Hydration Of CaO Research Articles

Properties of CaO-MgO-SiO2 insulating refractories prepared from phosphorus tailings

CaO-MgO-SiO2 insulating refractories were prepared using phosphorus tailings through in-situ pore-forming technology in this work, aiming to achieve the efficient utilization of solid waste phosphorus tailings. microsilica was used to react with free CaO generated by the decomposition of dolomite from phosphorus tailings during the calcination in order to prevent the hydration of CaO in the materials. The influence of microsilica addition and heating temperature on the properties of insulating refractories prepared from phosphorus tailings was investigated. The results showed that CaO-MgO-SiO2 insulating refractories with 8.8 wt% microsilica addition exhibited excellent comprehensive performance after heating at 1300 °C for 3 h. The insulation refractories prepared from phosphorus tailings were anti-hydration, had lower thermal conductivity, and could be used as insulation refractories for high-temperature kilns.

Hydration of MgO, CaO, SrO, and BaO (001) Surfaces from First Principles

AbstractIn this work we present a computational study of alkaline earth oxides (001) surfaces interfaced with water by means of density functional theory (DFT) in conjunction with ab initio molecular dynamics (AIMD) calculations. We studied the nature of MgO, CaO, SrO, and BaO (001) surfaces in contact with water. Results show that water dissociation is promoted as the alkaline earth metal becomes heavier. Similarly, the coordination number of the cation atoms with water molecules follows the same trend, indicating a more favorable interaction. The combined analysis of rumpling and pair distribution functions allows to provide a correspondence between reactivity of atoms on the surface and capability to coordinate water molecules. Except for MgO, the remaining surfaces display strong structural changes upon hydration, a relevant message when modelling hydrated surfaces. We also investigated the effect of the water thickness by adsorbing a water monolayer and bilayer to the surface. The findings of the study and the adopted approach could be of help for future studies of more complex systems or to provide fundamental rationalizations.

Passive direct air capture using calcium oxide powder: The importance of water vapor

Calcium oxide (CaO; lime) looping is a proposed technology with the potential to capture gigatonnes of carbon dioxide (CO2) from the atmosphere to help mitigate climate change. The importance of water in carbonation reactions is widely understood as it is needed for mineral and CO2 dissolution and carbonate precipitation. However, the effects of water vapor on CaO carbonation pathways and rates have yet to be elucidated in a systematic manner. Here, we examine the impact of relative humidity (RH) on CO2 removal using CaO powder at 20% to 95% RH. Higher RH resulted in faster hydration rates, forming Ca(OH)2 (portlandite); however, passivation limited carbonation in all experiments, with the greatest carbonation occurring at 80% RH (65% CaCO3; calcite). Thus, CaO powder is highly prone to passivation when using RH to drive hydration and carbonation. In RH swing experiments, RH was changed at different times (hours or days) and by different amounts (e.g., 40–99%). Humidity swings can yield >85% CaCO3 when complete CaO hydration at a low RH (<40%) occurs before carbonation at a high RH (∼99%). Importantly, separating these two processes yields nearly complete carbonation. In this case, we achieved a CO2 removal rate of 1 t CO2 for every 1.95 t of Ca(OH)2 per day.

Determination of the hydration and carbonation kinetics of CaO for low-temperature applications

In this work, the reaction kinetics of CaO were investigated with interest in its application as a sorbent in the sorption-enhanced methanation process. Therefore, hydration and carbonation kinetics were studied under operating conditions analogous to those required by the process (i.e. atmospheric pressure and temperature range between 200 °C and 350 °C). Two different CaO-based materials were considered in this study. The experimental results indicated that a shrinking core model with chemical reaction control successfully predicted hydration and carbonation conversion for both CaO-based sorbents. The activation energy obtained was similar for both sorbents, being 1.69 kJ/mol and 1.76 kJ/mol for the hydration reaction and 22.27 kJ/mol and 21.60 kJ/mol for the carbonation reaction. Additionally, the present study investigated the impact of simultaneous hydration and carbonation reactions. Both sorbents showed a reduction in hydration capacity when CO2 was introduced into the reaction atmosphere due to the formation of a CaCO3 product layer.

