Preparation of drying powder inorganic polymer cement based on alkali-activated slag technology

One-Part Alkali-Activated Materials: State of the Art and Perspectives

Alkali-activated materials (AAM) are recognized as potential alternatives to ordinary Portland cement (OPC) to limit CO₂ emissions and beneficiate several wastes into useful products. Compared with its counterparts involving the concentrated aqueous alkali solutions, the development of "just add water" one-part alkali-activated materials (OP-AAM) has drawn much attention, mainly attributed to their benefits in overcoming the hazardous, irritating, and corrosive nature of activator solutions. This study starts with a comprehensive overview of the OP-AAM; 89 published studies reported on mortar or concrete with OP-AAM were collected and concluded in this paper. Comprehensive comparisons and discussions were conducted on raw materials, preparation, working performance, mechanical properties, and durability, and so on. Moreover, an in-depth comparison of different material pretreatment methods, fiber types, and curing methods was presented, and their potential mechanisms were discussed. It is found that ground granulated blast-furnace slag (GGBS) provides the best mechanical properties, and the reuse of most aluminosilicate materials can improve the utilization efficiency of solid waste. The curing temperature can be improved significantly for precursor materials with low calcium contents. In order to overcome the brittleness of the AAM, fiber reinforcement might be an efficient way, and steel fiber has the best chemical stability. It is not recommended to use synthetic fiber with poor chemical stability. Based on the analysis of current limitations, both the recommendations and perspectives are laid down to be the lighthouse for further research.

Circular Economy of Coal Fly Ash and Silica Geothermal for Green Geopolymer: Characteristic and Kinetic Study

The study of geopolymers has become an interesting concern for many scientists, especially in the infrastructure sector, due to having inherently environmentally friendly properties and fewer energy requirements in production processes. Geopolymer attracts many scientists to develop practical synthesis methods, useful in industrial-scale applications as supplementary material for concrete. This study investigates the geopolymerization of fly ash and geothermal silica-based dry activator. The dry activator was synthesized between NaOH and silica geothermal sludge through the calcination process. Then, the geopolymer mortar was produced by mixing the fly ash and dry activator with a 4:1 (wt./wt.) ratio. After mixing homogeneously and forming a paste, the casted paste moved on to the drying process, with temperature variations of 30, 60, and 90 °C and curing times of 1, 3, 5, 7, 14, 21, 28 days. The compressive strength test was carried out at each curing time to determine the geopolymer’s strength evolution and simulate the reaction’s kinetics. In addition, ATR-FTIR spectroscopy was also used to observe aluminosilicate bonds’ formation. The higher the temperature, the higher the compressive strength value, reaching 22.7 MPa at 90 °C. A Third-order model was found to have the highest R² value of 0.92, with the collision frequency and activation energy values of 1.1171 day⁻¹ and 3.8336 kJ/mol, respectively. The utilization of coal fly ash and silica geothermal sludge as a dry activator is, indeed, an approach to realize the circular economy in electrical power generations.

Portland Versus Alkaline Cement: Continuity or Clean Break: "A Key Decision for Global Sustainability"

This review undertakes rigorous analysis of much of the copious literature available to the scientific community on the use of alkali-activated binders (AABs) in construction. The authors’ main intention is to categorically refute arguments of that part of the scientific community underestimating or even dismissing the actual potential of AABs as alternatives to Portland cement (PC). The main premise invoked in support of those arguments is a presumed lack of material resources for precursors that would make AAB industrial-scale production unfeasible anywhere on the planet (a substantial number of scientific papers show that the raw materials required for AAB manufacture are in abundance worldwide). The review also analyses the role of alkaline activators in the chemistry of AABs; it is important to clarify and highlight that alkaline activators are not, by any means, confined to the two synthetic products (caustic soda and waterglass) mostly employed by researchers; other sustainable and efficient products are widely available. Finally, the review deals with the versatility of AAB production processes. The technologies required for the large scale manufacturing of AABs are mostly already in place in PC factories; actually no huge investment is required to transform a PC plant in a AAB factory; and quality and compositional uniformity of Alkaline Cements (binders produced through an industrial process) would be guaranteed. The last conclusions extracted from this review-paper are related with: i) the low carbon footprint of one-part AABs and ii) the urgent need of exploring standardization formulas allowing the commercial development of (sustainable) binders different from PC.

