a mixed sodium hydroxide/sodium silicate alkali activator on the compressive strength, hydrolytic stability, and microstructure of alkali-activated materials (AAMs) were systematically investigated. XRD, FTIR, NMR, and NAD were used to investigate microstructural changes, and a water immersion test was used to show improvements in hydrolytic stability. For gold mine tailings activated with pure sodium hydroxide, the compressive strength was 15 MPa and a seven-day water immersion test caused a strength loss of 70%. With an addition of 1 M sodium silicate in the activator, the AAMs achieved a compressive strength of over 30 MPa and strength loss of only 45%. This paper proposes a mechanism explaining why the strength and hydrolytic stability of AAMs are dependent on the dosage of soluble silicate. A high dosage of sodium silicate inhibits the depolymerization of the source material, which results in a sample with less amorphous aluminosilicate gel and, therefore, lower hydrolytic stability.

Compositions and Microstructures of Carbonated Geopolymers with Different Precursors

It is thought that geopolymers are easy to carbonate, especially when they are cured in ambient temperatures. Matrix gel's composition and microstructure, and new products of geopolymers (GPs) after carbonation were investigated in this study on the basis of XRD and SEM-EDS measurements and ternary diagram analysis, which were prepared from low-lime fly ash (FA) and ground granulated blast-furnace slag (GGBS) alone or a blend, as a precursor. The specimens were hardened in a 20 °C environment with alkali activator solution (S/N = 1.1 in mole), followed by storage under sealing or accelerated carbonation. XRD patterns show that carbonation products were nahcolite for the sole FA-based GP and calcite for the GPs using GGBS alone or as a blend. The SEM images of carbonated samples show that there were cube-shaped calcite and small calcite particles in the GGBS-based GP, but hail-like particles in the FA/GGBS blend-based GP. The hail-like particles were complexes of calcite and C-A-S-H gels determined by ternary diagram analysis, and were found to plug the top of the pores of the spongy C-A-S-H gels. We also confirmed that combined ternary diagram analysis of S-(C + M + N)-A and A-(C + M)-N are very effective in determining the gel type of a geopolymer, as well as the products and compositional changes after carbonation, in which oxide components of gels are determined by SEM-EDS. In the former diagram, C-A-S-H gels were plotted linearly along the (C + M + N)-albite (Ab) join, while N-A-S-H gels showed a scattered distribution. In the latter diagram, the plots for N-A-S-H and C-A-S-H gels are distributed in different zones. N = Na2O, C = CaO, M = MgO, A = Al2O3, S = SiO2, H = H2O.

Exploring the Utilization of Activated Volcanic Ash as a Substitute for Portland Cement in Mortar Formulation: A Thorough Experimental Investigation

The manufacture of natural pozzolans as cement products is economically affordable and contributes to CO2 mitigation in the cement-based materials industry. Through two experimental stages, this study evaluates the feasibility of using volcanic ash (VA) to partially substitute portland cement (PC) in mortar production. In Stage 1, the effectiveness of different activation methods, such as calcination, alkali activation, and lime addition, in enhancing VA reactivity was assessed when the mortars were produced using 35% VA. The compressive strength (fcm) and physical properties of the mortars produced were determined at 7 and 28 days and compared with those of mortars without activated VA. In Stage 2, the most effective treatments obtained from Stage 1 were applied to produce mortars with 50% and 75% of VA replacements, focusing on their physical and mechanical properties. The findings revealed promising results, particularly when mortars were produced with up to 50% calcined VA (CVA) at 700 °C and 20 wt% lime addition, reaching a higher fcm than 45 MPa. Chemical activation with 2% CaCl or 1% NSi enhanced early-age strength in 35% VA-based mortars. Additionally, NSi-activated CVA-lime-based mortar at 50% VA achieved a notable fcm of 40 MPa at 28 days. Even mortars with 75% VA replacement achieved an adequate compressive strength of 33MPa at 28 days. This study determined that VA-based mortars have the potential for construction applications.

Interaction Mechanism between Slags and Alkali Silicate Activators: An Approach Based on the Al Phases

In this study, we examined the early-stage interaction of three types of slag and six activators with different chemical compositions. To determine the degree of hydration (DOH) and hydrate assemblage in alkali-activated slag (AAS), we employed EDX, XRD, and NMR analyses. We found that with increasing silicate concentration in the activator, the DOH in the AAS varied, whereas the proportion of C-(N)-A-S-H increased and the other Al-containing phase decreased. When examining the impact of the activator on glass dissolution, it is apparent that an index based on the degree of depolymerization of the glass structure correlates with the DOH and the proportion of hydrotalcite in the AAS. Coupled with the activator's modulus, this index can be utilised to elucidate the dissolution-reprecipitation mechanism that governs the interaction between the activator and slag.

Experimental Validation of the Cementation Mechanism of Wood Pellet Fly Ash Blended Binder in Weathered Granite Soil

In response to climate change, wood pellets have been increasingly utilized as a sustainable energy source. However, their growing utilization increases the production of wood pellet fly ash (WA) by-products, necessitating alternative recycling technologies due to a shortage of discharging landfills. Thus, this research seeks to utilize WA by developing a new sustainable construction material, called wood pellet fly ash blended binder (WABB), and to validate its stabilizing performance in natural soils, namely weathered granite soil (WS). WABB is made from 50% WA, 30% ground granulated blast-furnace slag (GGBS), and 20% cement by dry mass. WS was mixed with 5%, 15%, and 25% WABB and was tested for a series of unconfined compressive strength (qu), pH, and suction tests at 3, 7, 14, and 28 days. For the microstructural analyses, XRD, SEM, and EDS were employed. As the WABB dosage rate increased, the average qu increased by 1.88 to 11.77, which was higher than that of compacted WS without any binder. Newly cementitious minerals were also confirmed. These results suggest that the effects of the combined hydration mechanism of WABB are due to cement's role in facilitating early strength development, GGBS's latent hydraulic properties, and WA's capacity to stimulate the alkaline components of WABB and soil grains. Thus, this research validates a new sustainable binder, WABB, as a potential alternative to conventional soil stabilizers.

