Rapid Carbonation for Calcite from a Solid-Liquid-Gas System with an Imidazolium-Based Ionic Liquid

Aqueous carbonation of Ca(OH)₂ is a complex process that produces calcite with scalenohedral calcite phases and characterized by inadequate carbonate species for effective carbonation due to the poor dissolution of CO₂ in water. Consequently, we report a solid-liquid-gas carbonation system with an ionic liquid (IL), 1-butyl-3-methylimidazolium bromide, in view of enhancing the reaction of CO₂ with Ca(OH)₂. The use of the IL increased the solubility of CO₂ in the aqueous environment and enhanced the transport of the reactive species (Ca²⁺ and CO₃²⁻) and products. The presence of the IL also avoided the formation of the CaCO₃ protective and passivation layer and ensured high carbonation yields, as well as the production of stoichiometric rhombohedral calcite phases in a short time.     

Crystallization of Nano‐Calcium Carbonate: The Influence of Process Parameters

Precipitated calcium carbonate was synthesized by carbonation of calcium hydroxide in the presence and absence of ultrasound (conventional stirring) at atmospheric as well as at elevated pressures and different initial concentrations of Ca(OH)₂. Spherical morphology of the formed calcite was favored at high Ca(OH)₂ concentrations and low CO₂ pressures. The presence of ultrasound did not show any influence on the reaction rate in case of efficient mixing. A small increase of the reaction rate was observed at lower CO₂ pressures. Elevated pressures in combination with ultrasound did not lead to notable changes of reaction rate or particle morphology.     

Investigation of the carbonation front shape on cementitious materials: Effects of the chemical kinetics

Carbonation depth-profiles have been determined by thermogravimetric analysis and by gammadensitometry after accelerated carbonation tests on ordinary Portland cement (OPC) pastes and concretes. These methods support the idea that carbonation does not exhibit a sharp reaction front. From analytical modelling, this feature is explained by the fact that the kinetics of the chemical reactions become the rate-controlling processes, rather than the diffusion of CO₂. Furthermore, conclusions are drawn as to the mechanism by which carbonation of Ca(OH)₂ and C-S-H takes place. Carbonation gives rise to almost complete disappearance of C-S-H gel, while Ca(OH)₂ remains in appreciable amount. This may be associated with the CaCO₃ precipitation, forming a dense coating around partially reacted Ca(OH)₂ crystals. The way in which CO₂ is fixed in carbonated samples is studied. The results indicate that CO₂ is chemically bound as CaCO₃, which precipitates in various forms, namely: stable, metastable, and amorphous. It seems that the thermal stability of the produced CaCO₃ is lower when the carbonation level is high. It is also shown that the poorly crystallized and thermally unstable forms of CaCO₃ are preferentially associated with C-S-H carbonation.     

A breakthrough technique for the preparation of high-yield precipitated calcium carbonate

Precipitated calcium carbonate (PCC) is conventionally produced through the gas-solid-liquid carbonation route, which consists on bubbling gaseous CO₂ through a concentrated calcium hydroxide (Ca(OH)₂) slurry. However, atmospheric carbonation processes are slow and have low carbonation efficiency. A novel technology based on the combination of supercritical carbon dioxide (scCO₂) and ultrasonic agitation is described here for the preparation of high-yield PCC. The combination of both techniques has demonstrated to produce outstanding improvement for the conversion of Ca(OH)₂ to the stable calcite polymorph of calcium carbonate (CaCO₃). These experiments were carried out at 313 K and 13 MPa using a high-pressure reactor immersed in an ultrasounds cleaner bath. The process kinetics and the characteristics of the precipitated particles using ultrasonic agitation were compared with those obtained under similar experimental conditions using mechanical stirring and non-agitated systems. The crystal characteristics of the samples obtained using the three different agitation techniques were characterized by X-ray diffraction and scanning electron microscopy.     

The experimental investigation of concrete carbonation depth

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.     

