Direct Carbonation of Ca(OH)2 Using Liquid and Supercritical CO2: Implications for Carbon-Neutral Cementation

Investigation of Carbonation Kinetics in Carbonated Cementitious Materials by Reactive Molecular Dynamics Simulations

Calcium carbonate (CaCO3) precipitation plays a significant role during the carbon capture process; however, the mechanism is still only partially understood. Understanding the atomic-level carbonation mechanism of cementitious materials can promote the mineralization capture, immobilization, and utilization of carbon dioxide, as well as the improvement of carbonated cementitious materials' performance. Therefore, based on molecular dynamics simulations, this paper investigates the effect of Si/Al concentrations in cementitious materials on carbonation kinetics. We first verify the force field used in this paper. Then, we analyze the network connectivity evolution, the number and size of the carbonate cluster during gelation, the polymerization rate, and the activation energy. Finally, in order to reveal the reasons that caused the evolution of polymerization rate and activation energy, we analyze the local stress and charge of atoms. Results show that the Ca-Oc bond number and carbonate cluster size increase with the decrease of the Si/Al concentration and the increase of temperature, leading to the higher amorphous calcium carbonate gel polymerization degree. The local stress of each atom in the system is the driving force of the gelation transition. The presence of Si and Al components increases the atom's local stress and average charge, thus causing the increase of the energy barrier of CaCO3 polymerization and the activation energy of carbonation.

Analyzing the upscaling potential and geospatial siting of calcination-free calcium hydroxide production in the United States

This study evaluates the techno-economic feasibility and the embodied carbon dioxide intensity (eCI) of a novel process for producing nominally pure (>95 mass %) calcium hydroxide without the need for the thermal calcination of limestone. The process relies on the aqueous extraction of calcium from alkaline industrial wastes following which portlandite (Ca(OH)2: CH, a.k.a. slaked lime or hydrated lime) is precipitated by application of a waste-heat based thermal swing. This approach takes advantage of the temperature dependent solubility of CH at ambient pressure. We evaluated the feasibility of implementing this process in the U.S. based on the geospatial availability of waste heat and slags as a Ca-source. For the base case, the cost of production of "Low-Temperature Portlandite (LTP)" is 2-to-3 times that of traditional portlandite (∼$180/tonne). The main driver of cost is the electricity demand for reverse osmosis (RO) which is used to concentrate Ca-ions in solution, and the costs of membrane replacement. Our sensitivity analysis showed that parity with the cost of production of traditional portlandite is readily achievable by selecting membranes with better durability (i.e., better pH resistance) and flux (i.e., higher permeability) without sacrificing selectivity. Significantly, LTP features an eCI that is between 40%- and - 80 % lower than its traditional counterpart when electricity is sourced from natural gas combustion or wind power, respectively. Finally, our geospatial analysis reveals that there are three areas in the U.S. with the potential for implementation of industrial-scale facilities that could produce at least 50 tonnes of pure Ca(OH)2 per day, while achieving a production cost of ∼$270 per tonne of Ca(OH)2, owing to the proximity between slag feedstocks and waste heat sources.

Effects of temperature and CO2 concentration on the early stage nucleation of calcium carbonate by reactive molecular dynamics simulations

It is significant to investigate the calcium carbonate (CaCO3) precipitation mechanism during the carbon capture process; nevertheless, CaCO3 precipitation is not clearly understood yet. Understanding the carbonation mechanism at the atomic level can contribute to the mineralization capture and utilization of carbon dioxide, as well as the development of new cementitious materials with high-performance. There are many factors, such as temperature and CO2 concentration, that can influence the carbonation reaction. In order to achieve better carbonation efficiency, the reaction conditions of carbonation should be fully verified. Therefore, based on molecular dynamics simulations, this paper investigates the atomic-scale mechanism of carbonation. We investigate the effect of carbonation factors, including temperature and concentration, on the kinetics of carbonation (polymerization rate and activation energy), the early nucleation of calcium carbonate, etc. Then, we analyze the local stresses of atoms to reveal the driving force of early stage carbonate nucleation and the reasons for the evolution of polymerization rate and activation energy. Results show that the higher the calcium concentration or temperature, the higher the polymerization rate of calcium carbonate. In addition, the activation energies of the carbonation reaction increase with the decrease in calcium concentrations.