Effect of Aluminium Substitution on Physical Adsorption of Chloride and Sulphate Ions in Cement-Based Materials

When aluminium-rich phase minerals are added to Portland cement, Al atoms will enter the C-S-H and Al, then a substitution reaction will occur, forming a hydrated silica-calcium aluminate (C-A-S-H), which changes the molecular structure of the cement material. Due to limitations in experimental methods, the research on the bonding effect between corroded ions and Al-substituted structures is still unclear. Here, the mechanism of an Al substitution reaction affecting the adsorption of chloride and sulphate ions was studied using simulation. The C-A-S-H model of aluminium random substitution was built, evaluating the binding effects among the C-A-S-H, and sulphate and chloride ions. The results demonstrated that the C-A-S-H structure generated by the Al substitution reaction increased the physical adsorption capacity of the chloride and sulphate ions. The adsorption capacity of the sulphate ions was 13.26% higher than that before the Al substitution, and the adsorption capacity of chloride ions was 21.32% higher than that before the Al substitution. The addition of high aluminium phase minerals caused the interfacial flocculants C-A-S-H and C-S-H to connect and intertwine in the the interface transition zone (ITZ) structure. The addition of high-alumina phase minerals improves the microstructure of concrete hydration products, improving the physical and mechanical properties and durability of concrete. After the addition of 20% lithium slag, the sulphate ion erosion content and the chloride ion erosion content of the concrete decreased by 13.65% and 15.72%, respectively. This paper provides a deeper understanding of the effect of high-alumina phase admixtures on concrete at the micro-scale.

Freeze-thaw cycle and abrasion resistance of alkali-activated FA and POFA-based mortars: Role of high volume GBFS incorporation

Alkali-activated binders made from various waste products can appreciably reduce the emission of CO2 and enhance the waste recycling efficiency, thus making them viable substitutes to ordinary Portland cement (OPC)-based binders. Waste materials including fly ash (FA), palm oil fuel ash (POFA), and granulated blast furnace slag (GBFS) reveal favorable effects when applied to alkali-activated mortars (AAMs) that are mainly related to the high contents of silica, alumina, and calcium. Therefore, fifteen AAM mixes enclosing FA, POFA with high volume of GBFS were designed. The obtained GBFS/FA/POFA-based AAMs were subjected wet/dry and freeze/thaw cycles. The impact of various GBFS contents on the microstructures, freeze-thaw cycle, abrasion resistance, mechanical and durability features of the proposed AAMs were evaluated. The results showed that presence of Ca can significantly affect the AAMs durability features and long-term performance. The abrasion resistance of the AAMs was decreased with the decrease of CaO contents. Furthermore, the abrasion depth of 70% AAMs (0.8 mm) was lower in comparison to the mix made by replacing 50 wt% of FA with GBFS (1.4 mm). Generally, increase in the GBFS contents from 50 to 70% could largely impact the AAMs properties under aggressive environmental exposure. The expansion and physical impacts during the freezing-thawing cycles was argued to destroy the bonds in C-S-H and paste-aggregates, causing the formation of large cracks. It is asserted that the AAM mixes made from FA, POFA and high volume of GBFS may offer definitive mechanical, durable, and environmental benefits with their enhanced performance under aggressive environments.

Recent Advances in Alkali-Activated Materials with Seawater and Sea Sand

The development of sustainable cementitious materials is essential and urgent for the construction industry. Benefiting from excellent engineering properties and a reduced greenhouse gas footprint, alkali-activated materials (AAM) are among the robust alternatives to Portland cement for civil infrastructure. Meanwhile, concrete production also accounts for around 20% of all industrial water consumption, and the global freshwater shortage is increasing. This review discusses recent investigations on seawater-mixed AAMs, including the effects of seawater on workability, reaction mechanism, shrinkage, short and long-term strength, binding of chloride and corrosion of steel reinforcement. Attention is also paid to the utilization of sea sand as aggregate, as well as discussions on the challenges and further research perspectives on the field application of AAMs with seawater and sea sand.

Nanoscale Insight into the Effect of Calcium on Early-Age Polymerization of CNASH Gels

Sodium-containing calcium-alumino-silicate-hydrate (CNASH) gels, the primary binder phase of alkali-activated materials (AAMs), significantly impact the performance of the AAM. Although the effect of the calcium content on the AAM has been extensively studied in the past, few studies focus on the effect of calcium on the structure and performance of gels at a molecular scale. As an important element in gels, the effect of calcium in gels on its atomic-scale properties remains unclear. This study establishes a molecular model of the CNASH gel via reactive molecular dynamics (MD) simulation and verifies the feasibility of the gel model. By employing the reactive MD, the effect of calcium on the physicochemical properties of gels in the AAM is investigated. The simulation highlights that the condensation process of the system containing Ca is accelerated dramatically. This phenomenon is explained from the perspective of thermodynamics and kinetics. The increased calcium content enhances the thermodynamic stability and reduces the energy barrier of the reaction. Then, the phenomenon is further analyzed through the nanosegregation in the structure. It is proved that this behavior is driven by the weaker affinity of calcium for aluminosilicate chains than the particles in the aqueous environment. The difference in affinity leads to nanosegregation in the structure, making Si(OH)₄ and Al(OH)₃ monomers and oligomers closer for better polymerization.

Reuse of Oil Refining Sludge Residue Ash via Alkaline Activation in Matrices of Chamotte or Rice Husk Ash

The aim of this work is to investigate the possibility of reusing ashes obtained by the calcination of industrial sludge from the oil refining industry (ORSA) as a secondary raw material in the manufacture of alkaline activated cements or geopolymers. The incorporation behavior of 5-20 wt.% of residue in binary mixtures with rice husk ash (RHA) or chamotte (CHM) was evaluated. The cements were activated with a sustainable alternative activating solution obtained from NaOH (10 M) and diatomaceous earth. The specimens were cured at room temperature. Physical and mechanical properties were determined, and the reaction products were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX). The results indicate that the addition of ORSA (5-20 wt.%) to RHA and CHM improves the mechanical strength of alkaline activated cements with maximum compressive strengths of 30.6 MPa and 15.7 MPa, respectively, after 28 days of curing, with the incorporation of 20 wt.% waste. In these mixtures, the sludge acts as a source of aluminum, promoting the formation of a higher amount of geopolymer gel N-A-S-H in materials using RHA as a precursor and also (N)-(C)-A-S-H gel in cements using CHM.

