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The economic challenges arising from resource depletion and uneven distribution of Mg in land reserves have spurred interest in technologies for Mg extraction from seawater. Thus, this study introduces a novel method to recover high-purity magnesium carbonate, specifically hydromagnesite, from seawater through carbonation using Ca(OH)₂ as an alkali source. A significant achievement of this method is the production of high-purity hydromagnesite despite the presence of large amounts of Ca, realized only through carbonation without the need for expensive additives such as NaOH. Experimental results reveal that the Ca(OH)₂ content predominantly influences the Mg dissolution efficiency during carbonation and the overall Mg recovery rate. Maximizing the efficiency of this technology relies on using an appropriate amount of Ca(OH)₂. Notably, the appropriate amount of OH− supply enhances CO₂ dissolution, facilitating Mg dissolution. Conversely, an excess of Ca(OH)₂ impedes Mg dissolution. The overall Mg recovery rate reaches a peak of 67 %, enabling the recovery of 3.4 kg of hydromagnesite from one ton of seawater. The findings highlight the effectiveness of the proposed technology as a promising and cost-effective method for extracting high-purity Mg from seawater, utilizing Ca(OH) 2 , previously considered an impurity, as a cost-effective alkali source. • Recovered high-purity hydromagnesite from seawater via carbonation using Ca(OH) 2 • Used Ca(OH)₂, previously deemed an impurity, as a cost-effective alkali • Optimal amounts of Ca(OH)₂ enhanced Mg dissolution but excess content inhibited it • Mg recovery peaked at 67 %, yielding 3.4 kg hydromagnesite per ton of seawater [ABSTRACT FROM AUTHOR]

Calcium oxide, calcium hydroxide and calcium carbonate were tested for the CO 2 /CH 4 reaction in a dielectric barrier discharge plasma reactor under humid conditions. It appeared that the presence of water in the gas stream enhanced slightly the selectivity to CO and H 2 syngas on each support (2–5 %), contrary to the selectivities to CxHy that decreased (0.5 to 2%). The water showed little effect on oxygenates selectivities (in the order of 0.1%) compared to dry treatment. The three materials were characterized by thermogravimetric analysis and infrared spectroscopy prior and after, dry and humid, plasma treatments. Addition of humidity in the plasma gas stream led to the formation of calcium hydroxide on the surface of CaO and enhanced the carbonation of Ca(OH) 2. We showed that the carbonation of Ca(OH) 2 , H 2 O proceeded through simultaneous CO 2 capture and H 2 O release, highlighting an oscillating behavior. [ABSTRACT FROM AUTHOR]

Most commercially available disinfectants cause serious health side effects. This study investigates controlled carbonation of Ca (OH) 2 by mixing with dry ice and the use of the resultant particles as a more health friendly antimicrobial agent. In one mixing scheme, alternate layering of Ca (OH) 2 and dry ice is used with no additional mixing, while in another a Hamilton Beach mixer is utilized. X – ray diffraction confirmed the identity of the resultant particles and thermogravimetry measured their bulk composition. Fourier transform infrared spectroscopy displayed different surface composition than Ca (OH) 2 suggesting a core – shell structured product. Commercial Ca (OH) 2 − CaCO 3 core – shell (CSCC) particles prepared under quiescent conditions and the particles prepared in this work displayed different extent of aggregated structures, despite the different mixing strategies. Thicker Ca CO 3 shell and CSCC particle aggregation corresponded to higher content of Ca CO 3 as estimated by acid/base titration. The antibacterial activity of the CSCC particles was evaluated using a novel strategy where cells are mixed with a fixed density of particles as a slurry. Both gram – negative; namely P.aeruginosa and E.coli , and gram – positive; namely S.aureus , bacteria were tested. The results confirmed the superior antibacterial properties of the in – house prepared CSCC particles, where at least 2 times more effective particles than Ca (OH) 2 and commercial particles were reported against gram-positive bacteria. The antibacterial properties of the particles were correlated to their morphology. [Display omitted] ● Controlled carbonation of Ca (OH) 2 to form core – shell Ca (OH) 2 − Ca CO 3 is achieved. ● The product is more health friendly antimicrobial agent. ● Thicker CaCO 3 shell and aggregation corresponded to less antibacterial efficacy. ● The CaCO 3 shell prolonged the biocidal activity of the core-shell particles. [ABSTRACT FROM AUTHOR]