Effects of Organic Additives on Portlandite Microstructure during CaO Hydration

This study evaluates the effect of various organic additives on the microstructure of portlandite crystals during the hydration of CaO. Calcium hydroxide mixed with selected additives was prepared and characterised in terms of crystal size and habit, platelets abundance, and particle size distribution using various techniques such as XRD, SEM and Particle Size Analysis. The results suggest that the microstructure of portlandite is altered by the presence of carbohydrates and sulfonates, showing higher platelets abundance, smaller crystallite size, and higher specific surface area. The presence of hydroxyl-rich carbohydrates induces more marked effects compared to sulfonates. These results show that it is possible to tailor the properties of lime to specific applications in both, the conservation and the construction industry. Furthermore, the results show that it is possible to enable the development of materials specifically dedicated to other applications such as the synthesis of precipitated calcium carbonate and the design of CO2 sorbents, all of which will have the added value of reducing the global anthropogenic CO2 emissions.

Unveiling the dynamic thermal separation process of CO2 on the surface of calcium oxide: An ab-initio molecular dynamics study with experimental verification

Heating to release CO2 captured by Calcium oxide is one of the crucial steps in CO2 capture technology. However, the molecular mechanism underlying CO2 thermal desorption and the impact of CaO hydration on CO2 desorption remain unclear. Based on ab initio molecular dynamics (AIMD), a comprehensive analysis of the desorption and regeneration mechanism of carbonized CaO surfaces was conducted. Temperature is the primary factor responsible for CO2 thermal desorption on CaO surfaces. Simulations of CO2 thermal-dynamic desorption indicate that the desorption temperature range of CO2 from a dry CaO surface was between 400 and 500 °C. The explicit solvent model was utilised to simulate the desorption of CO2 under hydration condition. In the temperature range of 25–800 °C, the desorption behaviour of CO2 is substantially inhibited in the presence of water layers and Ca(OH)2. Consistent with the AIMD results, the thermal desorption experiment following CO2 adsorption confirmed that the desorption temperature of CO2 increases considerably when the adsorbed sample hydrates. Consequently, the desorption process of CO2 on CaO surface was comprehensively elucidated by a combination of theoretical study and experimental validation, thereby guiding the development and regeneration of CaO-based adsorbents.

Effects of carbohydrates and sulfonates during CaO hydration on portlandite microstructure

This study evaluates the effect of ten organic additives belonging to the classes of carbohydrates and sulfonates on the microstructure of portlandite crystals during CaO hydration. Calcium hydroxide modified with selected additives was prepared and characterised in terms of crystal size and habit, platelets abundance, crystallinity, specific surface area, particle size distribution, calcium concentration and organics in solution, using XRD, SEM, N2 absorption/BET, laser diffraction, ICP-OES and HPLC-MS. The results suggest that the microstructure of portlandite is altered by the presence of carbohydrates and sulfonates, mostly showing higher platelets abundance, smaller crystallite size, and higher specific surface area. The presence of hydroxyl-rich carbohydrates induced more marked effects compared to sulfonates. These results show that it is possible to tailor the properties of lime to specific applications. Consequently, this study can impact the conservation and the construction industry by enabling the development of novel binders for modern construction with optimal properties, and more broadly fields such as the synthesis of precipitated calcium carbonate and the design of CO2 sorbents, all of which will have the added value of reducing the global anthropogenic CO2 emissions.

