Accelerated Carbonation Experiments Research Articles

An investigation of the source of calcium carbonate from the carbonated Portland cement: Based on the transformation of hydrates

In this work, two PC pastes were used to carry out accelerated carbonation experiments, and phase assemblages of paste samples collected at different sampling depths were analyzed. The source of CaCO3 originating from the carbonation reaction of hydrates in cement was quantitatively determined by a novel method based on the transformation of hydrates. Experimental results indicated that the phase assemblages in the carbonated zone of two PC pastes showed minor differences. C(A)SH, ettringite, AFm-CO3, and unhydrated cement particles in the complete carbonation zone of two cement pastes were carbonated and transformed into other phases. For the surface of two PC pastes (0–3 mm), calculations showed that the CaCO3 originating from the carbonation of calcium hydroxide and C(A)SH accounted for 75.3% and 72.6% of the total CaCO3, respectively. A concentration of 20% CO2 was not beneficial for the diffusion of CO2 and the subsequent precipitation of CaCO3 at the later period of carbonation.

Influence of the Original Concrete Strength and Initial Moisture Condition on the Properties Improvement of Recycled Coarse Aggregate via Accelerated Carbonation Reactions.

The physical and mechanical properties of recycled coarse aggregate (RCA) are worse than those of natural coarse aggregate (NCA), and the overall performance of recycled concrete prepared from RCA is worse than that of natural aggregate concrete. Treatment of RCA by CO2-accelerated carbonation effectively improves the macroscopic properties of RCA. The degree of influence of raw material factors, i.e., the original concrete strength (OCS) and initial moisture content (IMC) of RCA, on the carbonation of RCAs is very complex. Herein, an accelerated carbonation experiment for RCA with different material factors as variables was carried out to explore the influence of the abovementioned factors on the physical properties of carbonated recycled coarse aggregate (CRCA). By analyzing the microstructure of the RCA with the best modification effect before and after carbonation, the carbonation modification mechanism of the RCA was revealed. The physical performance indexes, including the apparent density, water absorption and carbonation rate, of the dried RCA with an OCS of C40 and C50 were significantly improved. The research results can provide basic data and theoretical support for promoting the popularization and application of RCA and recycled concrete in practical engineering.

Degradation mechanism and evaluation of the carbonation resistance of concrete after high-temperature exposure

High-temperature exposure during fire decreases not only the mechanical properties of concrete structures but also their durability, especially their concrete carbonation resistance. However, the degradation of concrete carbonation resistance after fire exposure remains unclear. Concrete specimens with water/cement (w/c) ratios of 0.4, 0.5, and 0.6 were fabricated to solve such problems, and high-temperature experiments were performed at 400 °C, 500 °C, and 600 °C and sustained for one hour. Then, mechanical and accelerated carbonation experiments were conducted, and the carbonation depths and pH values of concrete were tested. X-ray diffraction, thermogravimetry analysis, and mercury intrusion porosimetry experiments were also performed on concrete with and without high-temperature treatment. Results showed that high-temperature exposure caused severe degradation in concrete compressive strength, relative dynamic modulus of elasticity, and carbonation resistance, and the carbonation resistance loss of concrete was higher than its mechanical property loss. Concrete with lower w/c ratio suffered more losses in mechanical properties, whereas w/c ratio had no obvious effect on the carbonation resistance degradation of fire-damaged concrete. The key mechanism of the decline in the carbonation resistance of concrete after fire was not mainly the alkalinity drop in concrete but the considerable increase in concrete porosity and harmful pores. Instead of the traditional phenolphthalein method, a new method of residual OH- concentration of superficial concrete after accelerated carbonation was proposed to evaluate the carbonation resistance of concrete after fire. The average carbonation resistance of the concrete with the three w/c ratios was reduced by 66.3 %, 84.2 % and 95.1 % after exposure to 400 °C, 500 °C, and 600 °C, respectively.

Synergistic Effects of Environmental Relative Humidity and Initial Water Content of Recycled Concrete Aggregate on the Improvement in Properties via Carbonation Reactions.