Correlating Hydration of Alkali-Activated Slag Modified by Organic Additives to the Evolution of Its Properties

This study investigates the relationships between the hydration kinetics of waterglass-activated slag and the development of its physical-mechanical properties, as well as its color change. To modify the calorimetric response of alkali-activated slag, hexylene glycol was selected from various alcohols for in-depth experiments. In presence of hexylene glycol, the formation of initial reaction products was restricted to the slag surface, which drastically slowed down the further consumption of dissolved species and slag dissolution and consequently delayed the bulk hydration of the waterglass-activated slag by several days. This allowed to show that the corresponding calorimetric peak is directly related to the rapid evolution of the microstructure and physical-mechanical parameters and to the onset of a blue/green color change recorded as a time-lapse video. Workability loss was correlated with the first half of the second calorimetric peak, while the most rapid increase in strengths and autogenous shrinkage was related to the third calorimetric peak. Ultrasonic pulse velocity increased considerably during both the second and third calorimetric peak. Despite the modified morphology of the initial reaction products, the prolonged induction period, and the slightly reduced degree of hydration induced by hexylene glycol, the overall mechanism of alkaline activation remained unchanged in the long-term perspective. It was hypothesized that the main issue of the use of organic admixtures in alkali-activated systems is the destabilizing effect of these admixtures on soluble silicates introduced into the system with an activator.

Synergistic gel formation in geopolymers of superior mechanical strength synthesized with volcanic ash and slag

The present work studies gel evolution and microstructure of geopolymers synthesized with volcanic ash (VA) and blast furnace slag (BFS). The synthesis parameters such as BFS proportions on geopolymer formation were investigated. Gel evolution and microstructure of the geopolymers were studied by FTIR, X-ray diffraction (XRD), ²⁹Si NMR spectroscopy and scanning electron microscopy measurements. Silicate gels (N-S-H) were mainly formed in VA-based geopolymers of low compressive strength (14.07 MPa). While with VA and BFS each account for 50%, VA-BFS-based geopolymers possessed a compressive strength of 55.6 MPa, as well as the homogeneous C-(A)-S-H and N-A-S-H gels were formed. The C-(A)-S-H and N-A-S-H gels show synergistic effects on the mechanical property of the geopolymers. This work provides a clue for the synthesis of geopolymers with superior mechanical properties in areas of architecture. Detailed characterization gel evolution and microstructure of geopolymers synthesized with volcanic ash (VA) and blast furnace slag (BFS) were studied. Silicate gels (N-S-H) were mainly formed in VA-based geopolymers of low compressive strength (14.07 MPa). When VA and BFS each account for 50%, VA-BFS-based geopolymers possessed a compressive strength of 55.6 MPa, as well as the homogeneous C-(A)-S-H and N-A-S-H gels formed. Synthesis protocol for VA-BFS-based geopolymers.

Sustainable development goals for industry, innovation, and infrastructure: demolition waste incorporated with nanoplastic waste enhanced the physicomechanical properties of white cement paste composites

The COVID-19 pandemic significantly impacts the increase in plastic waste from food packaging, masks, gloves, and personal protective equipment (PPE), resulting in an environmental disaster, if collected, processed, transported, or disposed inappropriately. Plastic waste has a very long deterioration time in the environment (soil and water), cheap, and plentiful. Additionally, construction waste disposal is a process that transfers debris to a state that does lead to any sustainable or environmental problems. The core objective of this current research work is to provide safety and efficacy by partial substitution of both ultrafine demolition waste (UDW), incorporated with nanoplastic waste (NPW), for eco-white cement (E-WC) composition. E-WC is designed by partially substituted WC with UDW (1.0, 5.0, 10.0, 15.0, and 20.0 wt.%); incorporated with NPW (1.0 and 3.0 wt.%); to adequately protect people and the environment over long periods. The context examines the high performance, physicomechanical properties and high durability of blends as presences of silica in UDW proposed a hydraulic filler material, plus; high surface area of NPW. The microstructure and workability are characterized by X-Ray Fluorescence (XRF), Scanning Electron Microscope (SEM), and Transmission Electron Microscope (TEM) measurements. The record results show greatly enhanced in the mechanical strength due to the combination of NPW and UDW (active silica). With the presence of NPW and UDW in WC matrix, the highest level of crystallization formed consequently a decrease in whiteness reflection (Ry) and total porosity. In summary, WC blend with NPW and UDW reflects better workability and energy saving qualities, which are economical and environmentally beneficial and may result in decreased construction budget and improve a long-term raw material sustainability.

A long-term study on structural changes in calcium aluminate silicate hydrates

Production of blended cements in which Portland cement is combined with supplementary cementitious materials (SCM) is an effective strategy for reducing the CO₂ emissions during cement manufacturing and achieving sustainable concrete production. However, the high Al₂O₃ and SiO₂ contents of SCM change the chemical composition of the main hydration product, calcium aluminate silicate hydrate (C-A-S-H). Herein, spectroscopic and structural data for C-A-S-H gels are reported in a large range of equilibration times from 3 months up to 2 years and Al/Si molar ratios from 0.001 to 0.2. The ²⁷Al MAS NMR spectroscopy and thermogravimetric analysis indicate that in addition to the C-A-S-H phase, secondary phases such as strätlingite, katoite, Al(OH)₃ and calcium aluminate hydrate are present at Al/Si ≥ 0.03 limiting the uptake of Al in C-A-S-H. More secondary phases are present at higher Al concentrations; their content decreases with equilibration time while more Al is taken up in the C-A-S-H phase. At low Al contents, Al concentrations decrease strongly with time indicating a slow equilibration, in contrast to high Al contents where a clear change in Al concentrations over time was not observed indicating that the equilibrium has been reached faster. The ²⁷Al NMR studies show that tetrahedrally coordinated Al is incorporated in C-A-S-H and its amount increases with the amount of Al present in the solution.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1617/s11527-022-02080-x.

Fresh and hardened properties of alkali-activated slag concrete: The effect of fly ash as a supplementary precursor

The present study explores the possibility of replacing blast furnace slag (BFS) with coal fly ash (FA) to produce alkali-activated material (AAM) concrete with hybrid precursors. With an increased FA replacement ratio, the reaction kinetics, fresh and hardened properties of AAM mixtures have been investigated. The retardation effect on the reaction kinetics with an increased FA content has been observed, which not only extended the induction period along with the heat flow evolution but also reduced the cumulative heat release up to 24 h. Spherical FA particles can provide a ball-bearing effect to improve the workability of the hybrid AAM mixtures, while FA also slows down the deterioration of fresh properties since they are less reactive compared to BFS particles. Regarding the strength development, FA results in the reduction at all curing ages in the mixtures with a low silicate modulus (Ms0.25). Similarly, reduction in 1-day compressive strength has been detected in high silicate modulus mixtures (Ms0.5) with FA replacement, while the mixture with 10% FA exhibits the highest compressive strength among Ms0.5 concretes at later curing ages. Bigger capillary pores have been detected in AAM mixtures with an increase in FA content. However, AAM with 10% FA shows the lowest porosity in Ms0.5 mixtures, which is in agreement with the compressive strength results.