Near-Infrared Spectroscopic Analysis─Formation of Ca(OH)2 and Ca(OD)2 by Hydration of CaO with H2O and D2O

The dehydration behavior of Ca(OH)2 was analyzed using near-infrared (NIR) and mid-infrared spectroscopy, with typical absorption bands attributed to the surface and interlayer hydroxyl groups of Ca(OH)2 being observed at 7260 and 7083 cm–1 in the NIR region, respectively. The dehydration reaction of Ca(OH)2 proceeded at approximately 470 °C, resulting in the disappearance of the interlayer hydroxyls of Ca(OH)2 and the formation of isolated hydroxyls of the CaO surface (7231 cm–1). The hydration behavior of CaO with H2O was also investigated using NIR spectroscopy, revealing that upon contact with H2O vapor at 50 °C [relative humidity (RH), 55%], CaO was immediately hydrated to form the interlayer hydroxyls of Ca(OH)2 after several minutes. Furthermore, since the difference between Ca(OH)2 and Ca(OD)2 could not be distinguished by X-ray diffraction measurements, the hydration behavior of CaO upon exposure to D2O vapor at room temperature was examined using NIR spectroscopy. This revealed the NIR absorption bands at 5357 and 5259 cm–1, corresponding to the surface and interlayer OD groups of Ca(OD)2, respectively. However, it should be noted that since the dehydration of Ca(OH)2 and the hydration of CaO were evaluated under atmospheric conditions, the formation of CaCO3 due to CO2 adsorption on the CaO surface could not be disregarded.

Understanding the acceleration effect of manganese and cerium doping on the hydration of CaO in CaO/Ca(OH)2 heat storage by density function theory

CaO/Ca(OH)2 heat storage system is one of the most promising candidates for large-scale thermochemical heat storage at temperature of 300–600 °C. In this work, the CaO/Ca(OH)2 heat storage performances of the Ce- and Mn-doped CaO were investigated and the mechanisms of promoting hydration by Ce and Mn doping were determined via density functional theory (DFT) calculations. The Mn-doped CaO exhibits the higher hydration conversion than CaO and Ce-doped CaO. The structural parameters, partial density of states, electron differential densities and energy barriers during hydration stage on the CaO, Ce- and Mn-doped CaO surfaces were compared to clarify the effects of Ce and Mn on hydration of CaO. The results show the detailed hydration reaction pathways on the CaO, Ce- and Mn-doped CaO surfaces. During the H2O adsorption stage, H2O on the Ce-doped CaO surface approaches the Ca atom first, while H2O on the Mn-doped CaO surface approaches the Os atom first. The adsorption energy for Ce- and Mn-doped CaO are −1.612 and −0.896 eV, respectively, which are higher than that for CaO. The doping of Ce and Mn accelerates the H2O adsorption on the CaO surface. The Mn doping causes the 84.3 % reduction in the energy barrier for hydration of CaO and the reduction by Mn doping is larger than that by Ce doping. Thus, Mn-doped CaO exhibits the higher hydration reactivity and hydration conversion than CaO and Ce-doped CaO. The DFT calculations determined the promotion mechanism of Ce and Mn doping on the hydration of CaO in CaO/Ca(OH)2 heat storage.

Influence of Silica Fume on High-Calcium Fly Ash Expansion during Hydration.

The purpose of this work was to study the possibility of neutralizing high-calcium fly ash expansion during hydration. The object of the study was the fly ash of Berezovskaya GRES, which is capable of independent setting and hardening. The test in the Le Chatelier molds showed that the divergence of indicator arms was 90–100 mm 1 day after mixing with water. The expansion and cracking of the fly ash could be completely prevented by silica fume addition in an amount of 42.9% by weight of the fly ash. At the same time, the compressive strength of specimens from the fly ash–sand paste in a ratio of 1:5 at the age of 28 days was 1.47 MPa. The isothermal heat release at a temperature of 20 °C for 10 days reached 500 kJ/kg. XRF and DTA results showed that free lime in the fly ash was completely hydrated in 11 days and gave the greatest expansion in the absence of silica fume. The presence of silica fume made the lime hydration incomplete and decreased the expansion. Unslaked free lime remained in the system. Exothermic data showed that silica fume inhibited CaO hydration from the reaction start.