Silica Polymerization Driving Opposite Effects of pH on Aqueous Carbonation Using Crystalline and Amorphous Calcium Silicates

The aqueous carbonation of calcium silicate (CS), a representative alkaline-earth silicate, has been widely explored in studies of carbon dioxide (CO2) mineralization. In this context, we conducted a specific comparison of the carbonation behaviors between the crystalline calcium silicate (CCS) and amorphous calcium silicate (ACS) across a pH range from 9.0 to 12.0. Interestingly, we observed opposite pH dependencies in the carbonation efficiencies (i.e., CaO conversion into CaCO3 in 1 M Na2CO3/NaHCO3 solution under ambient conditions) of CCS and ACS─the carbonation efficiency of CCS decreased with increasing the solution basicity, while that of ACS showed an inverse trend. In-depth insights were gained through in situ Raman characterizations, indicating that these differing trends appeared to originate from the polymerization/depolymerization behaviors of silicates released from minerals. More specifically, higher pH conditions seemed to favor the carbonation of minerals containing polymerized silica networks. These findings may contribute to a better understanding of the fundamental factors influencing the carbonation behaviors of alkaline earth silicates through interfacial coupled dissolution and precipitation processes. Moreover, they offer valuable insights for selecting optimal carbonation conditions for alkaline-earth silicate minerals.

Cool Concrete Incorporating Carbonated Periwinkle Shell: A Sustainable Solution for Mitigating Urban Heat Island Effects

The urban heat island effect has become a critical issue in urban areas, intensifying heat-related problems and increasing energy consumption. A sustainable cement formulation that combines ordinary Portland cement (OPC) with a carbonated aggregate derived from Periwinkle shell powder for the development of an efficient cool material is presented. Through a carbonation process, the aggregate undergoes a transformation, capturing carbon dioxide (CO2) and converting it into calcite. The resulting cement mixture exhibits high solar reflective properties, making it a potential candidate for cool pavement and roof applications. In this study, the raw materials, including the Periwinkle shell powder, were characterized, and the carbonation process was evaluated to quantify the CO2 capture efficiency. Additionally, a real test of the efficiency of this new cement on a roof demonstrated that the material achieved a significant cooling effect, being 6 °C cooler than that with standard OPC at the peak of solar radiation.

Performance and mechanism of CO2 absorption during the simultaneous removal of SO2 and NOx by wet scrubbing process

The co-removal of CO2 while removing SO2 and NOx from industrial flue gas has great potential of carbon emission reduction but related research is lacking. In this study, a wet scrubbing process with various urea solutions for desulfurization and denitrification was explored for the possibility of CO2 absorption. The results showed that the urea-additive solutions were efficient for NOx and SO2 abatement, but delivered < 10% CO2 absorption efficiency. The addition of Ca(OH)2 dramatically enhanced the CO2 absorption, remained the desulfurization efficiency, unfortunately restricted the denitrification efficiency. Among various operating parameters, pH of solution played a determining role during the absorption. The contradictory pH demands of CO2 absorption and denitrification were observed and discussed in detail. A higher pH of solution than 10 was favorable for CO2 absorption, while the oxidizing of NO to NO2, NO2- or NO3- by NaClO2 was inhibited in this condition. When 7 < pH < 10, it was favorable for the conversion and absorption of NO and NOx. However, the conversion of HCO3- to CO32- was significantly inhibited, hence preventing the absorption of CO2. Large part of Ca(OH)2 became CaCO3 with a finer particle size, which covered the unreacted Ca(OH)2 surface after the reaction. Kinetic analysis showed that the CO2 absorption in urea-NaClO2-Ca(OH)2 absorbent was controlled by chemical reaction in early stage, then by ash layer diffusion in later stage.

Performance Assessment of Carbon Dioxide Sequestration in Cement Composites with a Granulation Technique

The cement industry emits a significant amount of carbon dioxide (CO2). Therefore, the cement industry should recycle the emitted CO2. However, sequestration by carbonation in cement composites absorbs a very small amount of CO2. Therefore, a direct way of achieving this is to improve the absorption performance of CO2 in cement composites. In this study, to improve absorption, unlike in existing studies, a granulation technique was applied, and the material used was calcium hydroxide (CH). In addition, granulated CH was coated to prevent a reaction during the curing of cement paste. The coated CH granule (CCHG) was applied to 5% of the cement weight as an additive material, and the specimens were cured for 91 days to wait for the coating of CCHG to fully phase-change. The experiment of CO2 absorption showed an unexpected result, where the use of blast furnace slag (BFS) and fly ash (FA) had a negative effect on CO2 sequestration. This was because BFS and FA had a filler effect in the cement matrix, and the filler effect caused the blocking of the path of CO2. In addition, BFS and FA are well-known pozzolanic materials; the pozzolan reaction caused a reduction in the amount of CH because the pozzolan reaction consumed the CH to produce a calcium silicate hydrate. Therefore, the pozzolan reaction also had a negative effect on the CO2 sequestration performance combined with the filler effect. The CO2 sequestration efficiency was decreased between ordinary cement paste and BFS-applied specimens by 45.45%. In addition, compared to cases of ordinary cement paste and FA-applied specimens, the CO2 sequestration performance was decreased by 63.64%. Comprehensively, CO2 sequestration performance depends on the porosity and amount of CH.