Effect of Organic Polymers on Mechanical Property and Toughening Mechanism of Slag Geopolymer Matrix

In this work, two series of chemically reactive polymers, silane coupling agents (SCAs) and water-soluble polymers, were specifically designed as an additive to improve the ductility of slag geopolymer paste by vibration pressure technique. The influences of organic polymers on the fluidity, rheological behavior, mechanical property, porosity, and toughening mechanism of slag geopolymer were investigated. The polycondensation and bonding characteristics of organic-inorganic products were calculated by ¹H liquid nuclear magnetic resonance (NMR) technology and Fourier transform infrared (FT-IR). The polymerization degree of composite geopolymer was evaluated by ²⁹Si NMR and X-ray photoelectron spectroscopy (XPS). The microscopic morphology of the geopolymer matrix was analyzed using scanning electron microscopy (SEM). The results showed that the dosage of the KH570 and PAA-Na with 5 wt% behaved best in improving the flexural strength and the compressive strength of geopolymer in their corresponding organic series, respectively. The addition of polymers decreased the fluidity and the fluidity loss ratio of geopolymer slurry but reduced the harmful pores of hardened geopolymer. The organic polymers acting as bridge-fixed water molecules weakened the repulsion force, and formed a three-dimensional network through molecular interweaving in a geopolymer matrix. Methacryloxy in silane coupling agents and carboxyl group in water-soluble polymers may contribute to the improvement of hydration product structure through strong bonding with C-A-S-H. Microscopic measurements indicated that the addition of KH570 and PAA-Na in geopolymer could form 73.55% and 72.48% Si-O-Si with C-A-S-H gel, higher than the reference, and increase the polycondensation degree of C-A-S-H phase, reflected by the increased generation of Q² and Q²(1Al) and the longer chain length, leading to a higher densified geopolymer matrix with high ductility.

Preparation and characterization of a new alkali-activated binder for superfine-tailings mine backfill

Recently, the increasing of ultrafine-tailings increases the amount of ordinary Portland cement (OPC) in cemented paste backfill (CPB), which leads to the rise of CPB cost and carbon emission. As a result, it is necessary to develop alternative binders. The present work focuses on the preparation of a new binder, which is activated by a mixture of calcined quarry dust (CQD) and NaOH at a mass ratio of 1:1. The results indicated that CQD/NaOH was more effective than using NaOH or CQD alone in activating blast furnace slag (BFS) and also showed better performance than OPC. The compressive strength of the CPB samples using 10% CQD/NaOH was around 3.78 MPa after curing for 90 days, around 42% higher than the OPC-based CPB samples. The reaction products of CQD/NaOH-activated BFS consisted mainly of C-(A)-S-H, hydrotalcite like phases (Ht), and M-S-H. The generation of Ht phases lowered the Al incorporation into the structure of C-S-H, resulting in lower average Al/Si ratio and mean chain length.

Advances in the use of recycled non-ferrous slag as a resource for non-ferrous metal mine site remediation

The growing global demand for non-ferrous metals has led to serious environmental issues involving uncovered mine site slag dumps that threaten the surrounding soils, surface waters, groundwater, and the atmosphere. Remediation of these slags using substitute cement materials for ordinary Portland cement (OPC) and precursors for alkali-activated materials (AAMs) can convert hazardous solid wastes into valuable construction materials, as well as to attain the desired solidification and stabilization (S/S) of heavy metal(loid)s (HM). This review discusses the current research on the effect of non-ferrous slags on the reaction mechanisms of the OPC and AAM. The S/S of HM from the non-ferrous slags in AAM and OPC is also reviewed. HM can be stabilized in these materials based on the complex salt effect and isomorphic effects. The major challenges faced in AAMs and OPC for HM stabilization include the long-term durability of the matrix (e.g., sulfate attack, stability of volume). The existing knowledge gaps and future trends for the sustainable application of non-ferrous slags are also discussed.

Shear strength, water permeability and microstructure of modified municipal sludge based on industrial solid waste containing calcium used as landfill cover materials

In order to prepare a new type of landfill covering material for closure, we used industrial calcium-containing waste (slag, desulfurised gypsum and fly ash) to modify the municipal dewatered sludge. Shear, infiltration and rainfall infiltration model tests were performed to obtain the shear strength parameters of the modified sludge, the hydraulic conductivity during the wet-dry cycle, and the service performance against rainfall breakdown to evaluate the service performance of the modified sludge cover (MSC). Comprehensive characterisation of the modified sludge was analysed by XRD, FTIR and SEM-EDS to revealed the mineral structure, microstructure, and modification mechanism of the sludge. The MSC samples had high shear strength and shown the characteristics of evolving from plasticity to brittleness. After curing for 14 d, the values of cohesion c and internal friction angle φ reached 150.75-384.69 kPa and 37.60-57.29°, respectively. The MSC exhibited excellent anti-seepage service performance under dry and wet cycle conditions. Compared with traditional compacted clay, its hydraulic conductivity dropped by an order of magnitude, and after six wet and dry cycles, the hydraulic conductivity of the modified sludge reached stability at 1.4-7.2 × 10^-7 cm/s. The 60-cm-thick MSC layer can completely withstand the impact of long-term rainfall during the rainy season in the middle reaches of the Yangtze River in China. Analysis results also show that the modification mechanism of the sludge could be ascribed to the generation of dense blocks and clusters of C-S-H and C-A-S-H gels originated from SiO₂, Al₂O₃, and CaO phases in industrial calcium-containing waste and sludge by the activation of the alkali.

Experimental Research on the Properties and Formulation of Fly Ash Based Geopolymer Grouting Material

This paper experimentally investigated the effects of varying contents of Na2O in a modified sodium silicate, sodium silicate moduli (Ms), and contents of granulated blast furnace slag (GBFS) on the compressive strength and drying shrinkage of fly ash (FA)-based geopolymer grouting materials at different ages. X-ray diffraction (XRD), mercury intrusion porosimetry (MIP), and scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM–EDS) were used to study the influences of different amounts of GBFS on the microstructure and product compositions of FA-based geopolymer grouting materials. The results show that the content of Na2O in modified sodium silicate, Ms, and the content of GBFS play a significant role in compressive strength and drying shrinkage of FA-based geopolymer grouting materials. In addition, the influence of Ms as well as the content of GBFS on the compressive strength and drying shrinkage of FA-based geopolymer grouting materials was deeply affected by curing age. The micro-performance tests and analysis clearly showed that incorporating 30 wt% GBFS can decrease the proportion of pores with large pore sizes, improve pore size distribution, and enhance the solubility of FA and further promote the formation of C-A-S-H gel within FA-based geopolymer grouting materials.