Experimental investigation of a novel mechanically fluidized bed reactor for thermochemical energy storage with calcium hydroxide/calcium oxide

The reaction system based on the reversible hydration of CaO is promising for thermochemical energy storage since the material is a non-toxic, cheap industrial mass product with a comparatively high reaction enthalpy. However, the fine cohesive powder has a low thermal conductivity as well as a limited flowability and is not easily fluidized. Therefore, a reactor realization is challenging especially for higher capacities when the reactor cannot be the storage unit simultaneously. We developed a novel reactor concept based on a plow share mixer, demonstrated its feasibility and investigated its heat and mass transfer performance. In this concept, a rotating mixing device mechanically fluidize the bed in the reactor without the necessity of a gas flow, which might be especially advantageous for smaller power reactors. Both reaction directions have been successfully demonstrated. However, in the present configuration, the formation of a CaO/Ca(OH)2 layer on the heat transferring surface of the reactor reduces the heat transfer coefficient. Another layer formed on the filter, separating the reactor from the condenser, limited the gas transport and thereby dominated the conversion rate of the dehydration. Despite these limitations the mechanical fluidization yields significantly improved heat transfer compared to fixed bed reactors. One main parameter characterizing the performance of the reactor is the effective heat transfer coefficient from the electrically heated wall to the mechanically fluidized bed which was determined to be 156 ± 16 W/m2/K and 243 ± 52 W/m2/K for the heating up of CaO and the dehydration of Ca(OH)2, respectively.

The Influencing Factors for Volume Stability of Ladle Slag

The purpose of this study was to investigate the mechanism causing the unsoundness of ladle slag. Calcination temperature may have an impact on the level of reactivity of f-CaO. When CaO was produced at a higher temperature, the reactivity of CaO was lower. For example, dead burnt CaO (DCaO) was produced at higher temperatures than light burnt CaO (LCaO); therefore, DCaO had less reactivity than LCaO. In a hydration test, DCaO (1500 °C) showed 62 times lower reactivity than LCaO (900 °C), which meant that DCaO would result in the delay of hydration of CaO easily. Additionally, DCaO would cause unsoundness more easily than LCaO when adding the same number of cementitious materials. For this reason, using ASTM C114-18 (Standard Test Methods for Chemical Analysis of Hydraulic Cement) to quantify DCaO content may underestimate DCaO content by up to 20%. Conversely, this method was more suitable for f-CaO since it had high reactivity. Moreover, this study demonstrated that ladle slag would cause unsoundness when added into the cementitious material because it was produced from a higher temperature process (over 1500 °C), which generates the DCaO. Therefore, when reusing ladle slag, the problem of low reactivity of DCaO should be considered.

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.

Experimental study on material ratio and strength performance of geopolymer-improved soil

Geopolymer binder is a kind of green cementitious material with fast hardening, high strength, low shrinkage, and acid-alkali corrosion resistance. It is very beneficial to ensure the firmness, stability, and durability of subgrade structure in cold regions. In this study, the unconfined compression strength test of the clay soil improved by metakaolin-based geopolymer binder was carried out. The material ratio of the metakaolin-based geopolymer binder was analyzed, the optimal mixing ratio of the metakaolin-based geopolymer binder in the clay soil was investigated, and the impacts of metakaolin and alkali-activator on the mechanical properties of the geopolymer-improved soil were discussed. Experimental results indicate that the unconfined compression strength of the geopolymer-improved soil increases first and then decreases with the contents of metakaolin and alkali-activator (a mixture of unslaked lime (CaO) and sodium bicarbonate (NaHCO3) with the mass ratio of 1:1). The ideal mixing ratio of metakaolin and alkali-activator in the geopolymer binder is about 2:1, and the cost-optimal mixing ratio of the geopolymer binder in the clay soil is about 12%. By comparing with pure clay soil, lime soil, and ordinary Portland cement soil, the strength performance and stabilization effect of the geopolymer-improved soil were further studied through unconfined compression strength test, direct shear test, and Brazilian split-cylinder test. Outcomes show that the unconfined compression strength, shear strength, and Brazilian splitting strength of the geopolymer-improved soil have good advantages over the other three kinds of soils. The microstructure analysis by SEM with EDS illustrates that the agglomeration and stabilization effects of the geopolymer-improved soil, ordinary Portland cement soil, lime soil, and pure clay soil are weakened in turn, and the polymerization of the geopolymer binders can be in progress in the alkali-activated environment formed from the hydration of CaO and its reaction with NaHCO3. The failure of the geopolymer-improved soil with the increasing contents of metakaolin and alkali-activator shows that there is a tendency from plastic shear failure to brittle split failure. The results of this study can provide a parameter basis for the application and popularization of the soil improved by metakaolin-based geopolymer binder in engineering.