An optimized approach towards bio-capture and carbon dioxide sequestration with microalgae Phormidium valderianum using response surface methodology

As carbon dioxide emissions rise, there's need for alternative strategies, including microorganisms, to capture and mitigate them. The present study investigated on the capability and tolerance of microalgal strain, Phormidium valderianum to capture gaseous CO2 at varying levels (5-30 %). A biomass productivity of 0.0216 ± 0.027 gL-1day-1 and rate of CO2 fixation of 0.035 gL-1day-1 was obtained for 25 % CO2 concentration. From this study, it is evident that higher CO2 levels led to elevated carbohydrate concentration. In addition, protein concentration doubled with the introduction of 25 % CO2. In optimization studies, pH 10, 25 % CO2, and 200 mg/L of Ca(OH)2 concentration was found to be optimal for biomass growth. A higher rate of CO2 fixation of 0.315 gL-1day-1 was achieved at these optimum conditions using response surface methodology. Furthermore, the study demonstrated that microalgae, Phormidium valderianum has the potential to serve as a promising alternative for capturing CO2 emissions.

Room temperature bio-engineered multifunctional carbonates for CO2 sequestration and valorization

This contribution reports, for the first time, on an entirely green bio-engineering approach for the biosynthesis of single phase crystalline 1-D nano-scaled calcite CaCO3. This was validated using H2O as the universal solvent and natural extract of Hyphaene thebaica fruit as an effective chelating agent. In this room temperature green process, CaCl2 and CO2 are used as the unique source of Ca and CO3 respectively in view of forming nano-scaled CaCO3 with a significant shape anisotropy and an elevated surface to volume ratio. In terms of novelty, and relatively to the reported scientific and patented literature in relation to the fabrication of CaCO3 by green nano-chemistry, the current cost effective room temperature green process can be singled out as per the following specificities: only water as universal solvent is used, No additional base or acid chemicals for pH control, No additional catalyst, No critical or supercritical CO2 usage conditions, Only natural extract of thebaica as a green effective chelating agent through its phytochemicals and proper enzematic compounds, room Temperature processing, atmospheric pressure processing, Nanoscaled size particles, and Nanoparticles with a significant shape anisotropy (1-D like nanoparticles). Beyond and in addition to the validation of the 1-D synthesis aspect, the bio-engineered CaCO3 exhibited a wide-ranging functionalities in terms of highly reflecting pigment, an effective nanofertilizer as well as a potential binder in cement industry.

Rheological Characteristics of Starch-Based Biodegradable Blends

Thermoplastic starch was blended with commercially available biodegradable polyesters of poly(butylene adipate-co-terephthalate) (PBAT) and poly(lactic acid) (PLA) for its improved performance and processability. The morphology and elemental composition of these biodegradable polymer blends were observed by scanning electron microscopy and energy dispersive X-ray spectroscopy, respectively, while their thermal properties were analyzed using thermogravimetric analysis and differential thermal calorimetry. For rheological analysis, the steady shear and dynamic oscillation tests of three samples at various temperatures were investigated using a rotational rheometer. All three samples exhibited significant shear thinning at all measured temperatures, and their shear viscosity behavior was plotted using the Carreau model. The frequency sweep tests showed that the thermoplastic starch sample exhibited a solid state at all temperatures tested, whereas both starch/PBAT and starch/PBAT/PLA blend samples exhibited viscoelastic liquid behavior after the melting temperature such that their loss modulus at low frequencies was greater than the storage modulus, and inversion occurred at high frequencies (storage modulus > loss modulus).

Effect of the Concrete Slurry Waste Ratio on Supercritical CO2 Sequestration

To prevent drastic climate changes due to global warming, it is necessary to transition to a carbon-neutral society by reducing greenhouse gas emissions in all industrial sectors. This study aimed to develop carbon utilization sequestration technology that uses the concrete slurry water generated during the production of concrete as a new CO₂ sink to reduce CO₂ emissions from the cement industry. This was achieved by performing supercritical CO₂ carbonation by varying the concrete slurry waste (CSW) ratio. The study's results confirmed that, according to the CSW ratio (5 to 25%), complete carbonation occurred within only 10 min of the reaction at 40 °C and 100 bar.

New insights into the early stage nucleation of calcium carbonate gels by reactive molecular dynamics simulations

The precipitation of calcium carbonate (CaCO₃) is a key mechanism in carbon capture applications relying on mineralization. In that regard, Ca-rich cementitious binders offer a unique opportunity to act as a large-scale carbon sink by immobilizing CO₂ as calcium carbonate by mineralization. However, the atomistic mechanism of calcium carbonate formation is still not fully understood. Here, we study the atomic scale nucleation mechanism of an early stage amorphous CaCO₃ gel based on reactive molecular dynamics (MD) simulations. We observe that reactive MD offers a notably improved description of this reaction as compared to classical MD, which allows us to reveal new insights into the structure of amorphous calcium carbonate gels and formation kinetics thereof.