Alteration in molecular structure of alkali activated slag with various water to binder ratios under accelerated carbonation

Carbonation of alkali activated materials is one of the main deteriorations affecting their durability. However, current understanding of the structural alteration of these materials exposed to an environment inducing carbonation at the nano/micro scale remains limited. This study examined the evolution of phase assemblages of alkali activated slag mortars subjected to accelerated carbonation (1% CO₂, 60% relative humidity, up to 28 day carbonation) using XRD, FTIR and ²⁹Si, ²⁷Al, and ²³Na MAS NMR. Samples with three water to binder (w/b) ratios (0.35, 0.45, and 0.55) were investigated. The results show that the phase assemblages mainly consisted of C-A-S-H, a disordered remnant aluminosilicate binder, and a minor hydrotalcite as a secondary product. Upon carbonation, calcium carbonate is mainly formed as the vaterite polymorph, while no sodium carbonate is found after carbonation as commonly reported. Sodium acts primarily as a charge balancing ion without producing sodium carbonate as a final carbonation product in the 28-day carbonated materials. The C-A-S-H structure becomes more cross-linked due to the decalcification of this phase as evidenced by the appearance of Q⁴ groups, which replace the Q¹ and Q² groups as observed in the ²⁹Si MAS NMR spectra, and the dominance of Al(IV) in ²⁷Al MAS NMR. Especially, unlike cementitious materials, the influence of w/b ratio on the crystalline phase formation and structure of C-A-S-H in the alkali activated mortars before and after carbonation is limited.

Compressive and Flexural Properties of Ultra-Fine Coal Gangue-Based Geopolymer Gels and Microscopic Mechanism Analysis

Geopolymer gel that possesses advantageous features of fast setting, high strength, and good durability is increasingly used in civil engineering, including rapid retrofit projects, roadway, and other construction projects. Furthermore, geopolymer gel is also a green and economical material as it derives from solid wastes. In this study, activators with different sodium silicate modulus and alkali content were used to activate ultrafine coal gangue and slag powder to prepare coal-gangue-based geopolymers with high strength. To study the influence of slag powder content, sodium silicate modulus, and alkali activator content on strength, a two-stage design was adopted. In the first stage, the orthogonal test with three factors and four levels (10−40% slag, 0.4−1.0 modulus, 16−22%) was used to obtain the influence of each factor on the strength and select the design range of the specimen mix ratio with higher strength. In the second stage, based on the orthogonal experiment, the scope was narrowed to continue to find the optimal excitation scheme and the relationship between the influencing factors and strength. Further, mineral compositional, microstructural, functional group and elemental analyses were performed using X-ray diffraction technique, IR infrared diffraction, electron microscope observation and energy spectrum analysis to elucidate the mechanisms of the strength development. The results show that the factors affecting the geopolymer’s strength were in the order of slag content > alkali content > modulus. The optimum dosage of alkali activator was 18−20%, and the sodium silicate modulus was 0.6−0.8, and the compressive and flexural strength could reach above 40 MPa and 5.9 MPa, respectively. The compressive strength and modulus were in a parabolic relationship. Three types of cementing gels (N-A-S-H, C-A-S-H, and C-N-A-S-H) that were characterized with dense structure and high strength were identified from coal gangue and slag powder after alkali excitation.

Review on the Application of Supplementary Cementitious Materials in Self-Compacting Concrete

For the sustainable development of construction materials, supplementary cementitious materials (SCMs) are commonly added to self-compacting concrete (SCC). This paper reviewed the application techniques and hydration mechanisms of SCMs in SCC. The impacts of SCMs on the microstructure and performance of SCC were also discussed. SCMs are used as a powder material to produce SCC by replacing 10% to 50% of cement. Hydration mechanisms include the pozzolanic reaction, alkaline activation, and adsorption effect. Moreover, the filling effect and dilution effect of some SCMs can refine the pore structure and decrease the temperature rise of concrete, respectively. Specifically, the spherical particles of fly ash can improve the fluidity of SCC, and the aluminum-containing mineral phase can enhance the resistance to chloride ion penetration. Silica fume will increase the water demand of the paste and promote its strength development (a replacement of 10% results in a 20% increase at 28 days). Ground-granulated blast furnace slag may reduce the early strength of SCC. The adsorption of Ca2+ by CaCO3 in limestone powder can accelerate the hydration of cement and promote its strength development.

Impact of Na/Al Ratio on the Extent of Alkali-Activation Reaction: Non-linearity and Diminishing Returns

To address the high CO₂ footprint associated with cement production, many alternative, sustainable binders are now gaining worldwide attention-including alkali-activated materials. The alkali-activation reaction of metakaolin is a fairly complex process involving transformation of one amorphous reactant (precursor metakaolin) into another amorphous product or products (N-A-S-H gel and/or disordered zeolite type phases). In spite of this complexity, researchers in the past 2 decades have gained significant knowledge on the nature of this reaction at multiple scales. Understanding and developing a clear relationship between the alkalinity of the mix and the extent of reaction is of high interest for practical applications. However, detailed and thorough investigations on this important relationship are limited. Here, in this study, we address this gap by systematically investigating a series of alkali-activated materials samples with a wide range of Na/Al ratios (0.5-1.8) using seven different yet complementary analytical techniques (isothermal calorimetry, FTIR, XRD, TGA, NMR, and Raman imaging). Applied in tandem, these tools reveal a clear but non-linear relationship between the Na/Al ratio and the extent of alkali-activation reaction indicating diminishing returns at higher Na/Al ratios, where higher Na/Al ratios cause an increase in the degree of reaction until a certain point at which the increase in Na/Al ratio does not significantly affect the reaction kinetics, but may affect the gel polymerization. These findings could potentially aid decision making for commercial applications of AAMs where alkalinity of the mix is an important parameter for performance as well as safety.