Influence of relative humidity on the carbonation of calcium hydroxide nanoparticles and the formation of calcium carbonate polymorphs

A consolidating product based on nanoparticles of slaked lime (Ca(OH)₂) dispersed in isopropyl alcohol was exposed under different relative humidities (RH), 33%, 54%, 75% and 90% during 7, 14, 21 and 28 days. The characterization of the calcium hydroxide nanoparticles and the formed calcium carbonate polymorphs have been performed by Micro Raman spectroscopy, Transmission Electron Microscopy (TEM), Environmental Scanning Electron Microscopy (ESEM) with Energy Dispersive X-ray Spectroscopy (EDS) and X-ray Diffraction (XRD). Precipitation and transformation of calcium carbonate polymorphs strongly depend on the relative humidity (RH). Higher RH (75%–90% RH) gives rise to amorphous calcium carbonate and monohydrocalcite, calcite, aragonite and vaterite, faster carbonation and larger particles sizes with higher crystallinity compared to lower RH (33%–54% RH) that gives rise mainly to portlandite and vaterite, slower carbonation and smaller particle sizes with lower crystallinity.     

Preparation of ultrafine calcium carbonate particles with micropore dispersion method

Ultrafine particles of CaCO₃ were synthesized by dispersing the mixture of CO₂ and N₂ into the Ca(OH)₂/H₂O slurry with a micropore-plate. Because the micropore is micrometers scale, process of momentum transfer, mass transfer and reaction was significantly enhanced. The carbonation process of Ca(OH)₂/H₂O system was monitored with pH and conductivity. Operation conditions were investigated on the specific surface area of particles, such as initial slurry concentration and volume, gas flowrate and concentration, and temperature. The crystal structure of particles was characterized with BET, IR, TEM, SEM, etc. Results showed ultrafine particles were calcite with general shape of cube, whose size was about 40 nm and specific surface area was more than 25 m²/g. This preparation method is easy to operate.     

New insights into the mechanisms of carbon dioxide mineralization by portlandite

Portlandite (Ca(OH)₂; also known as calcium hydroxide or hydrated lime), an archetypal alkaline solid, interacts with carbon dioxide (CO₂) via a classic acid–base “carbonation” reaction to produce a salt (calcium carbonate: CaCO₃) that functions as a low-carbon cementation agent, and water. Herein, we revisit the effects of reaction temperature, relative humidity (RH), and CO₂ concentration on the carbonation of portlandite in the form of finely divided particulates and compacted monoliths. Special focus is paid to uncover the influences of the moisture state (i.e., the presence of adsorbed and/or liquid water), moisture content and the surface area-to-volume ratio (s_a/v, mm⁻¹) of reactants on the extent of carbonation. In general, increasing RH more significantly impacts the rate and thermodynamics of carbonation reactions, leading to high(er) conversion regardless of prior exposure history. This mitigated the effects (if any) of allegedly denser, less porous carbonate surface layers formed at lower RH. In monolithic compacts, microstructural (i.e., mass-transfer) constraints particularly hindered the progress of carbonation due to pore blocking by liquid water in compacts with limited surface area to volume ratios. These mechanistic insights into portlandite's carbonation inform processing routes for the production of cementation agents that seek to utilize CO₂ borne in dilute (≤30 mol%) post-combustion flue gas streams.     

The effects of carbonation and drying during intermittent leaching on the release of inorganic constituents from a cement-based matrix

A Portland cement mortar was submitted to cycles of intermittent wetting (IW) in which tank leaching was interspersed with periods of storage in either an inert or a reactive atmosphere. Relative humidity (RH) (23%, 48% and 98%) was maintained during storage to control the drying process. The effects of IW were qualified by comparing flux and cumulative release of matrix constituents (Ca, OH, Na, K and Cl) to that of continuous water saturation. The carbonation process was associated with the degree of drying occurring due to storage. Cumulative release of most major constituents was suppressed in samples storage under 100% CO₂ in comparison to the inert atmosphere (100% N₂). Results suggest that accurate long-term performance assessment must account for the potential impact of phenomena associated with IW.     