Moisture is the basis of CO2 transport and carbonation reactions in the internal pores of cement-based materials. Too much or too little moisture influences the effect of the carbonation modification of CO2 on recycled concrete aggregate (RCA). During the carbonation reaction process of RCA, moisture is mainly derived from the environmental relative humidity (RH) and the initial water content (IWC) of the RCA itself. According to the available literature, most of the studies on the effect of moisture on the carbonation modification of RCA considered either RH or IWC. Further investigations of the synergistic effects of RH and IWC on the improvement in the properties of carbonated recycled concrete aggregate (CRCA) are needed. In this study, accelerated carbonation experiments were conducted for RCA samples with different IWCs under different environmental RHs. The results showed that the best moisture conditions for CRCA property improvement were confirmed as RH = 70% for the dry-state IWC and RH = 50% for the saturated-state IWC. When the RCAs were carbonized under the conditions of high RH with low IWC and low RH with high IWC, CO2 had good abilities to permeate and diffuse, with the improvement in CRCA properties achieving excellent levels of performance.

Reverse-diffusion phenomenon of high ductility fly ash-based geopolymer against carbonation

Geopolymer has been a research hot spot in the field of building materials because of its superior performances, less pollution, low energy consumption and wide range of raw materials. In this study, the carbonation resistance of high ductility fly ash-based geopolymer was researched by means of accelerated carbonation experiment. The compressive strength, carbonation depth and pH value were adopted to macroscopically evaluate the carbonation enhancing influence of modified geopolymer. Meanwhile, the mercury intrusion porosimetry (MIP), Fourier transforms infrared spectroscopy (FT-IR) and X-ray diffraction (XRD) were adopted to evaluate the micropore structures, geopolymerization process and mineralogical phases, respectively. The experimental results showed that under rapid carbonization condition, the carbonization resistance of control geopolymer was obviously lower than that of cement mortar, and the incorporation of modified MWCNTs and PVA fibes could enhance the carbonization resistance of high ductility geopolymer from micron scale and nano scale. Importantly, under natural carbonization condition, a reverse-diffusion phenomenon of geopolymer was observed (the complete carbonization zone became the uncarbonization zone), and the pH value of complete carbonization zone was improved ranging from the 8.1 to 11.5, manifesting that the carbonization resistance could be improved significantly. The micro-analysis indicated that the main reason of causing reverse-diffusion phenomenon was the reverse migration of large amount of OH−, which was from the dissolution of magadiite by means of the physical phase transform from solid to liquid under low CO2 concentration.

Effect of Superabsorbent Polymer (SAP) Internal Curing Agent on Carbonation Resistance and Hydration Performance of Cement Concrete

To clarify the influencing mechanism of superabsorbent polymer (SAP) internal curing agent on the carbonation resistance of cement concrete, accelerated carbonation experiment was conducted to explore the effect of particle size and dosage of SAP on carbonation depth. The hydration performance of internally cured concrete at different ages was studied by Fourier transform infrared spectroscopy (FTIR) test and X-ray diffraction (XRD) test. Combined with the scanning electron microscope (SEM) test, the hydration filling effect of internal curing on the microstructure of concrete was analyzed; meanwhile, the influence mechanism of SAP on carbonation resistance was revealed. The results showed that (i) when the particle size and dosage of SAP were 100 mesh and 0.200%, the carbonation depth of internally cured concrete was only 56.5% of the control group on day 28; (ii) the Ca(OH)2 absorption peak area of SAP-concrete in the FTIR spectra could be increased by 3.38 times than that of the control group, and more C2S and C3S were translated into Ca(OH)2, which helped to improve the hydration degree of cement concrete; (iii) the hydration products of day 56 were increased significantly and the remaining pores formed by SAP gels were gradually filled by hydration products, which enhanced the compactness and carbonation resistance of cement concrete.