Co-solidification of bauxite residue and coal ash into indurated monolith via ambient geopolymerisation for in situ environmental application

Bauxite residues generated from alumina refineries worldwide have accumulated to more than 4 billion tons, at an annual increment of ~ 0.15 billion tons. It is imperative and urgent for the alumina sector to develop field-operable disposal solutions for rapid and cost-effective stabilisation of alkaline bauxite residues (BR) in the storage facility to minimise/prevent potential environmental risks. Taking advantage of the availability of coal ash (CA) on site, we studied a feasible way to synthesise geopolymer from active (amorphous) aluminosilicate components of BR and CA via the alkaline hydrolysis under ambient conditions. The new geopolymeric binder effectively solidifies BR-CA mixtures into indurated monoliths whose unconstrained compressive strength (UCS) can reach as high as ~ 20 MPa after 8 weeks. The Full Factorial Experimental Design was used to study relative influences of BR:CA ratio, modulus of activating solution, and H₂O/Na₂O ratio on UCS. Micro-spectroscopic structural analyses using electron-dispersive X-ray spectroscopy and X-ray Photoelectron Spectroscopy suggested a co-occurrence of cement-like calcium aluminosilicate hydrate (C-A-S-H) and Na-rich aluminosilicate 3D-extended network (geopolymer) within the binder phase. The advantage of this ambient geopolymerisation, without resorting to elevated temperature curing, renders a feasible way of valorising BR and CA for environmental management of alkaline wastes at alumina refineries.

Hybrid Cements: Mechanical Properties, Microstructure and Radiological Behavior

The use of more eco-efficient cements in concretes is one of the keys to ensuring construction industry sustainability. Such eco-efficient binders often contain large but variable proportions of industrial waste or by-products in their composition, many of which may be naturally occurring radioactive materials (NORMs). This study explored the application of a new gamma spectrometric method for measuring radionuclide activity in hybrid alkali-activated cements from solid 5 cm cubic specimens rather than powder samples. The research involved assessing the effect of significant variables such as the nature of the alkaline activator, reaction time and curing conditions to relate the microstructures identified to the radiological behavior observed. The findings showed that varying the inputs generated pastes with similar reaction products (C-S-H, C-A-S-H and (N,C)-A-S-H) but different microstructures. The new gamma spectrometric method for measuring radioactivity in solid 5 cm cubic specimens in alkaline pastes was found to be valid. The variables involved in hybrid cement activation were shown to have no impact on specimen radioactive content. The powder samples, however, emanated ²²²Rn (a descendent of ²²⁶Ra), possibly due to the deformation taking place in fly ash structure during alkaline activation. Further research would be required to explain that finding.

Characterization of an aged alkali-activated slag roof tile after 30 years of exposure to Northern Scandinavian weather

The phase assemblage and nanostructural characterization results reported here further elucidate the long-term changes occurring in alkali activated blast furnace slag binders.

The Effect of Various Si/Al, Na/Al Molar Ratios and Free Water on Micromorphology and Macro-Strength of Metakaolin-Based Geopolymer

The current work aimed to explore the effect of Na/Al ratios of 0.43, 0.53, 0.63, 0.73, 0.83, and 0.93, using NaOH to alter the molar ratio, on the mechanical properties of a geopolymer material, with fixing of the Si/Al molar ratio. While fixing the Na/Al molar ratio, alteration of the Si/Al ratios to 1.7, 1.75, 1.8, 1.85, 1.9, 1.95 was used, with silica fume and sodium silicate as a silica corrector. The influence on the micromorphology and macro-strength of samples was characterized through SEM, EDS, and compressive strength characterization methods. The results show that Si/Al and Na/Al molar ratios play a significant role in the microstructure and mechanical behavior of MK-based geopolymers, and revealed that the optimal molar Si/Al and Na/Al ratios for attaining maximum mechanical strength in geopolymers are 1.9 and 0.73, respectively. Under various Si/Al ratios, the macro-strength of the geopolymer mainly relies on the formation of NASH gel, rather than zeolites or silicate derivatives. The appropriate Na/Al molar ratio can contribute to the geopolymerization, but a ultra-high Na/Al molar ratio caused a high alkali state that destroyed the microstructure of the geopolymers. Regardless of the amount of water contained in the initial geopolymer raw material, the water content of Si/Al = 1.65 and Si/Al = 1.75 after curing for 10 days was almost the same, and the bound water content of the final geopolymer was maintained at about 15%. Structural water exists in geological polymer gels in the form of a chemical structure. It has effects on the structural performance strength, while free water affects the volume stability of the geological polymer. Overall, the current work provides a perspective on the elemental composition analysis, combined with the molecular structure and micromorphology, to explore the mechanical performance of geopolymers.

Development of a Geochemical Speciation Model for Use in Evaluating Leaching from a Cementitious Radioactive Waste Form

Cast Stone has been developed to immobilize a fraction of radioactive waste at the Hanford Site; however, constituents of potential concern (COPCs) can be released when in contact with water during disposal. Herein, a representative mineral and parameter set for geochemical speciation modeling was developed for Cast Stone aged in inert and oxic environments, to simulate leaching concentrations of major and trace constituents. The geochemical speciation model was verified using a monolithic diffusion model in conjunction with independent monolithic diffusion test results. Eskolaite (Cr₂O₃) was confirmed as the dominant mineral retaining Cr in Cast Stone doped with 0.1 or 0.2 wt % Cr. The immobilization of Tc as a primary COPC in Cast Stone was evaluated, and the redox states of porewater within monolithic Cast Stone indicated by Cr are insufficient for the reduction of Tc. However, redox states provided by blast furnace slag (BFS) within the interior of Cast Stone are capable of reducing Tc for immobilization, with the immobilization reaction rate postulated to be controlled by the diffusive migration of soluble Tc in porewater to the surface of reducing BFS particles. Aging in oxic conditions increased the flux of Cr and Tc from monolithic Cast Stone.

Cement Interfaces: Current Understanding, Challenges, and Opportunities

Cement and concrete are rapidly growing in demand and pose many unresolved chemistry questions at particle interfaces, during hydration reactions, regarding the role of electrolytes and organic additives. Solutions through developing greener, more sustainable formulations are needed to reduce the high carbon footprint that amounts to 11% of global CO₂ emissions. Cement is a multiphase material composed of calcium silicates, aluminates, and other mineral phases, produced from natural and low-cost industrial sources, which undergoes complex hydration reactions. This perspective highlights current research challenges and opportunities for new chemistry insight, including intriguing colloid and interface science problems that involve mineral surfaces, electrolytes, polymers, and hydration reactions. Specifically, we discuss (1) characteristics of cement phases, supplementary cementitious materials, and other constituents, (2) hydration reactions and the characterization by imaging and NMR spectroscopy, (3) the structure of hydrated cement phases including calcium-silicate-hydrates at different scales, (4) quantitative simulation techniques from the atomic scale to microscale kinetic models, and (5) the function of organic additives. Focusing on new directions, we explain the benefits of integrating knowledge from inorganic chemistry, acid-base chemistry, polymer chemistry, reaction mechanisms, and theory to describe mesoscale cement properties and bulk properties upon manufacturing.

