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Effects of calcium carbide slag on properties and carbon sequestration efficiency of cement pastes mixed under direct CO2 injection conditions
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Calcium silicate cement (CSC) can be a promising cementitious binder for its reduced CO2 footprint in comparison
with ordinary Portland cement, while its detailed chemical information remains undisclosed. Herein, we
present thermodynamic calculation results that illustrate the effect of reaction and carbonation degrees on the
reaction products of CSC that undergoes the hardening process at an elevated atmospheric CO2 concentration.
The obtained simulation results are discussed in relation to stability of phases. It is revealed that C-S-H can be a
stable and strength-giving phase when the carbonation degree is less than one-third of the reaction degree.
Amorphous aluminosilicate and calcite become the main binding phases at higher carbonation degrees when C-SH
is depleted.
With the utilization of mineral CO2 sequestration technology, the volume stability of cement paste exposed to carbonation is catching significant interest. This review attempts to summarize the state-of-the-art knowledge on the volume stability of cement paste upon carbonation and thereby benefit the decision-making process in the utilization of CO2 sequestration technology in the concrete industry. In this work, the dimensional change caused by carbonation of each major cement hydration product was analyzed individually, with a special focus on the carbonation of calcium silicate hydrate (C-S-H). Key parameters affecting carbonation reactions and associated volume stability of cement paste were examined, e.g. CO2 concentration, temperature, relative humidity, and supplementary cementitious materials. On that basis, the typical evolution of dimensional change of cement paste upon carbonation was presented and elucidated, with the critical research gaps identified. Overall, carbonation of calcium hydroxide and calcium silicate (C2S and C3S) tends to cause a volume expansion, whereas carbonation of ettringite and C-S-H phases leads to the opposite. The change in micropore structures of cement paste upon carbonation depends mainly on the competition between the clogging of capillary pores filled with calcium carbonate and the formation of additional capillary pores induced by the decalcification of C-S-H. To assess the volume stability of cement paste carbonated at early ages, the coupling effect between hydration and carbonation requires further studies. For the measurement of linear carbonation deformation of cement-based materials, a standard test procedure is urgently needed.
This study investigated the effect of temperature (5 • C-50 • C) on the carbonation process, compressive strength and microstructure of CO 2-cured cement paste. Results showed that the carbonation process and rate were significantly affected by temperature and time. When the curing temperature increased, the rate of improvement of cement paste's properties was accelerated. The carbonation reaction was mainly kinetically controlled by product layer diffusion with an activation energy of about 10.8 kJ/mol. Temperature had greatly affected the structural transformation, morphological changes, size and amounts of calcium carbonate polymorphs (calcite, aragonite and vaterite) as well as their degree of crystallinity and decomposition temperature. Alongside calcite, vaterite and aragonite were formed at low and high curing temperatures, respectively. Apart from microstruc-ture, the compressive strength was also found to be very sensitive to temperature and carbonation products. The relationship between the amount of different carbonate polymorphs and the compressive strength was also provided.
Accelerated carbonation treatment as an effective approach of reducing the carbon footprint in the concrete industry is being investigated extensively. Such carbonation treatment converts gaseous CO2 into carbonate minerals (or mineral CO2 sequestration). The present review provided a state-of-the-art summary on the relevant carbonation strategies that have been investigated, with a special focus on identifying the remaining industrial challenges and critical research gaps that need further investigation. Key parameters affecting pre-carbonation treatments of recycled concrete aggregates, fly ash, and slag were examined systematically, including chemical compositions and particle sizes of the materials, temperature, pressure, and CO2 concentration. The CO2 uptake capacity and efficiency of the materials were compared with each other, and the environmental impact of pre-carbonation treatment of fly ash and slag was approximated. After carbonation treatment, the incorporation of those carbonated materials in cement-based composites was assessed. It was found that the rising CO2 uptake of ingredient materials subjected to pre-carbonation treatment does not necessarily benefit the mechanical properties of the cement-based composites with the carbonated materials. For a specific material, there may exist a CO2 uptake threshold over which high CO2 uptake tends to be deleterious, e.g. for the mechanical properties. In terms of the influence of accelerated carbonation curing on the compressive strength of cement-based composites at early ages, both positive and negative influences were identified. Corresponding mechanisms behind such contrary influences were critically discussed. It was noted that over-intensified early-age carbonation curing could compromise the strength properties of cement-based composites both in the short and long terms.
Nano-CaCO3 was producted by using carbide slag in the self-designed jet-reactor. The effect of different operating parameter such as CO2 flow rate and concentration, liquid flow rate and concentration of Ca(OH)2 on the CaCO3 particle size and morphology has been investigated in this paper. The obtained calcite particle were characterized using x-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that the calcite about 50–200 nm was obtained under the reaction conditions of the saturation of calcium hydroxide as 1, the flow rate as 1.5 l min⁻¹, the CO2 flow rate as 50 ml min⁻¹, the concentration as 100%, and the SDS amount 2%.