Supercritical CO2-Induced Evolution of Alkali-Activated Slag Cements

The phase changes in alkali-activated slag samples when exposed to supercritical carbonation were evaluated. Ground granulated blast furnace slag was activated with five different activators. The NaOH, Na₂SiO₃, CaO, Na₂SO₄, and MgO were used as activators. C-S-H is identified as the main reaction product in all samples along with other minor reaction products. The X-ray diffractograms showed the complete decalcification of C-S-H and the formation of CaCO₃ polymorphs such as calcite, aragonite, and vaterite. The thermal decomposition of carbonated samples indicates a broader range of CO₂ decomposition. Formation of highly cross-linked aluminosilicate gel and a reduction in unreacted slag content upon carbonation is observed through ²⁹Si and ²⁷Al NMR spectroscopy. The observations indicate complete decalcification of C-S-H with formation of highly cross-linked aluminosilicates upon sCO₂ carbonation. A 20-30% CO₂ consumption per reacted slag under supercritical conditions is observed.

Carbon-negative cement manufacturing from seawater-derived magnesium feedstocks

This study describes and demonstrates key steps in a carbon-negative process for manufacturing cement from widely abundant seawater-derived magnesium (Mg) feedstocks. In contrast to conventional Portland cement, which starts with carbon-containing limestone as the source material, the proposed process uses membrane-free electrolyzers to facilitate the conversion of carbon-free magnesium ions (Mg²⁺) in seawater into magnesium hydroxide [Mg(OH)₂] precursors for the production of Mg-based cement. After a low-temperature carbonation curing step converts Mg(OH)₂ into magnesium carbonates through reaction with carbon dioxide (CO₂), the resulting Mg-based binders can exhibit compressive strength comparable to that achieved by Portland cement after curing for only 2 days. Although the proposed "cement-from-seawater" process requires similar energy use per ton of cement as existing processes and is not currently suitable for use in conventional reinforced concrete, its potential to achieve a carbon-negative footprint makes it highly attractive to help decarbonize one of the most carbon-intensive industries.

Supercritical CO2 Curing of Resource-Recycling Secondary Cement Products Containing Concrete Sludge Waste as Main Materials

This study aims to develop highly durable, mineral carbonation-based, resource-recycling, secondary cement products based on supercritical carbon dioxide (CO2) curing as part of carbon capture utilization technology that permanently fixes captured CO2. To investigate the basic characteristics of secondary cement products containing concrete sludge waste (CSW) as the main materials after supercritical CO2 curing, the compressive strengths of the paste and mortar (fabricated by using CSW as the main binder), ordinary Portland cement, blast furnace slag powder, and fly ash as admixtures were evaluated to derive the optimal mixture for secondary products. The carbonation curing method that can promote the surface densification (intensive CaCO3 formation) of the hardened body within a short period of time using supercritical CO2 curing was defined as “Lean Carbonation”. The optimal curing conditions were derived by evaluating the compressive strength and durability improvement effects of applying Lean Carbonation to secondary product specimens. As a result of the experiment, for specimens subjected to Lean Carbonation, compressive strength increased by up to 12%, and the carbonation penetration resistance also increased by more than 50%. The optimal conditions for Lean Carbonation used to improve compressive strength and durability were found to be 35 °C, 80 bar, and 1 min.

Fundamental Studies on CO2 Sequestration of Concrete Slurry Water Using Supercritical CO2

To prevent drastic climate change due to global warming, it is necessary to transition to a carbon-neutral society by reducing greenhouse gas emissions in all industrial sectors. This study aims to prepare measures to reduce the greenhouse gas in the cement industry, which is a large source of greenhouse gas emissions. The research uses supercritical CO₂ carbonation to develop a carbon utilization fixation technology that uses concrete slurry water generated via concrete production as a new CO₂ fixation source. Experiments were conducted using this concrete slurry water and supernatant water under different conditions of temperature (40 and 80 °C), pressure (100 and 150 bar), and reaction time (10 and 30 min). The results showed that reaction for 10 min was sufficient for complete carbonation at a sludge solids content of 5%. However, reaction products of supernatant water could not be identified due to the presence of Ca(HCO₃)₂ as an aqueous solution, warranting further research.