Measuring carbonation is increasingly important, especially for developing novel low-CO 2 cements and carbon-capture technologies. This study shows for the first time, the feasibility and advantages of confocal Raman microscopy (CRM) for measuring carbonation depth in cement-based materials, providing high spatial resolution (down to <100 μm), by mapping CaCO 3 and Ca(OH) 2. Pastes and mortars of different binders (CEM I, 30 % PFA, 50 % GGBS) and w/b ratios (0.45, 0.60) exposed to natural (440 ppm CO 2) and accelerated carbonation (4 % CO 2) at 65 % RH, 21 °C for up to 3 months were tested. CRM shows a sharp carbonation front, without the transition zone commonly supposed. Carbonation depths measured with image analysis of CRM-CaCO 3 maps and phenolphthalein-treated surfaces are in excellent agreement, however the latter is less reliable for depths <5 mm. Profile-based methods (TGA, XRD, FTIR, RS and BSE) systematically over-estimate carbonation depth; a simple method to correct this error is proposed. [ABSTRACT FROM AUTHOR]

Cultural heritage objects and structures are subjected to a range of weathering processes that result in their decay and destruction. To slow weathering rates and/or mitigate their effects, several protective and consolidant materials have been used during conservation interventions. Treatments based on organic polymers and alkoxysilanes, as well as some traditional inorganic treatments such as lime water, are in many cases either incompatible and/or show limited efficacy. In recent years nanolimes, that is, dispersions of Ca(OH)2 nanoparticles in alcohol (as well as alcohol dispersions of other alkaline-earth metal hydroxide nanoparticles), have emerged as an effective and compatible conservation material. Here we review recent advances in the synthesis and application of nanolimes in the field of heritage conservation. First, we present an overview of lime-based conservation materials, with an emphasis on the earliest reports on the use of nanolimes. Subsequently, we present the different methods used to synthesize nanolimes. Afterwards, we describe their carbonation and its consolidation effects. Practical application of nanolimes in heritage conservation are summarized, including consolidation of stone, ceramics, lime mortars and mural painting, as well as deacidification of paper, canvas, and wood. The advantages and limitations of this novel nanotechnology for cultural heritage conservation are outlined. Finally, some conclusions and areas for future research are presented. [ABSTRACT FROM AUTHOR]

Highlights: [•] Reaction zone of carbonated concrete is determined to chemical component change. [•] The partially carbonated zone width was greater than that of pH value variation. [•] Carbonated concrete is divided into four zones. [•] Carbonation process of concrete is classified into three stages. [ABSTRACT FROM AUTHOR]

Abstract: The aim of the study is to investigate the calcium hydroxide (Ca(OH)2) contents in pozzolanic cement pastes analyzed by the chemical extraction method and thermal analysis (DTA/TG). The second part of the study involves the carbonation of pozzolanic cement pastes and its influence on Ca(OH)2 reduction. The Ca(OH)2 contents in cement pastes after being subjected to accelerated carbonation were investigated by a chemical extraction method and compared to those values from DTA/TG analysis. The experimental results show that the chemical method reveals overall results comparable to those from DTA/TG analysis. However, in case of pastes containing high amounts of mineral admixtures or pastes subjected to carbonation which contain very low Ca(OH)2, only the chemical method yields precise reliable results by showing the pseudo-negative contents. [Copyright &y& Elsevier]

Abstract: A thermochemical cyclic process and associated reactor is presented for the continuous removal of CO2 from ambient air via consecutive CaO-carbonation and CaCO3-calcination steps using concentrated solar energy as the source of high-temperature process heat. A fluidized-bed solar reactor is applied to accomplish the carbonation at 365–400°C and the calcination at 800–875°C, with reacting particles directly exposed to high-flux solar irradiation. Water vapor was introduced during the carbonation step to enhance its kinetics. Five consecutive cycles were performed, yielding complete removal of CO2 from a continuous airflow containing 500ppm of CO2 within a residence time of 1.3s during each carbonation step, and subsequent complete release of CO2 and regeneration of the CaO reacting particles during the calcination step. The reactor design, set-up, and experimentation using a high-flux solar simulator are described. [Copyright &y& Elsevier]