Carbonation reaction of lime, kinetics at ambient temperature

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.     

Controllable preparation of nano-CaCO3 in a microporous tube-in-tube microchannel reactor

In this work, nano-CaCO₃ particles with tunable size have been synthesized via CO₂/Ca(OH)₂ precipitation reaction in a microporous tube-in-tube microchannel reactor (MTMCR) with a throughput capacity up to 400 L/h for CO₂ and 76.14 L/h for liquid. The overall volumetric mass-transfer coefficient (K_La) of CO₂ absorption into Ca(OH)₂ slurry in the MTMCR has been deduced and analyzed. To control the particle size, the effect of operating conditions including initial Ca(OH)₂ content, gas volumetric flow rate, liquid volumetric flow rate, micropore size, and annular channel width was investigated. The results indicated that the mass transfer in the MTMCR can be greatly enhanced in contrast with a stirred tank reactor, and the particle size can be well controlled by tuning the operating parameters. The nano-CaCO₃ particles with an average size of 28 nm and a calcite crystal structure were synthesized, indicating that this process is promising for mass production of nanoparticles.     

Co-utilisation of alkaline solid waste and compressed-or-supercritical CO2 to produce calcite and calcite/Se red nanocomposite

The coal combustion fly-ash and alkaline paper mill waste were previously used to sequester CO₂ via waste-water-CO₂ interactions. For this case, a solid mixture (calcite and un-reacted waste) was obtained after carbonation process. In the present study, we propose a solid-water separation of free lime (CaO) or free portlandite (Ca(OH)₂) contained in waste prior to carbonation experiments in order to produce pure calcite or calcite/Se⁰ red composite. The calcite and carbonate composite syntheses have been also independently studied, but for both cases, a commercial powdered portlandite was used as calcium source.For this study, the extracted alkaline-solution (pH = 12.2-12.4 and Ca concentration = 810-870 mg/L) from alkaline solid waste was placed in contact with compressed or supercritical CO₂ at moderate or high temperature, leading a preferential nucleation-growth of submicrometric particles of calcite (<1 μm) with rhombohedral morphology at 90 °C and 90 bar (9 MPa), whereas a preferential nucleation-growth of nanometric particles of calcite (<0.2 μm) with scalenohedral morphology at 30 °C and 20 bar (2 MPa) were observed. When, the extracted alkaline-solution was placed in contact with supercritical CO₂ (90 bar) at high temperature (90 °C) and in presence of unstable seleno-l-cystine compound, the nucleation-growth of calcite/Se⁰ red nano-composite taken place. The composite consisted predominantly of spherical, amorphous nanometric-to-submicrometric of elemental red selenium (<500 nm) deposited on the calcite matrix. Here, the calcite was constituted by nano- to microrhombohedral crystals (<2 μm) and micrometric agglomerates and/or aggregates (<5 μm). These results on the particle size and morphology of crystal faces are very similar to calcite produced using commercial powdered portlandite as alkaline reactant and calcium source. This study is a nice example of feasibility to obtain possible ecological and economical benefits from waste co-utilisation.     

Indirect mineral carbonation of titanium-bearing blast furnace slag coupled with recovery of TiO2 and Al2O3