Enflamed CO2 emissions from cement production in Nepal

Cement industry is one of the main contributors to greenhouse gas (GHG) emissions, specifically carbon dioxide (CO₂). This paper presents the cement production and the CO₂ emissions from the cement industry in Nepal. We compute emissions for the process-related, combustion-related (fuel use), and electricity-related activities during the cement production. We used eight emission factors (EFs) for the process-related, two EFs for the combustion or fuel-related, and two for the electricity-related activities using the previous researches. We computed the emissions as a product of the activities and the EFs. The estimated CO₂ emission in 2019 from the cement production is 3.45 ± 0.50 million metric tons (mMt) for Nepal. In 2019, the emissions are 1.87 ± 0.16 mMt from the process-related, 1.52 ± 0.34 mMt from the combustion-related, and 0.062 ± 0.004 mMt from the electricity use activities during the cement production in Nepal. Cumulative CO₂ emission was 22.73 ± 3.82 mMt from 1987 to 2019. Per capita CO₂ emission is 0.12 mMt for Nepal in 2019. Nepal contributes about 0.06% CO₂ emission from cement production to the global CO₂ emission (2.08 Gt) from the cement industry. By evaluating per capita gross domestic product (GDP) (from 1987/1988 to 2019/2020) and the human development index (HDI) (from 1990 to 2019) with the cement production, the result shows that cement production increases significantly (p < 0.01) with an increase in the GDP and the HDI. We emphasize that the study's outputs are directly relevant to the country's emission inventory, mitigation planning, and developing a strategy for cleaner production.

MgO-Based Cementitious Composites for Sustainable and Energy Efficient Building Design

Concrete made with Portland cement is by far the most heavily used construction material in the world today. Its success stems from the fact that it is relatively inexpensive yet highly versatile and functional and is made from widely available raw materials. However, in many environments, concrete structures gradually deteriorate over time. Premature deterioration of concrete is a major problem worldwide. Moreover, cement production is energy-intensive and releases a lot of CO2; this is compounded by its ever-increasing demand, particularly in developing countries. As such, there is an urgent need to develop more durable concretes to reduce their environmental impact and improve sustainability. To avoid such environmental problems, researchers are always searching for lightweight structural materials that show high performance during both processing and application. Among the various candidates, Magnesia (MgO) seems to be the most promising material to attain this target. This paper presents a comprehensive review of the characteristics and developments of MgO-based composites and their applications in cementitious materials and energy-efficient buildings. This paper starts with the characterization of MgO in terms of environmental production processes, calcination temperatures, reactivity, and micro-physical properties. Relationships between different MgO composites and energy-efficient building designs were established. Then, the influence of MgO incorporation on the properties of cementitious materials and indoor environmental quality was summarized. Finally, the future research directions on this were discussed.

Nano Calcium Carbonate (CaCO3) as a Reliable, Durable, and Environment-Friendly Alternative to Diminishing Fly Ash

Fly ash is widely used in the cement industry to improve the performance and durability of concrete. The future availability of fly ash, however, is a concern, as most countries are inclining towards renewable energy sources as opposed to fossil fuels. Additional concerns have been raised regarding the impact of strict environmental regulations on fly ash quality and variability. This paper, therefore, evaluates if nano calcium carbonate (nano CaCO₃) can be used as an alternative to fly ash. This paper presents comprehensive testing results (fresh, hardened, and durability) for OPC (Ordinary Portland Cement) and PLC (Portland Limestone Cement) concretes with 1% nano CaCO₃ and compares them to those for concretes with fly ash (both Class F and C). Compared to concretes with fly ash, OPC and PLC with nano CaCO₃ presented improved testing results in most cases, including later age strength, permeability, and scaling resistance. As nanotechnology in concrete is a relatively new topic, more research on the efficient use of nanotechnology, such as for proper dispersion of nano CaCO₃ in the concrete, has potential to offer increased benefits. Further, nano CaCO₃ is environmentally and economically viable, as it has the potential to be produced within the cement plant while utilizing waste CO₂ and generating economic revenue to the industry. Thus, nano CaCO₃ has the potential to serve as an alternative to fly ash in all beneficial aspects—economic, environmental, and technical.

Carbonation Reaction Mechanisms of Portlandite Predicted from Enhanced Ab Initio Molecular Dynamics Simulations

Geological carbon capture and sequestration (CCS) is a promising technology for curbing the global warming crisis by reduction of the overall carbon footprint. Degradation of cement wellbore casings due to carbonation reactions in the underground CO2 storage environment is one of the central issues in assessing the long-term success of the CCS operations. However, the complexity of hydrated cement coupled with extreme subsurface environmental conditions makes it difficult to understand the carbonation reaction mechanisms leading to the loss of well integrity. In this work, we use biased ab initio molecular dynamics (AIMD) simulations to explore the reactivity of supercritical CO2 with the basal and edge surfaces of a model hydrated cement phase—portlandite—in dry scCO2 and water-rich conditions. Our simulations show that in dry scCO2 conditions, the undercoordinated edge surfaces of portlandite experience a fast barrierless reaction with CO2, while the fully hydroxylated basal surfaces suppress the formation of carbonate ions, resulting in a higher reactivity barrier. We deduce that the rate-limiting step in scCO2 conditions is the formation of the surface carbonate barrier which controls the diffusion of CO2 through the layer. The presence of water hinders direct interaction of CO2 with portlandite as H2O molecules form well-structured surface layers. In the water-rich environment, CO2 undergoes a concerted reaction with H2O and surface hydroxyl groups to form bicarbonate complexes. We relate the variation of the free-energy barriers in the formation of the bicarbonate complexes to the structure of the water layer at the interface which is, in turn, dictated by the surface chemistry and the degree of nanoconfinement.