Abstract: Phenolphthalein indicator has traditionally been used to determine the depth of carbonation in concrete. This investigation uses the thermalgravimetric analysis (TGA) method, which tests the concentration distribution of Ca(OH)2 and CaCO3, while the X-ray diffraction analysis (XRDA) tests the intensity distribution of Ca(OH)2 and CaCO3. The Fourier transformation infrared spectroscopy (FTIR) test method detects the presence of C–O in concrete samples as a basis for determining the presence of CaCO3. Concrete specimens were prepared and subjected to accelerated carbonation under conditions of 23 °C temperature, 70% RH and 20% concentration of CO2. The test results of TGA and XRDA indicate that there exist a sharp carbonation front. Three zones of carbonation were identified according to the degree of carbonation and pH in the pore solutions. The TGA, XRDA and FTIR results showed the depth of carbonation front is twice of that determined from phenolphthalein indicator. [Copyright &y& Elsevier]

Abstract: We have studied the carbonation process in different types of mortars, with and without pozzolana or air-entraining additives, subject to a CO2-rich atmosphere and compared the results with those of similar naturally carbonated mortars. We used X-ray diffraction technique to demonstrate that high CO2 concentrations favour a faster, more complete carbonation process with 8 days being sufficient to convert portlandite into 90 wt.% calcite. Full carbonation, however, is not reached during the life-span of the tests, not even in forced carbonation experiments. This could be due to at least one of the following phenomena: a premature drying of samples during carbonation reaction, the temperature at which the carbonation process was carried out or the reduction of pore volume occupied by newly formed calcite crystals. This last option seems to be the least probable. We observed a more prolific development of calcite crystals in the pores and fissures through which the carbonic anhydride flows. Under natural conditions, carbonation is much slower and similar levels are not reached for 6 months. These differences suggest that the carbonation process is influenced by the amount of CO2 used. Both the mineralogy and texture of mortars vary depending on the type of additive used but the speed of the portlandite–calcite transformation does not change significantly. Pozzolana produces hydraulic mortars although the quantity of calcium aluminosilicate crystals is low. The air-entraining agent significantly alters the texture of the mortars creating rounded pores and eliminating or reducing the drying cracks. [Copyright &y& Elsevier]

Abstract: Calcium and calcium magnesium putty-limes (C-L and C-M-L) were characterized, through SEM, Porosimeter and Chemical Analysis, to obtain information on the microstructure of their solid-phases. Irregular agglomerates of portlandite grains and large acicular crystallites of brucite are differently interconnected to form a cellular solid matrix dispersed into the saturated aqueous solution of the hydroxides. The setting of these putty-limes was followed also in a thermobalance with a thermostatic chamber designed to keep the temperature in the range 15–30 °C. The tests were done at a constant humidity of 80%, under isothermal conditions, and a wet and carbonated N2 flux with 1% of CO2 was added. The microstructure of the final samples was characterized by the usual methods of investigation at micro–macro scale. Drying kinetics and the related shrinkage processes were discussed on the basis of a modified Kelvin equation, which predicts for the C-L lime putty a larger shrinkage (50.3%) than for C-M-L (35.5%). The binding mechanisms of the calcium carbonate in the strengthening of the putty-lime systems is due to the interconnected texture formed by the calcium carbonate fine crystallites formed during the precipitation process. The scientific reasons for this microstructure evolution have been discussed and explained on the basis of experimental data and theory, leading to a better understanding of the complex relationships between drying, shrinkage and the chemical processes occurring in the setting of putty-lime. [Copyright &y& Elsevier]

The uptake of carbon dioxide due to the carbonation reaction of Ca(OH)2 in ambient temperature of approximately 20 °C has been studied. Different types of lime have been used and the CO2 concentration has been varied to identify the influence of different variables on the kinetics of the reaction. A closed loop system has been developed and validated that allows measurement of the carbonation progress directly from monitoring CO2 uptake. Thermal analysis (TA) was used to verify the degree of carbonation that reached up to 83%. Factor analysis on the data set has demonstrated that reaction speed is not dependent upon the CO2 concentration within the limits tested. Carbonation speed depends on the specific surface of the lime. The results of this study contribute to research carried out on lime mortar carbonation models and on the carbonation process in general. [Copyright &y& Elsevier]

The increasing use of lime-based mortars for the restoration of historic buildings and structures justifies the research on these materials. The focus of this paper is the effect of technological variables on pore structure and mechanical properties of lime-based mortars. The influence of curing time, binder–aggregate (B/Ag) ratio, aggregate attributes and porosity is discussed. Mortars prepared with aerial lime, varying aggregate types and B/Ag ratios ranging from 1:1 to 1:5 by volume were tested. Compressive and flexural strength measurements, as well as X-ray diffraction (XRD) and thermal studies, were performed after 3, 7, 28, 91, 182 and 365 days. A strong increase in strength of mortar mixtures after 365 curing days (as compared to 28 curing days) is found. In spite of the fact that larger amounts of binder increase the total porosity, the strength of these mixtures is also increased. A good interlocked structure is obtained as binder contents increase. Also, higher porosities allow better portlandite carbonation. A relationship between mechanical properties and pore structure was established. However, in case of binder excess, the increase in voids leads to a strength reduction. The use of calcareous aggregates improves strength more as compared to the use of siliceous aggregates. Factors as grain size distribution and grain shape of the aggregates have also been considered. [Copyright &y& Elsevier]