Large quantities of CO_2 and blast furnace slag are discharged in the iron and steel industry. Mineral carbonation of blast furnace slag can offer substantial CO_2 emission reduction and comprehensive utilization of the solid waste.This paper describes a novel route for indirect mineral carbonation of titanium-bearing blast furnace(TBBF) slag,in which the TBBF slag is roasted with recyclable(NH_4)_2SO_4(AS) at low temperatures and converted into the sulphates of various valuable metals, including calcium, magnesium, aluminium and titanium. High value added Ti-and Al-rich products can be obtained through stepwise precipitation of the leaching solution from the roasted slag. The NH_3 produced during the roasting is used to capture CO_2 from flue gases. The NH_4HCO_3 and(NH_4)_2CO_3 thus obtained are used to carbonate the CaSO_4-containing leaching residue and MgSO_4-rich leaching solution, respectively. In this study, the process parameters and efficiency for the roasting, carbonation and Ti and Al recovery were investigated in detail. The results showed that the sulfation ratios of calcium,magnesium, titanium and aluminium reached 92.6%, 87% and 84.4%, respectively, after roasting at an AS-to-TBBF slag mass ratio of 2:1 and 350 °C for 2 h. The leaching solution was subjected to hydrolysis at 102 °C for 4 h with a Ti hydrolysis ratio of 95.7% and the purity of TiO_2 in the calcined hydrolysate reached 98 wt%.99.7% of aluminium in the Ti-depleted leaching solution was precipitated by using NH_3. The carbonation products of Ca and Mg were CaCO_3 and(NH_4)_2 Mg(CO_3)_2·4H_2O, respectively. The latter can be decomposed into MgCO_3 at 100–200 °C with simultaneous recovery of the NH_3 for reuse. In this process, approximately 82.1% of Ca and 84.2%of Mg in the TBBF slag were transformed into stable carbonates and the total CO_2 sequestration capacity per ton of TBBF slag reached up to 239.7 kg. The TiO_2 obtained can be used directly as an end product, while the Al-rich precipitate and the two carbonation products can act, respectively, as raw materials for electrolytic aluminium,cement and light magnesium carbonate production for the replacement of natural resources.     

Analysis of carbonation behavior in concrete using neural network algorithm and carbonation modeling

Carbonation on concrete structures in underground sites or metropolitan cities is one of the major causes of steel corrosion in RC (Reinforced Concrete) structures. For quantitative evaluation of carbonation, physico-chemo modeling for reaction with dissolved CO₂ and hydrates is necessary. Amount of hydrates and CO₂ diffusion coefficient play an important role in evaluation of carbonation behavior, however, it is difficult to obtain a various CO₂ diffusion coefficient from experiments due to limited time and cost.In this paper, a numerical technique for carbonation behavior using neural network algorithm and carbonation modeling is developed. To obtain the comparable data set of CO₂ diffusion coefficient, experimental results which were performed previously are analyzed. Mix design components such as cement content, water to cement ratio, and volume of aggregate including exposure condition of relative humidity are selected as neurons. Training of learning for neural network is carried out using back propagation algorithm. The diffusion coefficient of CO₂ from neural network are in good agreement with experimental data considering various conditions such as water to cement ratios (w/c: 0.42, 0.50, and 0.58) and relative humidities (R.H.: 10%, 45%, 75%, and 90%). Furthermore, mercury intrusion porosimetry (MIP) test is also performed to evaluate the change in porosity under carbonation. Finally, the numerical technique which is based on behavior in early-aged concrete such as hydration and pore structure is developed considering CO₂ diffusion coefficient from neural network and changing effect on porosity under carbonation.     

How carbonation curing influences ca leaching of Portland cement paste: Mechanism and mathematical modeling

Carbonation curing provides a promising method for both CO₂ sequestration and strength improvement of cement-based materials. To date, there is little knowledge about the influence of carbonation curing on Ca leaching resistance of cement-based materials due to the occurrence of both physical and chemical transformation. It was the first time that Ca solid-liquid equilibrium curves were experimentally established for cement pastes with different carbonation degrees in this paper. Experimental results demonstrated that on the one hand, carbonation curing improves the leaching resistance of cement paste by sequestrating Ca in insoluble CaCO₃; on the other hand, potential negative effects may occur due to the accelerated decalcification and increased solubility of C–S–H after carbonation curing. Results of NMR showed that both carbonation curing and Ca leaching can increase the Si chain length and polymerization degree of C–S–H. Additionally, a modified mathematical model was established to simulate the leaching process of carbonation-cured cement paste and it was also verified by energy-dispersive spectroscopy (EDS) results. Therefore, the long-term leaching resistance of cement-based materials is possibly degraded by the carbonation curing treatment.     