The impact of carbonation at different CO2 concentrations on the microstructure of phosphogypsum-based supersulfated cement paste

This study investigates mechanisms of carbonation on the microstructure of phosphogypsum-based supersulfated cement (PSSC) paste at different CO2 concentrations (atmospheric, 5% and 20%). The products was characterized using XRD, TGA, FTIR and 27Al MAS NMR. It is observed that the main hydration products of PSSC are ettringite, C-A-S-H gels and third aluminate hydrate (TAH), and the carbonation products are calcium carbonate (mainly vaterite and calcite), secondary gypsum, and C-A-S-H gels and separate TAH with varying degrees of decalcification. The quantitative analysis of BSEM images shows that the volume change of products occurring on carbonation significantly increases porosity and pore size of the PSSC paste. Results indicate that the carbonation of PSSC at different CO2 concentrations results in a progressive carbonation degree, 5% CO2 supports the accelerated carbonation experiment in PSSC to simulate the natural carbonation process due to the same phase composition and similar microstructure with atmospheric CO2 and 20% CO2 further loosens the microstructure of PSSC for the complete decomposition of ettringite and the severe decalcification of gel products.

Carbonation behaviors of calcium silicate hydrate (C-S-H): Effects of aluminum

The cement systems containing supplementary cementitious materials (SCMs) usually present lower resistance to carbonation compared with ordinary Portland cement (OPC) systems. However, the contribution of the main component in these systems, calcium aluminum silicate hydrate (C-A-S-H), to carbonation still remains unclear. In this study, the carbonation behaviors of C-S-H and C-A-S-H were compared by accelerated carbonation experiments. FTIR, XRD, TGA, NMR, and electron microscope were integrated to reveal the kinetics and thermodynamics of C-(A)-S-H carbonation. It is concluded that the carbonation of C-(A)-S-H experiences three periods including dissolution, diffusion, and stationary period, turning into calcium carbonate with different polymorphs, calcium and aluminum modified silica gel. In the end product of carbonation, aluminum modified silica gel is observed to have a similar Al/Si to the initial C-A-S-H. But such small incorporation of aluminum into C-S-H structure (maximum Al/Si ∼ 0.1) can hardly have a significant effect on its resistance to carbonation.

Carbonation Model and Prediction of Polyvinyl Alcohol Fiber Concrete with Fiber Length and Content Effects

Durability is an important aspect of reliability of concrete structures. In order to establish the carbonation model of polyvinyl alcohol (PVA) fiber concrete including the influences of PVA fiber length and volume content, a series of accelerated carbonation experiments were carried out on the normal concrete specimens and PVA fiber concrete specimens with fiber lengths of 3 mm, 6 mm, 12 mm and 18 mm and fiber volume contents of 0.1%, 0.3%, 0.5%, 0.75%, 1.0% and 1.5%, respectively. The experimental conditions remained at temperature of 20 ℃, humidity of 70%, and carbon dioxide concentration of 20%. It was found that the addition of PVA fibers could improve the carbonation resistance of concrete considerably. Within the investigated range of fiber length and content, the carbonation resistance of concrete is first strengthened and then weakened with increasing PVA fiber length or content. The quadratic carbonation depth prediction model could characterize the influences of PVA fiber length and content better than the linear model. The carbon dioxide diffusion equation was established by introducing a PVA fiber influence factor and its parameters were determined from the carbonation depth prediction model. The carbonation process of PVA fiber concrete under the accelerated carbonation experiment condition was simulated. Both the model and the numerical method were validated by comparison between the experimental and numerical results. Finally, the influences of added PVA fibers on the carbonation life and durability of reinforced concrete components were further studied numerically. The results showed that compared with the normal concrete component, the durability of PVA fiber concrete components is significantly improved.

Impact of climate change on the carbonation in concrete due to carbon dioxide ingress: Experimental investigation and modelling

Greenhouse gases (GHG), in terms of CO2 emissions, cause a greenhouse impact leading to changes in the planet's temperature and relative humidity (RH). These changes in CO2, temperature and RH (leading to changes in the internal moisture content) have considerable impacts on the depth of carbonation (DoC) in existing concrete structures. However, the models to forecast the DoC as a function of time in the cracked and uncracked concrete members are scarce and have limitations in terms of incorporating key variables relevant to the climate change. This study aims to develop an integrated deterioration model of carbonation in concrete. The combined impacts of variations in the internal factors (such as mechanical properties of concrete, porosity and crack width) and external factors (such as %CO2, %RH and temperature), on chemical reaction rates due to carbonation in concrete were considered. This model is based on simultaneous solutions of the diffusivity and reaction activites of CO2(aq) and Ca(OH)2(aq). The proposed model was validated using the accelerated carbonation experiments involving different properties of concrete and cracks widths. Finally, this model was employed to forecast the DoC in order to identify corrosion state owing to impact of climate change scenarios of the Inter-governmental Panel of Climate Change (IPCC 2014) and the UKCP′09 climate projections.