Measurement of heat release during hydration and carbonation of ash disposed in landfills using an isothermal calorimeter

Temperatures as high as 100 °C have been reported at a few municipal solid waste (MSW) landfills in the U.S. A recently published model describing landfill heat accumulation identified reactions that contribute significant heat to landfills including the hydration and carbonation of Ca-containing wastes such as ash from MSW and coal combustion. The objective of this study was to develop a method to measure heat release from Ca-containing ash by isothermal calorimetry. The method was confirmed by comparing measured heat release from hydration and carbonation of pure CaO and Ca(OH)₂ to the theoretical heat. Theoretical heat release was determined by characterizing test materials before and after experiments using thermogravimetric analysis (TGA) and X-ray diffraction (XRD). Heat recovery efficiencies with both water and synthetic leachate ranged from 79 to 90% for CaO hydration and between 65 and 74% for Ca(OH)₂ carbonation, with no effect attributable to leachate. Additionally, simultaneous hydration and carbonation of CaO/Ca(OH)₂ mixtures resulted in efficiencies of 65 to 74%. The developed method was applied to eight samples that were excavated from a landfill and known to contain coal ash, and the ratio of measured to theoretical heat was 0.5 to 4. Thus, calculation of theoretical heat release from XRD data was not a good predictor of the experimentally measured heat release. The developed method can be used by landfill operators to evaluate the heat potential of a waste, thereby facilitating decisions on the quantity of a waste that can be buried in consideration of landfill temperatures.

Effects of pH and metal composition on selective extraction of calcium from steel slag for Ca(OH)2 production

This research article explains the effects of pH and metal composition on the selective calcium extraction from steel slag. The operating parameters including extraction solvent type, solvent concentration, metal composition of steel slag, substance type and pH were investigated. HCl, NH₄Cl, NH₄OH and NaOH were employed as solvents to extract Ca from steel slag. It has been shown that hydrochloric acid effectively extracts Ca. The high metal content in steel slag reacted sensitively to the solvent concentration, and a specific concentration was derived to selectively extract Ca. The optimal solvent for calcium extraction was 2 M HCl, which induced the extraction of 97% of Ca; 46% of Mg; 35% of Al; and 1% of Si from the steel slag. In order to separate Ca in the leaching solution from other metal ions, various acidic/basic substances were added to regulate the pH. The optimal pH level for removing the impurities without calcium was found to be 9.5. The precipitated impurities were removed by filtration, and the pH was adjusted to 13 or higher for Ca(OH)₂(s) production. In conclusion, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) revealed that the Ca content produced through the process was more than 99%. It is expected that high-purity Precipitated Calcium Carbonate (PCC) will be achieved when the generated Ca(OH)₂ is used as a source of calcium for mineral carbonation.

This research article explains the effects of pH and metal composition on the selective calcium extraction from steel slag.

Calcination-free production of calcium hydroxide at sub-boiling temperatures

Calcium hydroxide (Ca(OH)₂), a commodity chemical, finds use in diverse industries ranging from food, to environmental remediation and construction. However, the current thermal process of Ca(OH)₂ production via limestone calcination is energy- and CO₂-intensive. Herein, we demonstrate a novel aqueous-phase calcination-free process to precipitate Ca(OH)₂ from saturated solutions at sub-boiling temperatures in three steps. First, calcium was extracted from an archetypal alkaline industrial waste, a steel slag, to produce an alkaline leachate. Second, the leachate was concentrated using reverse osmosis (RO) processing. This elevated the Ca-abundance in the leachate to a level approaching Ca(OH)₂ saturation at ambient temperature. Thereafter, Ca(OH)₂ was precipitated from the concentrated leachate by forcing a temperature excursion in excess of 65 °C while exploiting the retrograde solubility of Ca(OH)₂. This nature of temperature swing can be forced using low-grade waste heat (≤100 °C) as is often available at power generation, and industrial facilities, or using solar thermal heat. Based on a detailed accounting of the mass and energy balances, this new process offers at least ≈65% lower CO₂ emissions than incumbent methods of Ca(OH)₂, and potentially, cement production.

A calcination-free route to produce calcium hydroxide from alkaline industrial wastes including leaching, concentration, and temperature-swing precipitation.

The role of gas flow distributions on CO2 mineralization within monolithic cemented composites: coupled CFD-factorial design approach

Optimizing the spatial distribution of contacting gas and the gas processing conditions enhances CO₂ mineralization reactions and material properties of carbonate-cementitious monoliths.