This paper presents a study on the carbonation behaviors of hydraulic and non-hydraulic calcium silicate phases, including tricalcium silicate (3CaO·SiO2 or C3S), γ-dicalcium silicate (γ-2CaO·SiO2 or γ-C2S), β-dicalcium silicate (β-2CaO·SiO2 or β-C2S), rankinite (3CaO·2SiO2 or C3S2), and wollastonite (CaO·SiO2 or CS). These calcium silicate phases were subjected to carbonation reaction at different CO2 concentration and temperatures. Thermogravimetric analysis (TGA) tests were performed to quantify the amounts of carbonates formed during the carbonation reactions, which were eventually used to monitor the degree of reactions of the calcium silicate phases. Both hydraulic and non-hydraulic calcium silicates demonstrated higher reaction rate in case of carbonation reaction than that of the hydration reaction. Under specific carbonation scenario, non-hydraulic low-lime calcium silicates such as γ-C2S, C3S2 and CS were found to achieve a reaction rate close to that of C3S. Fourier transformed infrared (FTIR) spectroscopy and scanning electron microscope (SEM) tests were performed to characterize the carbonation reaction products of the calcium silicate phases. The FTIR spectra during the early stage of carbonation reaction showed formation of calcium silicate hydrate (C–S–H) from C3S, γ-C2S, β-C2S, and C3S2 phases with a similar degree of polymerization as that of the C–S–H that forms during the hydration reaction of C3S. However, upon further exposure to CO2, these C–S–H phases decompose and eventually converted to calcium-modified (Ca-modified) silica gel phase with higher degree of silicate polymerization. Contradictorily, CS phase started forming Ca-modified silica gel phase even at the early stage of carbonation reaction. This paper also revealed the stoichiometry of the Ca-modified silica gel that formed during the carbonation reaction of the calcium silicate phases using the SEM/energy dispersive spectroscopy (EDS) and TGA results.
The purpose of this article is to investigate the carbonation mechanism of CH and C-S-H within type-I cement-based materials in terms of kinetics, microstructure changes and water released from hydrates during carbonation. Carbonation tests were performed under accelerated conditions (10% CO2, 25 °C and 65 ± 5% RH). Carbonation profiles were assessed by destructive and non-destructive methods such as phenolphthalein spray test, thermogravimetric analysis, and mercury intrusion porosimetry (destructive), as well as gamma-ray attenuation (non-destructive). Carbonation penetration was carried out at different ages from 1 to 16 weeks of CO2 exposure on cement pastes of 0.45 and 0.6 w/c, as well as on mortar specimens (w/c = 0.50 and s/c = 2). Combining experimental results allowed us to improve the understanding of C-S-H and CH carbonation mechanism. The variation of molar volume of C-S-H during carbonation was identified and a quantification of the amount of water released during CH and C-S-H carbonation was performed.
In this paper, effect of nano-CaCO3 (NC) on properties of cement paste was studied. Experimental results showed that NC had no effect on water requirement of normal consistency of cement. However, with the increase of NC content, the flowability decreased and the setting time of fresh cement paste was shortened. The early hydration of cement was activated by NC. The strength and the early age shrinkage of cement paste were also studied. Mechanical experiments indicated that the flexural and compressive strength of hardened cement paste increased with the addition of NC at the age of 7d and 28d, and the optimal content of NC was 1%. The early age (12 h) shrinkage of cement paste containing 1% NC was the most prominent which could decrease the shrinkage strain obviously.
Conventionally, the aragonite polymorph of CaCO3 possesses a ‘needle-like’ morphology. Aragonite crystals with unconventional morphologies, like ‘cauliflower’ and ‘flake’, have been synthesised by combining aqueous solutions of CaCl2 and Na2CO3 under ambient reaction conditions. The unambiguous characterisation of these aragonite polymorphs is based on their characteristic powder X-ray diffraction patterns, Raman and FTIR spectra. The morphologies of these aragonite crystals have been confirmed by scanning electron microscopy. Both ‘cauliflower’ and ‘flake-like’ aragonites are spectroscopically similar to their needle-like congener, except that they are less crystalline in nature and they exhibit minor differences in their FTIR spectra. A plausible mechanism for the formation of these aragonites has been proposed based on the phase transformation behaviour during their syntheses.
The current state of knowledge of cement hydration mechanisms is reviewed, including the origin of the period of slow reaction in alite and cement, the nature of the acceleration period, the role of calcium sulfate in modifying the reaction rate of tricalcium aluminate, the interactions of silicates and aluminates, and the kinetics of the deceleration period. In addition, several remaining controversies or gaps in understanding are identified, such as the nature and influence on kinetics of an early surface hydrate, the mechanistic origin of the beginning of the acceleration period, the manner in which microscopic growth processes lead to the characteristic morphologies of hydration products at larger length scales, and the role played by diffusion in the deceleration period. The review concludes with some perspectives on research needs for the future.