Sustainable building material from CO2 mineralization slag: Aggregate for concretes and effect of CO2 curing

Carbonation curing for cement-based materials can be applied as a green concrete production technology, which provides a promising route to economic CO2 sequestration. The substitution of cement by industrial solid waste could further reduce the carbon footprint of concrete. This study proposed a sustainable building material of CO2 mineralization slag (CMS) from the wet process. Based on the accelerated carbonation experiment of Portland cementitious materials, the feasibility of blending CMS as enhanced addition is evaluated. This study discussed the effects of CMS aggregate on CO2 sequestration capacity and mechanical property during CO2 curing, and further investigated the related physicochemical mechanisms. It is revealed that the increase of CaCO3/SiO2 ratio would promote the nucleation and crystallization of CaCO3 microcrystal and result in higher CO2 uptake. With optimum CMS addition ratio of 30% and CaCO3/SiO2 ratio of 4:1, the carbonation degree can be increased by 74.2% in comparison with pure cement paste. Unlike natural curing (28d), a significant strength increment of 40% is observed after pure CO2 curing at 40℃ and 1.5 MPa with a 20% addition of CMS. It is considered that the strength gain is primarily contributed by the intensifying of the interfacial transition zone after CO2 curing. The Klinkenberg permeability tests verified the densification effect of carbonation. These findings provide a guidance for the industrial application of CO2 mineralization slag as the sustainable building material.

Carbonation Resistance Performance and Micro-Structure Analysis of Glazed Hollow Bead Insulation Concrete

In this paper, the carbonation depths of glazed hollow bead insulation concrete (GHBC) and normal concrete (NC) at different carbonation ages are tested. The microstructure of GHBC and NC before and after carbonation were observed and compared by mercury intrusion porosimetry (MIP), energy dispersive spectrometer (EDS), and X-ray diffraction (XRD). The results showed that NC had better carbonation resistance than GHBC, and GHBC had a carbonation depth of 1.61 times than that of NC at 28 days accelerated carbonation experiment. The microstructural analysis showed that with the decrease of porosity of the samples, the carbon content and CaCO3 content increased after carbonation. The porosity of NC decreased from 14.36% to 13.53%, the carbon content increased from 4.42% to 5.94%, and the CaCO3 content increased from 18.5% to 56.0%. The porosity of GHBC decreased from 22.94% to 20.71%, the carbon content increased from 4.97% to 5.31%, and the CaCO3 content increased from 70.0% to 82.0%. The above results showed that carbon reacts with hydration products 3CaO·SiO2, 2CaO·SiO2, and Ca(OH)2 to produce a large amount of CaCO3 which causes a large amount of pores to be filled and refined hence the porosity and pore size were reduced leading to increase in the compactness of the material. From the results obtained, the carbonation depth prediction formula of glazed hollow bead insulation concrete was developed, and carbonation life was predicted.

Long-term effectiveness of carbonation resistance of concrete treated with nano-SiO2 modified polymer coatings

For studying the influences of nano-SiO2 on the long-term carbonation resistance of concrete treated with polymer coatings, three types of organic film-forming paints, such as polyurethane, epoxy resin, and chlorinated rubber, were selected, and nano-modified paints with the addition of 0.5, 1.5, and 3.0 wt% of nano-SiO2 particles were prepared and applied on concrete specimens. Ultraviolet radiation, natural exposure, and accelerated carbonation experiments were performed on coated concrete specimens. In addition, scanning electron microscopy and Fourier transform infrared spectroscopy experiments were conducted. The results indicate that the carbonation resistance of concrete with polymer coatings deteriorates gradually with aging. Nano-SiO2 particles can reduce the micro-defects in coating, and enhance the compactness of coating, thus increasing the carbonation resistance of coated concrete. Furthermore, the addition of nano-SiO2 can alleviate the damages of ultraviolet irradiation on polymer molecules, and then enhance the long-term effectiveness of carbonation resistance of coated concrete. However, a suitable nano-SiO2 dosage of about 1.5 wt% exists, and excessive dosage decreases or even loses the useful functions. Based on carbonation resistance of coated concrete and an S-shaped curve, a new service life prediction model for polymer coatings is established. Owing to the contribution of nano-SiO2, the average service life of chlorinated rubber, epoxy resin, and polyurethane coatings can be lengthened by 78%.