Controlled preparation of cerium oxide loaded slag-based geopolymer microspheres (CeO2@SGMs) for the adsorptive removal and solidification of F- from acidic waste-water

A new cerium oxide loaded slag-based geopolymer microspheres (CeO₂@SGMs) was prepared by a two-step i.e. dispersion-suspension-solidification and in-situ co-precipitation method. The optimal parameters for the preparation of 0.02CeO₂@SGMs were slag (30 g), 1.7 M water glass (12.86 g), water (8 g) and 0.02 mol/L of Ce⁴⁺. 0.02CeO₂@SGMs was characterized by SEM, XRD, BET, EDX, FTIR, XPS and PSD techniques. The leaching concentration of Ca²⁺ (95.65 mg/L) was only 1/5 of the SGMs at pH 2 after the modification of CeO₂. Adsorption data fitted well with Freundlich isotherm model suggesting multilayer adsorption mechanism with a maximum adsorption capacity for F^- by 0.02CeO₂@SGMs of 121.77 mg/g at 298 K. The negative values of thermodynamic parameters (ΔH⁰ and ΔS⁰) indicated the exothermic nature of the adsorption process with reduced chaos of the whole system. 0.02CeO₂@SGMs exhibited excellent dynamic adsorption performance at 4 mL/min F^- solution flow rate. The influence of various co-existing anions on adsorption of F^- over 0.02CeO₂@SGMs followed an order of: Cl^- ≈ NO₃^- < SO₄^2- << PO₄^3-. Attributed to the facile preparation process, cost-effectiveness and environmental friendliness, the newly designed 0.02CeO₂@SGMs can be deemed of promising industrial applications for the abatement of F^- from wastewater.

Inhibition of Efflorescence in Na-Based Geopolymer Inorganic Coating

Coating is one of the most important high-value-added application cases in geopolymer materials. However, efflorescence can easily cause discoloration and reduce the esthetic impression of the coating surface, thus limiting its application; hence, inhibition of efflorescence is one of the most important techniques in the application of geopolymer coatings. Efflorescence is a spontaneous behavior in a Na-based geopolymer, involving the migration of soluble alkalis. Alkalis are dissolved by water and diffuse to the material surface through nocuous pores, and then react with CO₂ to produce white carbonate products. To inhibit efflorescence in geopolymer coating, this article reports a structure modification method using polydimethy siloxane (PS) and mica. To explore the inhibition mechanism, the effects of PS and mica on the pore structure, water absorption, alkali leaching, and efflorescence product were investigated. The experimental results showed that a harmful pore structure and instinctive water absorption of the geopolymer strongly contributed to efflorescence. PS and mica could reduce the pore size distribution and porosity and are helpful to establish a waterproof structure, leading to water absorption and the alkali leaching rate being significantly suppressed. Both high water glass and water content play a critical role in the increase of efflorescence, but even under a high content of water glass and water used in geopolymer coating, this method shows an 80-90% efflorescence reduction, which is much higher than that of other studies. In practical engineering, when the geopolymer coating is applied after modification, even if it is exposed to the field environment for a long time, there is no efflorescence deposit on the coating surface. It is feasible to limit water ingression in a geopolymer, which effectively blocks the efflorescence reaction process. This method is simple and practical and can be applied in practical engineering applications of geopolymer coatings conveniently.

Preparation of a Composite Calcium Silicate Board with Carbide Slag and Coal-Based Solid Waste Activated by Different Alkali Activators

Overall performance of composite calcium silicate boards (CCSBs) was investigated to further promote their application. The alkali activators were used to fully hydrate the calcium and silicon raw materials, which further improved the comprehensive performance of the CCSBs made of four pure industrial solid wastes. Within the range of dosage in this study, single doping of different proportions of the alkali activator improved the flexural strength of the CCSB. Based on this, the mechanical properties of the CCSB were further improved as the compounded alkali activator was optimized. Flexural strength is improved when the average pore diameter was refined. The freeze-thaw cycle test shows that a compound-doped alkali activator can effectively reduce the mass loss and strength loss, thereby improving the frost resistance of this material. This research discussed an economically affordable approach to prepare the CCSB material made of industrial solid waste.

The Application of Electrical Parameters to Reflect the Hydration Process of Cement Paste with Rice Husk Ash

This paper aims to study the electrical parameters (electrical resistivity and alternating current (AC) impedance spectroscopy) of cement paste with rice husk ash (RHA). The water to cement (Mass ratio of water to cement (w/c)) ratios of the paste in this study varied from 0.4 to 0.5. The mass ratio of rice husk ash in each w/c ratio of specimens ranged from 0% to 15% by t mass of cement. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to determine the microstructures of specimens. Moreover, the slump flow and plastic viscosity of fresh paste were determined. The results indicated that with the increasing dosage of RHA, the fluidity decreased, while the plastic viscosity increased. Meanwhile, a high w/c ratio led to a low plastic viscosity and high slump flow. The electrical resistivity of RHA cement paste gradually ascended with the increasing curing period. The conduction of specimens intricately changed by mixing RHA, a reasonable equivalent circuit was selected to describe the conduction mechanism by AC impedance spectroscopy. Additionally, the results of XRD and SEM showed that RHA could effectively promote the hydration process as well as decrease the size and number of cracks in hardened cement paste.