This study aims to investigate the influence of CO2-mixing dose (mass fractions of 0.3%, 0.6%, and 0.9%) and prolonged mixing time on the fresh and hardened properties of cement pastes. The CO2-mixing can act as coagulant in fresh cement mixtures, resulting in a significant reduction in workability associated with the formation of a rich calcium carbonate network on the surface of cement particles. The CO2-mixing cement pastes were found to be much stiffer and more difficult to handle, place, and compact than the control mixture, which had a negative effect on the mechanical strength performance of the hardened pastes. However, prolonging the mixing time for 1 min (immediately after CO2-mixing) can effectively improve the workability (by ∼53%–85%) by breaking up the flocculation network of deposited calcium carbonates. As a result, the presence of detached calcium carbonate accelerated early cement hydration and densified the microstructure; this improved early-age compressive strength by ∼6%–32%, depending on the CO2-mixing dose used. Therefore, it seems that the CO2-mixing dose should be controlled at ⩽0.6% with the mixing time prolonged in order to attain satisfactory workability and mechanical strength.
Aiming to promote applications of carbonated recycled concrete fines (CRCF) as a low-carbon alternative to ordinary Portland cement (OPC), this study explored the effect of CRCF on the early hydration kinetics of OPC paste. The results show that CRCF is not only a highly active pozzolanic material but also an effective accelerator affecting the early hydration kinetics of OPC paste, thereby leading to an increased early compressive strength and a comparable 28-d compressive strength, even at a replacement ratio of 20%. Although the rheological properties and workability are negatively affected due to the high surface area and fine particle size of CRCF, the amorphous silica and silica-alumina gels and calcite present in CRCF facilitate initial ions dissolution and provide additional active nucleation sites for hydrates. As a result, the microstructure is densified due to the formation of additional C-S-H gels with high Si/Ca and Al/Ca ratios and carboaluminate phases, along with the inert filler effect of unreacted calcite particles. The incorporation of 20% CRCF to replace OPC can achieve a CO 2 emissions reduction up to 25%. These indicate that the CRCF has a promising application scenario in transforming wasted concrete into high-value industrial products by CO 2 activation.
Cementitious binders are porous materials with significant portlandite content. Potentially, this portlandite content can react with CO 2 and form carbonate compounds with binding ability. Using molecular dynamics simulations and density functional theory, in this paper the kinetics of CO 2 inside the portlandite pores were studied. The results indicate that the larger dry pores within the portlandite-enriched binder matrix are the most effective at enabling the passage and adsorption of CO2 molecules. Also the pores smaller than 21 Å are capable of adsorbing CO2 on the surface of portlandite, leading to the formation of CaCO 3. Two main attraction forces between Ca(portlandite)-O(calcite) and O(portlandite)-H(calcite) pair the newly formed Ca(OH) 2 matrix and CaCO 3 walls together, with the Ca-O providing the majority of binding energy. The existence of these attractions enhances the mechanical cohesion as the sliding resistance between portlandite, the main body of the pores, and the calcite, the carbonation product, reaches as high as 1.7 GPa in theory.
A series of new low calcium CO2 sequestration cementitious materials (LCC) with different Ca/Si ratios were
prepared by sintering waste concrete fine powder (WCP) and calcium carbide slag (CCS) at different ratios. Their
carbonation reaction activity, carbonation hardening properties, phase assemblages, and microstructure were
systematically characterized. The results showed that when the WCP: CCS ratio was 70:30, the compressive
strength of LCC reached 38 MPa after carbonation for 24h. With the decrease in the WCP: CCS ratio, the
carbonation activity of LCC increased significantly. When the WCP: CCS ratio was 45:55, the compressive
strength was increased to 109 MPa. Microstructure tests showed that the minerals of LCC gradually changed from
CS and C2AS to C3S2 and C2S. After carbonation, unreacted clinker particles, crystalline CaCO3, and highly
polymerized silica formed a dense microstructure, and the impurities of Mg element in the LCC triggered the
formation of aragonite.
In the present study, we investigated the shell microstructures of the gastropod European abalone Haliotis tuberculata in order to clarify the complex spatial distribution of the different mineral phases. Our studies were carried out with a standardized methodology on thirty adult European abalone H. tuberculata (5-6 cm long) composed of 15 wild individuals and 15 individuals taken from the France Haliotis hatchery. The macroscopic (binocular) and microscopic observations coupled with Fourier Transform Infrared Spectroscopy (FTIR) and Raman vibrational analysis allowed to unambiguously detect and identify and localize calcite and aragonite. For the first time it has been shown that calcite is present in 100% of farmed and wild adult shell. The microstructural details of the calcite-aragonite interfaces were revealed by using both confocal micro-Raman mapping and Scanning Electron Microscopy (SEM) observations. Calcite zones are systematically found in the spherulitic layer without direct contact with the nacreous layer. The calcite area - nacreous layer interface is made of a thin spherulitic layer with variable thickness from a few micrometers to several millimeters.
In order to contribute to a better understanding of the biomineralization process, a model explaining the hierarchical arrangement of the different phases of calcium carbonate is presented and discussed. Finally, it has been shown that these calcitic zones can be connected to each other within the shells and that their spatial distributions correspond to streaks perpendicular to the direction of length growth.