Mechanistic insight into mineral carbonation and utilization in cement-based materials at solid-liquid interfaces

In order to ensure the viability of CO₂ mineralization and utilization using alkaline solid waste, a mechanistic understanding of reactions at mineral–water interfaces was required to control the reaction pathways and kinetics. In this study, we provided new information for understanding the reactions of CO₂ mineralization and utilization at mineral–water interfaces. Here we have carried out high-energy synchrotron X-ray analyses to characterize the changes of mineral phases in petroleum coke fly ash during CO₂ mineralization and their subsequent utilization as supplementary cementitious materials in cement mortars. The 2-D synchrotron patterns were converted to 1-D diffraction patterns and the results were then interpreted via the Rietveld refinement. The results indicated that there was a continuous source of calcium ions mainly due to the dissolution of CaO and Ca(OH)₂ in fly ash. This would actually enhance the driving force of saturation index at the solid–fluid interfacial layer, and then could eventually result in the nucleation and growth of calcium carbonate (calcite) at the interface. A small quantity of CaSO₄ (anhydrite) in fly ash was also dissolved and simultaneously converted into calcite. In addition, the calcium sulfate in fly ash would effectively prevent the early hydration of tricalcium aluminate in blended cement, and thus could avoid the negative impact on its strength development. The proposed reaction mechanisms were also qualitatively verified by X-ray fluorescence mapping and electron microscopy. These results would help to design efficient reactors and cost-effective processes for CO₂ mineralization and utilization in the future.

Synchrotron-based X-ray analyses for understanding the reactions at mineral–water interfaces for CO₂ mineralization and utilization using petroleum coke fly ash.

Heterogeneous chemistry in the 3-D state: an original approach to generate bioactive, mechanically-competent bone scaffolds

The present work investigates heterogeneous gas-solid reactions involved in the biomorphic transformation of natural wood into large 3-D hydroxyapatite (HA) scaffolds recapitulating physico-chemical, morphological and mechanical features typical of natural bone. In particular, we found that the use of a reactive CO2/H2O gas mixture, under supercritical conditions at high pressure, permits to control heterogeneous CaO-CO2 reactions throughout the whole bulk and to direct the nucleation-growth of CaCO3 at a relatively low temperature, thus obtaining a highly reactive 3-D precursor enabling the formation of a large biomorphic HA scaffold preserving fine nanostructure by a hydrothermal process. To the best of our knowledge, the application of heterogeneous chemical reactions in the 3-D state is an original way to generate large HA scaffolds maintaining bio-relevant ionic substitutions, with specific regard to Mg2+, Sr2+ and CO32- ions, conferring a superior ability to guide cell fate. We hypothesize that the original nanostructure of the final 3-D HA scaffold, not achievable by the classic sintering procedure, and the multi-scale hierarchical organization inherited by the original template, account for its high compression strength with damage-tolerant mechanical behaviour. The ability of the new scaffold to induce bone regeneration is attested by the overexpression of genes, early and late markers of the osteogenic differentiation pathway, and by the in vivo osteoinductivity. We hypothesize that the unique association of bioactive chemical composition, nanostructure and multi-scale hierarchy can synergistically act as instructing signals for cells to generate new bone tissue with organized 3-D architecture. These results point to its great applicative potential for the regeneration of large bone defects, which is a still unmet clinical need.

The carbonation kinetics of calcium hydroxide nanoparticles: A Boundary Nucleation and Growth description

HYPOTHESIS

The reaction of Ca(OH)₂ with CO₂ to form CaCO₃ (carbonation process) is of high interest for construction materials, environmental applications and art preservation. Here, the "Boundary Nucleation and Growth" model (BNGM) was adopted for the first time to consider the effect of the surface area of Ca(OH)₂ nanoparticles on the carbonation kinetics.

EXPERIMENTS

The carbonation of commercial and laboratory-prepared particles' dispersions was monitored by Fourier Transform Infrared Spectroscopy, and the BNGM was used to analyze the data. The contributions of nucleation and growth of CaCO₃ were evaluated separately.

FINDINGS

During carbonation the boundary regions of the Ca(OH)₂ particles are densely populated with CaCO₃ nuclei, and transform early with subsequent thickening of slab-like regions centered on the original boundaries. A BNGM limiting case equation was thus used to fit the kinetics, where the transformation rate decreases exponentially with time. The carbonation rate constants, activation energies, and linear growth rate were calculated. Particles with larger size and lower surface area show a decrease of the rate at which the non-nucleated grains between the boundaries transform, and an increase of the ending time of Ca(OH)₂ transformation. The effect of temperature on the carbonation kinetics and on the CaCO₃ polymorphs formation was evaluated.