Role of Curing Conditions and Precursor on the Microstructure and Phase Chemistry of Alkali-Activated Fly Ash and Slag Pastes

Understanding the role of curing conditions on the microstructure and phase chemistry of alkali-activated materials (AAMs) is essential for the evaluation of the long-term performance as well as the optimization of the processing methods for achieving more durable AAMs-based concretes. However, this information cannot be obtained with the common material characterization techniques as they often deliver limited information on the chemical domains and proportions of reaction products. This paper presents the use of PhAse Recognition and Characterization (PARC) software to overcome this obstacle for the first time. A single precursor (ground granulated blast-furnace slag (GBFS)) and a binary precursor (50% GBFS–50% fly ash) alkali-activated paste are investigated. The pastes are prepared and then cured in sealed and unsealed conditions for up to one year. The development of the microstructure and phase chemistry are investigated with PARC, and the obtained results are compared with independent bulk analytical techniques X-ray Powder Fluorescence and X-ray Powder Diffraction. PARC allowed the determination of the type of reaction products and GBFS and FA’s spatial distribution and degree of reaction at different curing ages and conditions. The results showed that the pastes react at different rates with the dominant reaction products of Mg-rich gel around GBFS particles, i.e., Ca-Mg-Na-Al-Si, and with Ca-Na-Al-Si gel, in the bulk paste. The microstructure evolution was significantly affected in the unsealed curing conditions due to the Na⁺ loss. The effect of the curing conditions was more pronounced in the binary system.

Precipitation of calcium-alumino-silicate-hydrate gels: The role of the internal stress

Concrete gains its strength from the precipitation of a calcium-alumino-silicate-hydrate (C-A-S-H) colloidal gel, which acts as its binding phase. However, despite concrete's ubiquity in the building environment, the atomic-scale mechanism of C-A-S-H precipitation is still unclear. Here, we use reactive molecular dynamics simulations to model the early-age precipitation of a C-A-S-H gel. We find that, upon gelation, silicate and aluminate precursors condensate and polymerize to form an aluminosilicate gel network. Notably, we demonstrate that the gelation reaction is driven by the existence of a mismatch of atomic-level internal stress between Si and Al polytopes, which are initially experiencing some local tension and compression, respectively. The polymerization of Si and Al polytopes enables the release of these competitive stresses.

The Atomic-Level Structure of Cementitious Calcium Aluminate Silicate Hydrate

Despite use of blended cements containing significant amounts of aluminum for over 30 years, the structural nature of aluminum in the main hydration product, calcium aluminate silicate hydrate (C-A-S-H), remains elusive. Using first-principles calculations, we predict that aluminum is incorporated into the bridging sites of the linear silicate chains and that at high Ca:Si and H₂O ratios, the stable coordination number of aluminum is six. Specifically, we predict that silicate-bridging [AlO₂(OH)₄]^5- complexes are favored, stabilized by hydroxyl ligands and charge balancing calcium ions in the interlayer space. This structure is then confirmed experimentally by one- and two-dimensional dynamic nuclear polarization enhanced ²⁷Al and ²⁹Si solid-state NMR experiments. We notably assign a narrow ²⁷Al NMR signal at 5 ppm to the silicate-bridging [AlO₂(OH)₄]^5- sites and show that this signal correlates to ²⁹Si NMR signals from silicates in C-A-S-H, conflicting with its conventional assignment to a "third aluminate hydrate" (TAH) phase. We therefore conclude that TAH does not exist. This resolves a long-standing dilemma about the location and nature of the six-fold-coordinated aluminum observed by ²⁷Al NMR in C-A-S-H samples.

Immobilization of cesium with alkali-activated blast furnace slag

Alkali-activated binders (AABs), as a promising alternative to Portland cement, are now being used on a commercial scale in various applications around the world, including hazardous and radioactive waste immobilization. In this paper, the leaching resistance, strength, and nanostructural alteration of alkali-activated blast furnace slag (AABFS) doped with 2 % and 5 % cesium were investigated. The addition of cesium caused a significant increase in the compressive strength of AABFS, followed by mild strength reduction after leaching. AABFS can be considered a potentially efficient matrix for cesium immobilization, since the mean leachability index in both cases (2 % and 5 % of Cs added) was above the threshold value of 6. Both doping with Cs and leaching caused the transformation of the AABFS nanostructure. The majority of the aluminum that was released from the C-A-S-H gel due to leaching remained within the AABFS matrix, initiating gel reconstruction: the C-A-S-H gel was converted to C-S-H gel, and an additional N-(C)-A-S-H gel was also formed. Cesium was preferentially associated with the N-(C)-A-S-H gel rather than with the C-A-S-H gel. The results of this research seem to be in good agreement with the Cross-linked Substituted Tobermorite Model (CSTM).

Defective analcime/geopolymer composite membrane derived from fly ash for ultrafast and highly efficient filtration of organic pollutants

Nanofiltration membranes (NFMs) are of great interest for water purification attributed by their excellent performance, while the high fabrication cost greatly limits their use. Herein, an ultra-low-cost zeolite-based NFM was developed by a simple hydrothermal method using fly ash as the raw material and used for the high-efficiency filtration of organic pollutants from wastewater. The as-obtained zeolite membrane was composed of crystalline analcime (ANA) type zeolite and amorphous geopolymer (GP) composite. Benefiting from the defects introduced large cavities and microporous channels in ANA, the ANA/GP composite membrane with a thickness of ∼60 μm exhibited permeation rates as high as 340-440 L/(m²·h·MPa), and the rejection rates are up to 97 % towards methylene blue. Moreover, the fabrication cost of the ANA/GP membrane is only $31.8/m², far lower than the reported efficient NFMs. The development of the ANA/GP-NFM paves the way for developing commercially applicable membranes for organics separation and water purification.