Carbonation hardening of Portland cement concrete can contribute to lower its CO2 footprint and to improve its strength. The reaction mechanisms involved in carbonation curing were investigated on fresh cement pastes. The quantitative results gained allowed to link the curing conditions to the reactions kinetics, phase assemblage and microstructure evolutions.
During the early carbonation curing, alite and belite react with dissolved CO2. The main carbonation products are calcium carbonate, an alumina-silica gel and a C-S-H phase with a low Ca/Si ratio. Hydration curing following the carbonation transforms the silica-rich phase into a C-S-H phase typical of hydrated Portland cements. Additionally, anhydrous calcium aluminates from clinker react to form ettringite and monocarbonate phases. The structure and composition of these phases is governed by the local equilibria in the dense microstructure. The carbonation and hydration reactions decrease the pore volume and refine the porosity resembling the microstructure development in the composite cements.
This paper investigated the properties of sulphoaluminate cement-based reactive powder concrete (RPC) incorporated with nano-CaCO3 (NC), including setting time, fluidity, strength, hydration, and microstructure. The results showed that the addition of NC shortened the setting time and reduced the fluidity of RPC mortar. The rate of heat flow and the cumulative heat of hydration increased along with the increased NC content, and reached the maximum when the NC content was 2.5%. Moreover, the addition of NC enhanced the compressive and flexural strengths of RPC. When the NC content was 2.5%, the experimental group’s 90d compressive and flexural strength increased by 25.8% and 19.9%, respectively, compared with the blank group. The results of XRD, TG, ICP and isothermal calorimetry had a good consistency and revealed the mechanism of NC promotes the hydration process of RPC, i.e., more hydration products, such as AFt and AH3, were formed owing to the sufficient ion exchange in earlier ages, which filled the pores and made the RPC microstructure more compact and the mechanical properties better.
Attempts have been dealt to stabilize the aragonite (CaCO3) and improve its environmental applications for high concentration of phosphate removal by the introduction of aluminum (Al) species. The nano-pellets of aragonite doped with Al (Al/aragonite) was therefore manufactured in this study. With a combination of XRD, FTIR, zeta potential, SEM-EDX and XPS analysis, the synthesized Al/aragonite materials before/after phosphate sorption were characterized. The incorporation of Al species in aragonite lattice significantly increased the high phosphate scavenging from solution. Compared with the Langmuir monolayer coverage of phosphate on aragonite and calcite, the multi-layer sorption onto Al/aragonite could be described well by Freundlich model. Thus, the phosphate removal of Al/aragonite was 6 time stronger than aragonite, and 15 times than calcite at alkaline pH. The likely reason was that the Al species doped in Al/aragonite nanoparticles increased the surface area and reduced the negative surface charges, especially, provided the more adsorption sites for the formation of irreversible inner-sphere phosphate complexes which induced the vastly unlimited growth of CaHPO4 in the pH range of 5.0-12.0. Overall, the Al/aragonite nanoparticles (Al/Ca molar ratio = 0.205) was found to be the optimal material to efficiently remove the high-level phosphate from wastewater within the wide pH ranges.
To tackle the rising CO2 concentration in atmosphere, technologies of carbon sequestration are emerging. As one of these technologies, carbonation curing of cement-based composite at early age shows a great potential because its wide application and benefits to provide cement-based composite with enhanced mechanical properties and durability. However, carbonation curing is mostly investigated on lab-scale, due to two main challenges that should be addressed before it can be widely applied for industrial-scale production: (1) CO2 sequestration rate is still low, which is around 20 %–50 % of the theoretical potential because of the limitation of CO2 diffusion into matrix during the carbonation curing; and (2) the effect carbonation curing on the durability of cement-based composites and the underlying mechanisms remains unclear, especially for some critical aspects such as alkali-silica reaction, corrosion, and sulfate attack. Therefore, this paper focuses on discussing: (1) factors affecting CO2 sequestration in cement-based composite during each stage of carbonation curing and the optimization methods of carbonation curing for high CO2 sequestration rate without mitigating mechanical properties and durability, and (2) effects of carbonation curing on different aspects of durability and the corresponding mechanisms. Based on the review, future studies on addressing current challenges are inspired to comprehend and promote the carbon sequestration by carbonation curing of cement-based composites.
The industrial injection of carbon dioxide into ready mixed concrete during mixing and batching can produce a measurable increase in hydration and a significant compressive strength increase. Physiochemical aspects of the mechanism were investigated through the analysis of a model tricalcium silicate system activated with a carbon dioxide addition immediately after hydration started. The attendant effects were examined through isothermal calorimetry, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). Carbonate reaction products around 70 nm were observed to have formed within 60 s of the CO2 gas injection. Within 10 min the carbonates have been covered by new hydration products. The initial product formed appears to be an amorphous or poorly crystalline calcium carbonate. The overall reaction over 24 h was the same for the hydrated and CO2-activated cases although the latter instance included the formation of carbonate reaction products.