Assessment of the protective effect of carbonation on portlandite crystals

The kinetics of many reactions important to cement hydration and use are not well understood: this is in part due to the great complexity of many supposedly “simple” processes. One such process, carbonation of portlandite, Ca(OH)₂, in moist air at ~ 23 °C has been investigated by microscopy and microchemical analysis. Single crystals of portlandite were grown, carbonated at relative humidities between ~ 25 and ~ 90%, and the transport properties of the self-generated calcite, CaCO₃, product film were determined.The calcite films thus grown within days or weeks varied in thickness but typically were polycrystalline and epitaxial: a variety of morphologies and surface features are recorded. Permeation was measured by determining the time taken for Ca^2 + ions, arising from the Ca(OH)₂ substrate, to diffuse through the calcite coat into initially pure water. The spontaneous formation of self-protecting films on concrete has long been envisaged: results demonstrate that passivation can actually be achieved.     

硅酸盐水泥混凝土的碳化分析

介绍了硅酸盐水泥混凝土的碳化反应和碳化过程,分析了Ca(OH)_2与水化硅酸钙(C-S-H)的碳化作用.Ca(OH)_2发生碳化反应的同时,C-S-H也会发生碳化反应;Ca(OH)_2的碳化产物是方解石,而C-S-H碳化后会转变成无定形硅胶,可能形成稳定性差、结晶度差的球霰石、文石,其分解温度低于方解石的分解温度;C/S低、结晶度差的C-S-H凝胶易于碳化;水泥浆体孔隙溶液中的碱含量越高,碳化速度越快,深度越大.     

Mechanisms of NOx entrapment into hydrated cement paste containing activated carbon — Influences of the temperature and carbonation

The NO_x pollution produced by road traffic in confined volumes, such as tunnels, is an issue for public health. This work focuses on the mechanisms of NO_x removal by modified cement pastes. The samples (made of pure synthetic powders or cement paste cylinders) are continuously exposed to 220 ppbv of NO and/or 110 ppbv of NO₂ gas. The ability for the main hydrates (such as Ca(OH)₂ and C–S–H) to trap NO₂ is then quantified. After the leaching of samples, the chromatography experiments show that the adsorbed NO₂ is transformed in nitrate and nitrite ions by a disproportionate mechanism. The addition of activated carbon into cement paste enhances the NO₂ abatement, which is not influenced by carbonation. The NO₂ abatement by the activated carbon materials is also stable between 20 and 50 °C, revealing a competition between the disproportionate reaction and the gas adsorption.     

Aging of Lime Putty: Effects on Traditional Lime Mortar Carbonation

The influence of storing slaked lime under water for extended periods of time (i.e., aging) on Ca(OH)₂ crystal morphology, texture, and carbonation evolution of various lime mortars has been studied by the combined use of X-ray diffractometry, phenolphthalein tests, porosity measurements, electron microscopy, and ultrasonic wave propagation analyses. Mortars prepared using traditional aged lime putties (up to 14 years storage under water) show rapid, extensive carbonation, resulting in porosity reduction and ultrasonic speed increase. The aged hydrated lime mortar carbonation reaction (i.e., Ca(OH)₂+ CO₂= CaCO₃+ H₂O) follows a complex diffusive path, resulting in periodic calcite precipitation as Liesegang rings. In this case, binder:aggregate ratios >1:4 result in crack development. Nonaged commercial hydrated lime mortars show slower carbonation and need a higher binder:aggregate ratio (1:3). The carbonation of nonaged lime mortars follows a normal diffusion-limited continuous path progressing from the mortar sample surface toward the core. Differences between aged and nonaged lime mortar carbonation evolution are explained considering Ca(OH)₂ crystal shape changes (from prisms to platelike crystals) and size reduction that occurs on aging of lime putty. Implications of these results on historic building conservation using traditional lime mortars are discussed.