Synthesis of calcium silicate as paper filler with desirable particle size from desilication solution of silicon-containing waste residues

Resource utilization of silicon is crucial to the sustainable reuse of the silicon-containing waste residues. In this study, a simple method to synthesize calcium silicate as paper filler with desirable particle size from the residues was proposed. Foil-shaped calcium silicate with the average particle size (APS) of about 10 μm and the span of below 1.2 was successfully synthesized. Compared with larger calcium silicate, the obtained product used as paper filler could greatly improve the bulk of paper while maintaining similar tensile strength. The online focused beam reflectance measurement (FBRM) was used to investigate the hydration of CaO and the formation of calcium silicate. It was found that the hydration of CaO in NaOH solution with the presence of silicate played a key role in synthesizing the 10 μm-sized calcium silicate. The formation of calcium silicate was speculated to be controlled by a liquid-liquid reaction.

Sorbent steam reactivation and methane-concentrated calcination for calcium-looping carbon capture: Compatibilities and limitations

This paper investigates the equilibrium performance of CaO hydration and CaCO3/Ca(OH)2 co-calcination in calcium-looping processes. A novel lime hydration configuration is proposed, introducing saturated steam as the reactivating reagent and direct-heat-removal medium. The proposed hydrator generates a substantial amount of superheated steam, used elsewhere as required. The slaked lime and limestone are calcined simultaneously in a methane-concentrated oxy-fuel calciner. The equilibrium performance of the co-calciner is studied at different reactor temperatures, operating pressures, solid/gas feed ratios and feed compositions. A correlation is proposed which estimates the required gaseous feed composition for autothermal, coke-free and complete sorbent regeneration using this novel technology.

Luminescent properties of CaSc2O4:Ce3+ green phosphor for white LED and its optical simulation

Green-emissive Ce3+-doped CaSc2O4 phosphors were synthesized by solid-state method. The water washing process of the as-prepared samples was several times conducted because of severe hydration of unreacted CaO and CaF2 flux residual. The washed CaSc2O4:Ce3+ phosphors showed a single phase with a crystallite size of 1–3 μm, and its photoluminescence intensity was 10% improved compared with the as-prepared. It shows the broad 450 nm excitation peak and the green emission peak at 520 nm with a half width of 105 nm. Its temperature dependence showed the similar thermal stability with a commercial silicate phosphor. The optical simulation and the real fabrication of white LEDs with a combination of our green phosphor and one of possible orange-red phosphors demonstrated that the white LED with Sr3SiO5:Eu2+ orange phosphor gives the best luminous efficacy and the appropriate color rendering index of 70 under the daylight color temperature of 6400 K.

Preparation and Application of Bioshell Calcium Oxide (BiSCaO) Nanoparticle-Dispersions with Bactericidal Activity.