The carbonation mechanisms of ground, hydrated cement pastes that mimic recycled concrete fines are investigated under direct wet carbonation. The carbonation is a sequential processes including dissolution of the hydrates, transport of the dissolved material to the precipitation sites and precipitation of the reaction products. During the first stages of carbonation, when portlandite reacts, the reaction is limited by the amount of dissolved CO2 and potentially by the calcite precipitation rate. When portlandite is depleted, the kinetics is dominated by the dissolution of other hydrates and diffusion of calcium into the solution. The main carbonation products are calcite and an alumina-silica (hydrate) gel. The hydrotalcite-like phase appears to be stable towards carbonation under the experimental conditions employed in this work. The CO2 concentration and the paste composition have a limited influence on the reaction mechanisms as well as on products formed.
In-depth understanding regarding the carbonation properties of pure minerals including β-dicalcium silicate (β-C2S), calcium hydroxide (CH), and tetracalcium aluminoferrite (C4AF) was conducted to explore the types, microstructure, crystallinity of products produced during carbonation, thereby developing a tentative microscopic mechanism of the contribution of carbonation to macroscopic properties. An investigation of changes in pH value and Ca leaching of the pure minerals was conducted to explore the effect of dissolution properties to the carbonation. Results showed that the descending order of the ultimate carbonation degrees of minerals was: (i) CH, (ii) β-C2S, (iii) C4AF. The carbonation degrees and rates were not only controlled by the dissolution properties of the minerals, but also the distribution of the produced CaCO3. The order of the contribution of minerals induced by carbonation to the macroscopic mechanical strength was, in descending order: (i) β-C2S, (ii) CH, (iii) C4AF. This difference was the combined effects of the microstructure, crystallinity, and particle size of calcite. Effects of the compact stack and the strong mechanical bond between the calcite particles were observed in carbonated β-C2S samples; In contrast, pronounced defects on the calcite particles were observed in carbonated C4AF samples. Carbonation is an effective way to reduce the density of which mainly contained phases of β-C2S and C4AF, and to store greenhouse gas CO2 permanently. These findings are meaningful for the further in-depth researches of the carbonation mechanism of materials which primarily contain β-C2S, CH and C4AF phases.
This study aims to investigate the effects of pre-curing and carbonation duration on compressive strength and microstructure characteristics of Portland cement paste. Experimental results showed that 30-40% water loss of cement paste is ideal for CO 2 uptake. The combination of appropriate pre-curing and carbonation duration could increase compressive strength of cement mortar effectively, especially at early age. The optimal pre-curing duration decreases as the carbonation duration increases. Thermogravimetric analysis indicated that carbona-tion could retard hydration of cement paste and higher concentration of CO 2 increases crystallinity of carbonate. According to MIP results, the influence of hydration and carbonation on pore structure of cement paste is different. Carbonation could decrease large capillary porosity obviously while hydration fills small capillary first. In addition, it is observed from EDS point analysis that excessive carbonation lead to decalcification of C-S-H. Therefore, suitable pre-curing and carbonation duration is of significance to improve the strength and micro-structure of cement pastes.
This paper aims to investigate the influences of limestone powder, fly ash and ground granulated blast-furnace slag (GGBS) on carbonation curing of cement pastes. These three mineral admixtures were incorporated at percentage of 20% by weight to replace cement and compressive strength and chloride ion permeability test were performed to evaluate the carbonation curing effects. On the other hand, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetry-differential thermal analysis (TG-DTA), mercury intrusion porosimeter (MIP) and scanning electron microscope (SEM) measurements were performed on typical samples. Experimental results show that the addition of mineral admixture decreased the compressive strength of uncarbonated specimens but strengthened the improvement effect of carbonation curing on compressive strength. This better improvement effect can partly compensate the decreasing strength induced by the incorporation of mineral admixtures. Among those mineral admixtures, fly ash exhibited the best improvement effect and GGBS ranked second. The chloride ion permeability of cement mortars were decreased by the addition of mineral admix-tures but improved by the carbonation curing. The formation of CaCO 3 upon carbonation curing refined the pore structure and presented a higher effectiveness for filling pores with diameters of 0.1 $ 1 lm. Therefore, carbonation curing provides a good method for both efficiently recycling industrial wastes as mineral admixtures and capturing more greenhouse gas.
CO 2 cured calcium silicate cement (CSC) can substantially improve the sustainability of concrete-like materials when used as an alternative to ordinary portland cement (OPC). CSC is produced from the same raw materials as the OPC but at kiln temperatures around 250 °C lower than those required for the production of the OPC. This paper presents a summary of the results from a comprehensive study on the evolution of microscopic phases, reaction kinetics, and strength development in CSC paste and mortar samples during carbonation. The primary carbonation products of CSC are calcium carbonate and Ca-modified silica gel. All three polymorphs of crystalline calcium carbonate (i.e. calcite, aragonite, and vaterite) were found to be present in carbonated CSC paste, calcite being the most abundant polymorph. Influences of water to cement ratio (w/c, by weight), temperature, relative humidity, and CO 2 concentration on the carbonation rate constants of CSC have been studied here. The carbonation rate constants of CSC were found to be the highest for w/c of 0.4 in a 99.9% CO 2 environment. The carbonation activation energies of these systems varied from about 44 kJ/mol to about 57 kJ/mol, depending on the carbonation curing conditions (i.e., w/c ratio, CO 2 concentration, etc.). The CSC mortar samples achieved compressive strength as high as 40 MPa after only 3 days of carbonation.