This paper presents theoretical models of the phase transformation and the change of mechanical properties of oil well cement due to underground carbonation reactions. Application of oil well cement has become common practice in CO 2 underground storage formations. However, the CO 2 stored underground may leak through damaged well cement, and it can cause carbonation reactions which may significantly decrease the mechanical properties of well cement. Extensive research has been conducted by using experimental methods in laboratories to characterize the carbonation reactions of cement underground. However, no theoretical formulations have been developed considering the mechanical properties change due to the carbonation effect. In this paper, the CO 2 profile was predicted first by the error function solution of a 1D CO 2 diffusion equation and further the carbonation reaction rate was calculated. The phase transformation of well cement due to carbonation were then analyzed by using a stoichiometric model. After that, the changes of mechanical properties of well cement were modeled using a two-phase generalized self-consistent model at different scales. Specifically, the decrease of the elastic modulus of well cement during the carbonation process was used as an example application of the model. Finally, the modeling result was validated with published experimental results. • The stoichiometric model has been applied to track the phase transformations of cement under the carbonation reaction. • The rate of carbonation reactions for each phase is calculated in the model with the application of CO 2 diffusion. • The two-phase general self-consistent model has been used to calculate the mechanical properties change of the cement. • The changes of of Class G cement has been predicted in the underground CO2 storage system. [ABSTRACT FROM AUTHOR]

• Agglomerating trend of nanoSiO 2 caused less increase in workability than nanoCa(OH) 2. • Adding nanoCa(OH) 2 lowered pozzolanic activity but increased carbonation ratio. • Rate of pozzolanicity increased when nanoSiO 2 was added alone or in combination with nanoCa(OH) 2. • The combined addition of nanoSiO 2 and nanoCa(OH) 2 increased porosity and capillary absorption. • The generation of less stable phases and microcracks decreased the strength but increased deformability. This research presents a novel approach based on the combination of nanotechnology and Roman technology by investigating how adding nanoCa(OH) 2 and nanoSiO 2 modify the performance of air lime mortars containing Roman ceramics. Microstructural, physico-mechanical properties were periodically controlled until 120 days of curing. XRD and TGA analyses showed that adding nanoSiO 2 either alone or with nanoCa(OH) 2 were more beneficial to improve the pozzolanic activity in the mortars. The less stable hydrated phases generated led to microcracks which eventually impaired compressive strength but enhanced deformability capacity. These results provide insight into the development of highly compatible mortars for cultural heritage. [ABSTRACT FROM AUTHOR]

• Influence of synthetic anhydrous CaCO 3 on nanolime carbonation was investigated. • Carbonation was accelerated, with a control exerted by specific surface of solids. • Use of synthetic CaCO 3 resulted an effective seeding method for polymorph control. • Addition of aragonite reduced desiccation cracks in the obtained carbonated product. The influence of synthetic anhydrous calcium carbonate polymorphs, calcite, vaterite and aragonite, on carbonation of nanolime suspension, has been investigated. The higher rates of carbonation observed after 7 days in mixtures containing vaterite and aragonite particles, found explanation in the control exerted by heterogeneous nucleation on the water-mediated reaction mechanism and in the enhanced carbon dioxide diffusivity in the sample volume, both facilitated by the higher specific surface of the powders and the microporous sample fabric. Introduction of synthetic calcium carbonate allowed for exerting control on formed polymorphs, subverting the order of their (meta-)stability. The peculiar microstructure obtained by adding aragonite rod-like particles, mitigates the propagation of desiccation cracks (which impair continuity of the carbonated nanolime film). The mechanism, although still not completely understood, likely involves the regulation of evaporation of liquid fraction and redistribution of the tensional stresses during drying and nanolime crystallization. The obtained results provide new insights into the crystallization of carbonates from nanolime particles and may help in the design of innovative compatible products for consolidation of lime-based materials and carbon dioxide sequestration. [ABSTRACT FROM AUTHOR]