Scallop-shell powder (SSP) heated at high temperature exhibits high pH and broad antimicrobial activity. Bioshell calcium oxide (BiSCaO) is an SSP composed mainly of calcium oxide. It is poorly water-soluble under alkaline conditions and the generated precipitate can plug spray nozzles. The aim of this study was to establish that BiSCaO dispersion caused no significant CaO loss and plugging of spray nozzles, and to evaluate its deodorization and microbicidal abilities and its ability to reduce the concentrations of NO2− and NO3−. BiSCaO dispersions were prepared by mixing various concentrations of BiSCaO suspension, while phosphate compounds such as Na3PO4, Na2HPO4 or NaH2PO4 and the pH, average diameter, zeta potential, and form of the compounds with cryo-SEM were evaluated. We evaluated deodorization using tainted pork meat and microbicidal efficacy using contaminated suspension with normal bacterial flora. The concentration of NO2− and NO3− after mixing BiSCaO dispersion and pure water containing a high proportion of NO2− and NO3− were measured. BiSCaO dispersion formed with Na2HPO4, whose ratio to BiSCaO was 60%, showed a high pH (>12), a small particle diameter (>181 nm) and was stable for seven days. The BiSCaO dispersion showed higher deodorization and microbicidal activities than SSP-Ca(OH)2, which was mainly composed of Ca(OH)2. BiSCaO, but not SSP-Ca(OH)2, could reduce the concentration of NO2− and NO3− by more than 90% within 15 min. We developed a stable BiSCaO dispersion, and it had high deodorization and microbicidal efficacy. These activities of BiSCaO might result from the high pH caused by CaO hydration and a reduction activity causing active radical species.

High-temperature thermochemical energy storage – heat transfer enhancements within reaction bed

Ca(OH)2/CaO reversible reaction system has high potentials to be used for high-temperature thermal energy storage. Endothermic dehydration of Ca(OH)2 and exothermic hydration of CaO can be carried out at high temperatures compatible with most of the concentrated solar power applications. One of the challenges in using Ca(OH)2/CaO is the slow rate of heat transfer inside the solid reaction beds. Heat transfer enhancement techniques are introduced in a circular fixed bed heated indirectly through heat exchanger walls. Two reactor configurations are studied: one in which the heat transfer fluid (HTF) flows through an outer annular shell and another in which the HTF flows through a central pipe. Dehydration and rehydration processes are simulated with and without fins attached to the heat exchanger wall extending into the reaction bed. Heat and mass transport equations coupled with reaction kinetics are solved numerically. The designed heat transfer enhancements (HTE) within the reaction beds resulted in a significant decrease in overall conversion time during the energy storage and discharge. It is shown that the reactor with an outer annular shell as the HTF channel has better performance compared to the reactor with internal pipe flow configuration for HTF.

Coal fly ash: linking immersion freezing behavior and physicochemical particle properties

Abstract. To date, only a few studies have investigated the potential of coal fly ash particles to trigger heterogeneous ice nucleation in cloud droplets. The presented measurements aim at expanding the sparse dataset and improving process understanding of how physicochemical particle properties can influence the freezing behavior of coal fly ash particles immersed in water. Firstly, immersion freezing measurements were performed with two single particle techniques, i.e., the Leipzig Aerosol Cloud Interaction Simulator (LACIS) and the SPectrometer for Ice Nuclei (SPIN). The effect of suspension time on the efficiency of the coal fly ash particles when immersed in a cloud droplet is analyzed based on the different residence times of the two instruments and employing both dry and wet particle generation. Secondly, two cold-stage setups, one using microliter sized droplets (Leipzig Ice Nucleation Array) and one using nanoliter sized droplets (WeIzmann Supercooled Droplets Observation on Microarray setup) were applied. We found that coal fly ash particles are comparable to mineral dust in their immersion freezing behavior when being dry generated. However, a significant decrease in immersion freezing efficiency was observed during experiments with wet-generated particles in LACIS and SPIN. The efficiency of wet-generated particles is in agreement with the cold-stage measurements. In order to understand the reason behind the deactivation, a series of chemical composition, morphology, and crystallography analyses (single particle mass spectrometry, scanning electron microscopy coupled with energy dispersive X-ray microanalysis, X-ray diffraction analysis) were performed with dry- and wet-generated particles. From these investigations, we conclude that anhydrous CaSO4 and CaO – which, if investigated in pure form, show the same qualitative immersion freezing behavior as observed for dry-generated coal fly ash particles – contribute to triggering heterogeneous ice nucleation at the particle–water interface. The observed deactivation in contact with water is related to changes in the particle surface properties which are potentially caused by hydration of CaSO4 and CaO. The contribution of coal fly ash to the ambient population of ice-nucleating particles therefore depends on whether and for how long particles are immersed in cloud droplets.