Many of the degradation problems of concrete can be attributed to the weak surface layer. Surface treatment of concrete has been proven a simple and effective method to enhance the durability of concrete. In the present study, two series of portland cement mortar specimens with w/c ratios of 0.4 and 0.3 were prepared. CO2 treatment was applied to he mortars 24 h after casting. The results indicated that CO2 treatment resulted a carbonated a layer of less than 1.6 mm in thickness, slightly increased compressive strength, but significantly reduced the water permeability, water-vapor transmission and chloride migration of the mortars. Environmental Scanning Electron Microscopy (SEM), Fourier Transform Infrared (FT-IR) Spectroscopy and thermogravimetric analysis (TGA) results indicated that calcium silicate hydrate with low Ca/Si ratio and CaCO3 were the products in the carbonated surface layer.
Paulo J. M. Monteiro, Sabbie A. Miller and Arpad Horvath provide an overview of the challenges and accomplishments in reducing the environmental burden of concrete production.
A facile and direct hydrothermal method for the crystallization of vaterite CaCO3 micro-spheres in the presence of carbide slag saturated limpid solution and a new CO2-storage material (CO2SM), which was obtained from an equimolar system of 1,2-ethylenediamine (EDA) + 1,2-ethylene glycol (EG) uptaking CO2 (ChemPhysChem., 16 (2015) 2106), was presented without any outside additives. It’s worth noting that the morphologies of CaCO3 precipitates could be controlled as homogeneous spherical-like (pure vaterite) at the 100 g L⁻¹ CO2SM concentration for 90 min at 100 °C, in which released EDA and/or EG from the CO2SM was used as surfactants. After the precipitation of CaCO3 crystals in 100 g L⁻¹ CO2SM solution, the filtered solution could not only be reused to absorb CO2, but also to prepare the same crystal phase CaCO3 micro-particles repeatedly with the addition of carbide slag. Thus, this novel synthesis process of CaCO3 micro-particles with carbide slag and the CO2SM offers an alternative way for the comprehensive utilization of CO2 and the solid waste carbide slag.
The cement industry is a major industrial producer of greenhouse gases, responsible for 5–8% of man-made CO2 emissions. The carbonation reaction, which occurs between CO2 and the corresponding reactive compounds from cement-based materials, can fix CO2 in the form of thermodynamically stable carbonates. Thus, the CO2 uptake technology using cement-based materials could be considered as an effective option for Carbon Capture, Utilization and Sequestration projects. This paper presents a review on state of the art studies in this emerging technology and provides information from a scientific and technical perspective on CO2 utilization and sequestration technologies using cement-based materials. The CO2 uptake mechanisms of cement-based materials are summarized and factors affecting the kinetic of the carbonation reaction are identified. Furthermore, this paper consolidates available information on CO2 uptake capacity and efficiency, industrial challenges, critical issue and the need for further investigation regarding the technologies. Lastly, the environmental impact and economic feasibility of using cement-based materials in the CO2 utilization and sequestration technology sector were analyzed and compared with geologic sequestration and mineral carbonation technologies.
Abstract The effect of carbonation curing on the mechanical properties and microstructure of concrete masonry units (CMU) with Portland limestone cement (PLC) as binder was examined. Slab samples, representing the web of a CMU, were initially cured at 25°C and 50% relative humidity for durations up to 18 h. Carbonation was then carried out for 4 h in a chamber at a pressure of 0.1 MPa. Based on Portland limestone cement content, CO2 uptake of PLC concrete after 18 h of initial curing reached 18%. Carbonated and hydrated concretes showed comparable compressive strength at both early and late ages due to the 18-h initial curing. Carbonation reaction converted early hydration products to a crystalline microstructure and subsequent hydration transformed amorphous carbonates into more crystalline calcite. Portland limestone cement could replace Ordinary Portland Cement (OPC) in making equivalent CMUs which have shown similar carbon sequestration potential.
Global warming is affecting each and every part of the world. Due to global warming, the glaciers are melting which are causing the rise in the sea level. When the level of the sea rises, it causes danger to the people living in the low lying areas. So, this causes a big problem for people, plants and animals living on the earth or the ecosystem all in all. Pollution whether vehicular, electrical or industrial is the main contributor to the global warming. Everyday billions of vehicles release various gases into the atmosphere. This causes earth to warm up and increase its average temperature. The main purpose of the study is to collect information about a real problem that has negative impact on something in order to be able to reach solution or decrease its impact. This study shows that global warming is the result of many factors including greenhouse gasses which can be reduced if people behave in a responsible way. We can state that pollution is the link between the greenhouse gases released to the air and get trapped in the atmosphere which cause the raise in temperature known as global warming and leading to a huge bulk of negative consequences to all living and non-living creatures on Earth’s surface.