Accelerated and natural carbonation of concrete with high volumes of fly ash: chemical, mineralogical and microstructural effects.

Today, a rather poor carbonation resistance is being reported for high-volume fly ash (HVFA) binder systems. This conclusion is usually drawn from accelerated carbonation experiments conducted at CO2 levels that highly exceed the natural atmospheric CO2 concentration of 0.03–0.04%. However, such accelerated test conditions may change the chemistry of the carbonation reaction (and the resulting amount of CH and C–S–H carbonation), the nature of the mineralogical phases formed (stable calcite versus metastable vaterite, aragonite) and the resulting porosity and pore size distribution of the microstructure after carbonation. In this paper, these phenomena were studied on HVFA and fly ash + silica fume (FA + SF) pastes after exposure to 0.03–0.04%, 1% and 10% CO2 using thermogravimetric analysis, quantitative X-ray diffraction and mercury intrusion porosimetry. It was found that none of these techniques unambiguously revealed the reason for significantly underestimating carbonation rates at 1% CO2 from colorimetric carbonation test results obtained after exposure to 10% CO2 that were implemented in a conversion formula that solely accounts for the differences in CO2 concentration. Possibly, excess water production due to carbonation at too high CO2 levels with a pore blocking effect and a diminished solubility for CO2 plays an important role in this.

Carbonation modeling analysis on carbonation behavior of sand autoclaved aerated concrete

Carbonation could not be ignored because of the significant deterioration of mechanical property and durability of sand autoclaved aerated concrete (SAAC). A better understanding and quantification are necessary for maintaining and repairing the existing structures of SAAC. A practicable mathematical model based on the Fick’s first law was established, which was evaluated by the data of accelerated carbonation experiment. Carbonation behavior was studied by means of phenolphthalein test, SEM and XRD. Because the tobermorite generated by hydration gradually transformed into calcite, the compressive strength of samples showed an obvious decline with carbonation depth increased. Furthermore, the experimental date of B05 bulk density class SAAC samples and the calculated values of model have shown good agreements.

Advances in understanding and analyzing the anti-diffusion behavior in complete carbonation zone of MSWI bottom ash-based alkali-activated concrete

This study investigated the carbonation resistance of municipal solid waste incinerator (MSWI) bottom ash alkali-activated concrete (BA) and Portland cement concrete (CC) through the accelerated carbonation test. The results show that BA has lower resistance against carbonation compared to CC under the conditions of the accelerated carbonation experiment. However, BA displayed a unique anti-diffusion behavior under natural conditions which significantly reinforced its carbonation resistance. The anti-diffusion behavior was closely related to the amount of liquid sodium silicate in the activator. Moreover, the pH value in complete carbonation zone of BA was much higher compared to that of CC. The main reason is that liquid sodium silicate turned into magadiite in BA and then the magadiite turned back to liquid sodium silicate to release a large amount of OH− under low concentrations of CO2. This resulted in a rapid increase in pH value of the pore fluid and generation of the anti-diffusion behavior.

Numerical Simulation on Carbonation Depth of Concrete Structures considering Time- and Temperature-Dependent Carbonation Process

In order to further understand the carbonation process of concrete structures, the time- and temperature-dependent diffusion process of CO2 in concrete is simulated based on the law of the CO2 mass conservation, and a two-dimensional mass transfer equation is established for the CO2 diffusion in concrete. The concrete block is discretized into triangular elements, and the CO2 concentrations at different positions are calculated based on finite element method. A computational algorithm is programed through the Matlab platform. The time- and temperature-dependent property and difference of the CO2 concentration at different positions of the structure are considered in the proposed model. Then, an accelerated carbonation experiment is carried out using concrete blocks with different mix proportions to investigate the influence of the water-cement ratio and temperature on the concrete carbonation. The experimental results effectively verify the correctness of the finite element model, and the proposed finite element method reasonably simulates the concrete carbonation through calculating the carbonation in practical engineering compared with other methods in references. An experimental-numerical correlation has been performed. The ratio of carbonation depth at the corner of the concrete members to the other positions is about 1.35. The carbonation depth is increased about 1.9 times when the temperature changes from 20°C to 40°C.