Thermally induced carbonation of Ca(OH)2 in a CO2 atmosphere: kinetic simulation of overlapping mass-loss and mass-gain processes in a solid-gas system

Thermally induced carbonation of Ca(OH)₂ in a CO₂ atmosphere is a reaction exhibiting particular features, including stoichiometric completeness to form CaCO₃ and a kinetic advantage over the carbonation of CaO particles. This study aims to gain further insight into the reaction mechanisms of CO₂ capture by Ca(OH)₂ and CaO. It focuses on the kinetic modeling of the carbonation of Ca(OH)₂ as a consecutive reaction in a solid-gas system. The kinetic behaviors of the thermal decomposition of Ca(OH)₂ in an inert gas atmosphere and of the overall process of thermally induced carbonation of Ca(OH)₂ in a CO₂ atmosphere were investigated using thermal analyses and other complementary techniques. Based on kinetic results, the overall reaction of the thermally induced carbonation of Ca(OH)₂ in a CO₂ atmosphere was separated by a kinetic deconvolution analysis into two consecutive reaction steps: the thermal decomposition of Ca(OH)₂ and the subsequent carbonation of the CaO intermediate. The relationship between the two component reaction processes was well illustrated by a consecutive shrinkage of the dual reaction interfaces of Ca(OH)₂-CaO and CaO-CaCO₃. The continuous supply of water vapor and CO₂ to the CaO-CaCO₃ interface from different directions was suggested to be the physico-geometrical advantageous feature of the carbonation of Ca(OH)₂.

Remediation of 137Cs-contaminated concrete rubble by supercritical CO2 extraction

The removal of cesium contamination is a critical issue for the recycling of concrete rubble in most decommissioning operations. The high solvent strength and diffusivity of supercritical CO₂ make it an attractive choice as vector for extractant system in this context. Experimental extraction runs have been carried out in a radioactive environment on rubble contaminated with ¹³⁷Cs. The best extraction system was found to be CalixOctyl (25,27-Bis(1-octyloxy)calix[4]arene-crown-6, 1,3-alternate) with pentadecafluorooctanoic acid as a modifier. The effects of various operating parameters were investigated, namely the coarseness of rubble, the temperature of supercritical CO₂, the residual water and initial cesium concentrations, and the amounts of extractant and modifier used. The yields from direct extraction were low (<30%), because of the virtually irreversible sorption of Cs in concrete. The best extraction yield of ∼55% was achieved by leaching concrete rubble with nitric acid prior to supercritical CO₂ extraction.

Additive Manufacturing: Unlocking the Evolution of Energy Materials

The global energy infrastructure is undergoing a drastic transformation towards renewable energy, posing huge challenges on the energy materials research, development and manufacturing. Additive manufacturing has shown its promise to change the way how future energy system can be designed and delivered. It offers capability in manufacturing complex 3D structures, with near-complete design freedom and high sustainability due to minimal use of materials and toxic chemicals. Recent literatures have reported that additive manufacturing could unlock the evolution of energy materials and chemistries with unprecedented performance in the way that could never be achieved by conventional manufacturing techniques. This comprehensive review will fill the gap in communicating on recent breakthroughs in additive manufacturing for energy material and device applications. It will underpin the discoveries on what 3D functional energy structures can be created without design constraints, which bespoke energy materials could be additively manufactured with customised solutions, and how the additively manufactured devices could be integrated into energy systems. This review will also highlight emerging and important applications in energy additive manufacturing, including fuel cells, batteries, hydrogen, solar cell as well as carbon capture and storage.

Cements in the 21st Century: Challenges, Perspectives, and Opportunities

In a book published in 1906, Richard Meade outlined the history of portland cement up to that point1. Since then there has been great progress in portland cement-based construction materials technologies brought about by advances in the materials science of composites and the development of chemical additives (admixtures) for applications. The resulting functionalities, together with its economy and the sheer abundance of its raw materials, have elevated ordinary portland cement (OPC) concrete to the status of most used synthetic material on Earth. While the 20th century was characterized by the emergence of computer technology, computational science and engineering, and instrumental analysis, the fundamental composition of portland cement has remained surprisingly constant. And, although our understanding of ordinary portland cement (OPC) chemistry has grown tremendously, the intermediate steps in hydration and the nature of calcium silicate hydrate (C-S-H), the major product of OPC hydration, remain clouded in uncertainty. Nonetheless, the century also witnessed great advances in the materials technology of cement despite the uncertain understanding of its most fundamental components. Unfortunately, OPC also has a tremendous consumption-based environmental impact, and concrete made from OPC has a poor strength-to-weight ratio. If these challenges are not addressed, the dominance of OPC could wane over the next 100 years. With this in mind, this paper envisions what the 21st century holds in store for OPC in terms of the driving forces that will shape our continued use of this material. Will a new material replace OPC, and concrete as we know it today, as the preeminent infrastructure construction material?