The purpose of this research is to investigate the effect of specific hydration reaction on the stiffening process of cement paste. The cement compositions are manipulated to cause specific hydration reactions (secondary gypsum and syngenite formation) responsible for false set, and the relationship between specific hydration reactions and the flow and stiffening behavior of cement paste were investigated using modified ASTM C 403 penetration resistance measurement and oscillatory shear rheology. X-ray powder diffraction (XRD) was used for the phase identification associated with premature stiffening of cement paste. Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were used for verification of syngenite formation. From the results, both secondary gypsum and syngenite formation caused faster stiffening and set. The amount of syngenite produced during 1 hour hydration was approximately 1 % of total mass of the cement paste, but cement paste with syngenite formation showed significantly accelerated stiffening behavior compared to normal cement paste.
The carbonization of 11 Å-tobermorite and CSH-gel produces in addition to SiO2-gel, first vaterite which is later transformed into the stable modification: calcite. The metastable phase-aragonite is formed only to a small extent. The formation of vaterite mainly depends on a relatively lower CO2 concentration (1, 10%) and on a moderate relative humidity (50, 75%); the opposite holds true for its rapid transformation into calcite (30% CO2, r.h. = 100%). As a consequence of pseudomorphosis, no substantial change occurs in the outer forms of the 11 Å-tobermorite crystals, even though complete decomposition has taken place. It was also proved that the pressure strength values of porous concrete test cubes maximally decreased by 10% after two years depositing when compared with the data before carbonizing.ZusammenfassungDurch die Karbonisierung von 11 Å-Tobermorit und CSH-Gel entsteht neben SiO2-Gel zuerst Vaterit, das in die stabile Modifikation: Calcit übergeht. In einem geringen Ausmass kommt es zur Entstehung einer weiteren metasbabilen Phase - Aragonit. Die Bildung von Vaterit ist hauptsächlich durch eine relativ niedrigere CO2-Konzentration (1, 10%) und durch eine mittlere relativ Feuchte (50, 75%) bedingt; entgegensesetzt ist es bei den Bedingungen seiner schnellen Umwandlung in Calcit (30% CO2, r.F. = 100%). Infolge einer Pseudomorphose werden die äusseren Formen der 11 Å-Tobermoritkristalle nicht wesentlicher verändert, wenn auch es zu einer vollkommenen Zersetzung von Tobermorit gekommn ist. Es wurde bewiesen dass die Druckfestigkeitswerte der Prüfkörper von Porenbetonen nach Lagerung von zwei Jahren maximal um 10% im Vergleich mit den Angaben vor der Karbonisierung gesunken sind.
A beneficial use of carbonation as an auxiliary curing regime for concrete pipes was studied in an attempt to reduce steam curing time, improve durability performance and explore the possibility of using concrete pipe to sequester carbon dioxide. Durability performance of the carbonated concretes was characterized by carbon uptake, strength gain, pH, calcium hydroxide content, permeability, sorptivity and sulfate and acid resistance. It was found that initial curing using steam is necessary to facilitate carbonation. Although the contribution of early carbonation to strength gain is not noticeable after initial steam curing, the process is unique in promoting enhanced durability performance of concrete. The early carbonation leads to a reduction in calcium hydroxide near the surface while maintaining a pH above the corrosion threshold value at the core. Carbonated concretes also exhibit improved resistance to sulfate attack, water absorption, and chloride ion penetration. A carbon uptake of 9% by cement mass makes concrete pipe an ideal candidate for carbon dioxide capture and storage.
An investigation on using municipal solid waste incinerator bottom ash (MSWI) and calcium carbide waste (CCW) as a part of the cement raw materials was performed. Cement raw meals were replaced by 5% and 10% of MSWI and CCW to study properties of the laboratory produced MSWI and CCW cements. Chemical composition, setting times, compressive strength and expansion in sulfate solution of the pastes and mortars made of MSWI cements and CCW cements were tested and compared with these made of conventional cement. It was found from the study that the chemical compositions of MSWI cements and CCW cements were similar to that of the control cement. However, SiO2 content of MSWI cements was higher than that of the control cement, whereas CaO content was lower. Setting times of cement pastes were slightly delayed when MSWI or CCW were used to replace a part of raw meal in cement production. The longer setting times of these cement pastes were observed due to the lower C3S but higher C2S content than those in the control cement. Compressive strength of CCW cement mortars was close to that of the control cement. However, compressive strength of the mortars produced from MSWI cements was smaller than that of the control cement mortar, especially when the percentage of MSWI in the raw meal was increased. When compared to the control cement, the performance of MSWI cement and CCW cement in sodium sulfate solution was superior due to the lower C3S and C3A.
The stability of ettringite (3CaO·Al2O3·3CaSO4·32H2O) in contact with CO2 gas has been studied using synthesized ettringite. Carbonation were carried out at water/solid ratios(W/S) of 0.6∼3.5 in a moist CO2-incubator. According to the SEM-EDAX examination, the carbonation without water makes compositional changes slightly in ettringite, maintaining fibrous form. By the carbonation with excess water, ettringite clearly decomposed to gypsum, calcium carbonate, and alumina gel. The apparent carbonation rates were calculated from the amount of calcium carbonate. Jander equation was taken to model the carbonation kinetics.