Stabilization/solidification of chromium-bearing electroplating sludge with alkali-activated slag binders

Chromium (Cr)-bearing electroplating sludge is a hazardous solid waste and has a detrimental effect on human health and the environment. In this study, an alkali-activated slag binders, namely, formed by the reaction of blast furnace slag (BFS) with alkali, was applied to the stabilization/solidification (S/S) of electroplating sludge. The effects of liquid-solid ratio, water glass modulus ratio (molar ratio of SiO₂ to Na₂O), water glass dosage, and electroplating sludge amount on the compressive strength and Cr leachability of binders were analyzed. The related mechanism of the S/S of electroplating sludge was discussed on the basis of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy coupled with energy-dispersive spectrometry (SEM-EDS). Results showed that the compressive strength of the alkali-activated slag binder first increased and then remained stable with the increase in liquid-solid ratio, water glass modulus ratio, and water glass dosage. By contrast, the leaching concentrations of Cr(VI) and total Cr decreased with the increase in liquid-solid ratio, water glass modulus ratio, water glass dosage, and curing time. In addition, XRD, FTIR, and SEM-EDS revealed that the hydration products of the binders were mainly low-crystallinity and dense calcium silicate hydrate gels, and Cr(VI) had been effectively immobilized in the structure. The reduction in Cr(VI) by the reductive components in the BFS boosted the stabilization of Cr-bearing electroplating sludge. Overall, the BFS binders containing electroplating sludge had relatively high compressive strengths and low Cr(VI) leaching concentrations. The physical encapsulation, chemical bonding, and absorption contributed the Cr immobilization during the S/S process of electroplating sludge.

Exploiting in-situ solid-state NMR spectroscopy to probe the early stages of hydration of calcium aluminate cement

We report a high-field in-situ solid-state NMR study of the hydration of CaAl₂O₄ (the most important hydraulic phase in calcium aluminate cement), based on time-resolved measurements of solid-state ²⁷Al NMR spectra during the early stages of the reaction. A variant of the CLASSIC NMR methodology, involving alternate recording of direct-excitation and MQMAS ²⁷Al NMR spectra, was used to monitor the ²⁷Al species present in both the solid and liquid phases as a function of time. Our results provide quantitative information on the changes in the relative amounts of ²⁷Al sites with tetrahedral coordination (the anhydrous reactant phase) and octahedral coordination (the hydrated product phases) as a function of time, and reveal significantly different kinetic and mechanistic behaviour of the hydration reaction at the different temperatures (20 °C and 60 °C) studied.

Effects of Ca/Si Ratio, Aluminum and Magnesium on the Carbonation Behavior of Calcium Silicate Hydrate

The effects of Ca/Si ratio, aluminum and magnesium on the carbonation behavior of calcium silicate hydrate (C-S-H) were investigated by using X-ray powder diffraction (XRD), nuclear magnetic resonance (NMR) and thermogravimetric analyzer (TGA). The results showed that the Ca/Si ratio, Al/Si ratio and Mg/Si ratio had a significant influence on the structure, carbonation products and carbonation resistance of C-(M)-(A)-S-H. The mean chain length of silicate chains in C-S-H increased as the Ca/Si ratio decreased. Aluminum uptake in C-S-H increased the content of bridging silicate tetrahedron (Q²). A cross-linked structure (Q³) appeared when magnesium uptake in C-S-H. The carbonation product of C-S-H was vaterite if the Ca/Si ratio was lower than 0.87. The carbonation products of C-S-H were vaterite and calcite if the Ca/Si ratio was higher than 1.02. C-M-S-H had more polymerized units, stronger bond strength and better carbonation resistance than C-S-H.

Long-term mineral transformation of Ca-rich oil shale ash waste

Power generation and other industries using solid fossil fuels like coal, lignite, oil shale and peat are responsible for producing large quantities of solid residues that are often chemically reactive and/or unstable and are disposed in holding ponds and deposition sites. Stability and long-term behaviour of such deposits are typically studied in short-term laboratory experiments that cannot describe nor predict long-term changes taking place in these materials. Here, we study long-term (>40 years) transformations, in highly alkaline conditions, of the Ca-rich ash deposit in Estonia composed of oil shale processing residues from the Eesti power plant. Detailed mineralogical, chemical and micromorphological analyses using X-ray diffraction, X-ray fluorescence, ²⁹Si nuclear magnetic resonance, scanning electron microscopy and other methods were applied in order to identify the composition of the waste with a focus on formation and transformation of semicrystalline phases in the deposit. The results show progressive formation of calcium-silicate-hydrate (C-S-H) type phase at the expense of silicate minerals and amorphous glass phases with increasing depth and age of the sediments, from about 25% in the upper part of the depository to over 60% in the oldest-deepest part. This demonstrates that over time the high alkalinity of the ash is responsible for initiating natural alkali-activation. The formation of C-S-H-type phases increases the mechanical strength of the sediment and ensures long-term stability of waste deposits. These findings may encourage the use of these ashes in binder or other construction material production or as construction aggregates.

Binders alternative to Portland cement and waste management for sustainable construction-part 1

This review presents "a state of the art" report on sustainability in construction materials. The authors propose different solutions to make the concrete industry more environmentally friendly in order to reduce greenhouse gases emissions and consumption of non-renewable resources. Part 1-the present paper-focuses on the use of binders alternative to Portland cement, including sulfoaluminate cements, alkali-activated materials, and geopolymers. Part 2 will be dedicated to traditional Portland-free binders and waste management and recycling in mortar and concrete production.

Blast furnace slag-Mg(OH)2 cements activated by sodium carbonate

The structural evolution of a sodium carbonate activated slag cement blended with varying quantities of Mg(OH)2 was assessed. The main reaction products of these blended cements were a calcium-sodium aluminosilicate hydrate type gel, an Mg-Al layered double hydroxide with a hydrotalcite type structure, calcite, and a hydrous calcium aluminate phase (tentatively identified as a carbonate-containing AFm structure), in proportions which varied with Na2O/slag ratios. Particles of Mg(OH)2 do not chemically react within these cements. Instead, Mg(OH)2 acts as a filler accelerating the hardening of sodium carbonate activated slags. Although increased Mg(OH)2 replacement reduced the compressive strength of these cements, pastes with 50 wt% Mg(OH)2 still reached strengths of ∼21 MPa. The chemical and mechanical characteristics of sodium carbonate activated slag/Mg(OH)2 cements makes them a potentially suitable matrix for encapsulation of high loadings of Mg(OH)2-bearing wastes such as Magnox sludge.