Prediction Model for the Carbonation of Post-Repair Materials in Carbonated RC Structures.

Concrete carbonation damages the passive film that surrounds reinforcement bars, resulting in their exposure to corrosion. Studies on the prediction of concrete carbonation are thus of great significance. The repair of pre-built reinforced concrete (RC) structures by methods such as remodeling was recently introduced. While many studies have been conducted on the progress of carbonation in newly constructed buildings and RC structures fitted with new repair materials, the prediction of post-repair carbonation has not been considered. In the present study, accelerated carbonation was carried out to investigate RC structures following surface layer repair, in order to determine the carbonation depth. To validate the obtained results, a second experiment was performed under the same conditions to determine the carbonation depth by the Finite Difference Method (FDM) and Finite Element Method (FEM). For the accelerated carbonation experiment, FDM and FEM analyses, produced very similar results, thus confirming that the carbonation depth in an RC structure after surface layer repair can be predicted with accuracy. The specimen repaired using inhibiting surface coating (ISC) had the highest carbonation penetration of 19.81, while this value was the lowest for the corrosion inhibiting mortar (IM) with 13.39 mm. In addition, the carbonation depth predicted by using the carbonation prediction formula after repair indicated that that the analytical and experimental values are almost identical if the initial concentration of Ca(OH)2 is assumed to be 52%.

Accelerated mineral carbonation curing of cement paste for CO2 sequestration and enhanced properties of blended calcium silicate

The mineral carbonation (MC) curing of cement-based materials could be employed to obtain both CO2 sequestration and accelerated strength gain. But there is still absence of understanding on the gas-solid reaction kinetics and mechanisms. This study aims to improve the understanding of gas-solid reaction between CO2 and cement-based materials during the MC curing, and evaluate the feasibility of blending calcium silicate (CS) as intensified addition. Based on the accelerated carbonation experiments, the impact of different factors (pressure, temperature and water-to-binder rate) on the reaction rate and the CO2 uptake were systematically investigated. The MC curing showed significant diffusion-controlled kinetics and the reaction gradually turned into the stable product-layer diffusion controlling process. The evolution process of kinetics indicated the increasingly dense structure of cement matrix, as verified by SEM analysis. The blended CS was found to significantly enhance the reaction rate and CO2 uptake capacity, accompanied with the intensified carbonation conversion of cement components. The increased Klinkenberg permeability of blended CS paste after MC curing demonstrated the diluted effect of CS. In terms of the practical feasibility, MC curing also contributed remarkably to the compressive performance improvement of cement paste with blended CS.

Predicting carbonation depth for concrete with organic film coatings combined with ageing effects

A new time-dependent model for concrete carbonation depth prediction combining with the ageing effects of a coating was established to predict the carbonation depth of concrete with organic film coatings under practical service environmental conditions. This model could be solved by introducing a function about the equivalent concrete thickness of a coating with ageing time. Three coated concrete specimens with epoxy, polyurethane, and chlorinated rubber coatings were fabricated and subjected to natural outdoor experiments of exposure degradation for one year. The coated specimens were collected and subjected to accelerated carbonation experiments with different corresponding ageing periods. Based on these experiments, development models of the coatings’ equivalent concrete thickness with exposure time were obtained through regression analysis, and the carbonation depths of concrete with organic film coatings combined with the ageing of the coatings and repainting effects were obtained through numerical calculation. Results indicated that organic film coatings could improve concrete carbonation resistance effectively, and a coating with denser equivalent concrete thickness could better improve the carbonation resistance of concrete compared with a thinner application. However, this protection would gradually decrease and reach zero because of coating degradation. Coatings would not affect the development of concrete carbonation after reaching the desired service life. The carbonation resistance of concrete could be improved again through effective coating repainting, which could prolong its service life against carbonation.