This paper proposes a new cementitious material from a mixture of calcium carbide residue and rice husk ash. Calcium carbide residue and rice husk ash consist mainly of Ca(OH)(2) and SiO2, respectively. The cementing property was identified as a pozzolanic reaction between the two materials without portland cement in the mixture. Properties such as setting times of pastes, flow, and compressive strength of mortars were investigated when calcium carbide residue and rice husk ash were used as cementitious material. The results show that the setting times of the new cementing pastes are longer than that of the portland cement paste. The ratio of calcium carbide residue to rice husk ash of 50:50 by weight obtains the highest compressive strength of mortar. The compressive strength of mortar could be as high as 15.6 MPa at curing age of 28 days and increased to 19.1 MPa at 180 days. According to the compressive strength of mortar, the mixture from calcium carbide residue-rice husk ash has a high potential to be used as a cementing material. However, more research and development especially on optimum mix design, setting times, early strength, and durability of concrete should be carried out.
Two structural propositions of vaterite are examined using infrared spectroscopy. The carbonate ion has Cs-site symmetry in the structure presented by Meyer (1959), but C2v according to Kamhi (1963). Observed infrared spectra are characterized by (1) the activation and splitting of v1 and (2) the splitting of v3, which support C2v symmetry.
The structures of partially carbonated hardened C3S cement pastes have been investigated by a combination of 29Si magic angle spinning nuclear magnetic resonance spectroscopy and analytical transmission electron microscopy, supported by X-ray diffraction and thermogravimetric analysis. Progressive changes in structure are reported for thin slices for a paste carbonated in pure CO2 for times from 1 to 16 h, and the results are compared with those for a paste carbonated for 2 months in air. C-S-H gel of reduced Ca:Si ratio and increased silicate polymerization was formed during the early stages of carbonation. The morphology of the original C-S-H was, in the main, retained. A cross-linked silica-rich gel formed at later times in paste carbonated in CO2 but not up to the time of 2 months in air. Calcium carbonate took the form of microcrystals of vaterite and calcite which formed dense masses between gel fibrils and around partially reacted CH crystals, possibly accounting for the observed slowing in the rate of reaction of CH with time.
This paper is a bibliographic tool reviewing experimental and theoretical studies related to cement hydration and microstructure development that have been published within the four years of the interim period between the 12th and 13th International Congress on the Chemistry of Cement.
The fundamental chemical hydration process of portland cement and its main mineral component, tricalcium silicate, was studied by investigating the effects of various additives. A relatively small amount (1-4 wt %) of well-dispersed calcium silicate hydrate (C-S-H), a pure form of the main hydration product, significantly increases both the early hydration rate and the total amount of hydration during the early nucleation and growth period (the first ∼24 h), as measured by calorimetry. This is attributed to a seeding effect whereby the C-S-H additive provides new nucleation sites within the pore space away from the particle surfaces. This mechanism is verified by a digital simulation of the hydration process that reproduces key features of the hydration kinetics. The results provide strong evidence that the hydration process is autocatalytic such that the C-S-H gel product stimulates its own formation. The seeding effect of C-S-H also provides a new explanation of the hydration-accelerating effects of various forms of reactive silica because these additives form C-S-H by reacting with aqueous calcium ions released by cement dissolution. Experiments involving sucrose, a hydration retarder, confirm that sucrose interferes with the normal nucleation process on the particle surface.
An understanding of the performance of portland cement-based materials requires knowledge at the microstructural level. Developments in the instrumentation of several techniques have led to improved understanding of the composition, morphology, and spatial distribution of the various products of cement hydration. In particular, our understanding of the nature of the nearly amorphous calcium silicate hydrate (C–S–H) phases – which are the principal binding phases in all portland cement-based systems – has been advanced by developments in solid-state NMR spectroscopy and analytical TEM. This paper presents an overview of the nature of the hydration products formed in hardened portland cement-based systems. It starts with the most straightforward cementitious calcium silicate systems, C3S and β-C2S, and then considers ordinary portland cement and blends of portland cement with silica fume, ground granulated iron blast-furnace slag, and finally alkali hydroxide-activated slag cements.
Carbonation caused by atmospheric carbon dioxide is one of the major physicochemical processes which can compromise the service life of reinforced concrete structures. While the bulk of the carbonation reaction is that of calcium hydroxide, other constituents of the porous matrix can also carbonate and compete with calcium hydroxide for carbon dioxide. Particularly the carbonation of calcium–silicate hydrates and unhydrated constituents are neglected by most authors in carbonation prediction models. In this paper, a mathematical model of carbonation is extended to include additional carbonation and hydration reactions. The competition of the several reactions and their effect on the carbonation depth is investigated by dimensional analysis and numerical simulations. A parameter study emphasises that multiple internal reaction layers appear. Their position and speed essentially depend on the strength of the different reactions. It is also observed that, for a wide range of parameters, the effect of some of the additional reactions on the carbonation depth is small. In particular, a comparison with data from laboratory experiments justifies the neglect of the carbonation of the unhydrated constituents in prediction models.