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Calcium silicate cements react with carbon dioxide (CO2) to form a concrete-like product. While several papers have focused on the properties of the solid material that forms, this study investigates the processing of carbonated calcium silicate systems. Specifically, this paper examines the drying of fresh calcium silicate cement/ water systems and the subsequent carbonation process. A new methodology is presented, based on neutron radiography, to quantify the drying and extent of carbonation that has occurred (degree of carbonation) and the spatial distribution of carbonated products within the sample. Mortar mixtures with high-purity calcium silicate-based cement significantly extend the initial drying period, enabling greater penetration of CO2, allowing it to react with the calcium silicate at greater depths in the sample. While the carbonation reaction is rapid immediately after the CO2 is introduced into the system, the carbonation reaction slows over time. The findings indicate that the degree of saturation and the potential formation of reaction products may limit the penetration of CO2 through the sample depth. [ABSTRACT FROM AUTHOR]

The properties of sodium sulfate-activated ground-granulated blast-furnace slag (shortened as slag) pastes under carbonation attack were analyzed and compared with the uncarbonated specimens in this paper. A slag with two different finenesses--namely, 250 and 500 m2/kg--was activated by sodium sulfate at two concentrations (1 and 3% Na2O equivalent). After the initial 28 days of curing, the hardened pastes were carbonated in 5% CO2 and relative humidity (RH) of 65% at 20 ± 1°C for 2, 4, and 12 weeks. The carbonation depth, compressive strength, and pH value of the carbonated specimens were measured and compared with the uncarbonated counterparts exposed to a natural concentration of CO2. X-ray diffraction, thermogravimetric analysis, and scanning electron microscopy were employed to characterize the reaction products and microstructure of both carbonated and uncarbonated alkali-activated slag (AAS) samples. The results indicated that the specimens prepared with the coarse slag and low Na2SO4 concentration (1% Na2O equivalent) showed the worst carbonation resistance. Increasing slag fineness has a leverage effect on increasing the carbonation resistance. However, increasing Na2SO4 concentration (3% Na2O equivalent) led to more notable carbonation resistance than increasing slag fineness. By combining the fine slag with high Na2SO4 concentration, almost no changes in the pH, carbonation depth, and compressive strength were noticed even after 12 weeks of carbonation, showing a superb resistance to carbonation attack. [ABSTRACT FROM AUTHOR]

This paper studies the effect of carbonation resistance on alkali-activated slag (AAS) concrete. To be specific, this paper investigates the effect of alkali content and type of activators on the carbonation depth of AAS concrete, and analyzes the changes in volume stability, water absorption rate, and carbonation depth of the AAS system after the addition of active MgO. As is shown by the results, while early carbonation is slower, the later carbonation rate of concrete with a high alkali content is higher than that of concrete with a low alkali content. After 28 days and 56 days, the carbonation depth with NaOH as the activator is smaller than that with sodium silicate as the activator; the strength of concrete after carbonation is slightly increased when sodium silicate is used as the activator. The addition of 3% active MgO decreased the water absorption rate of slurry, reduced the shrinkage of concrete, and improved the impermeability of concrete. Moreover, the incorporation of active MgO promoted the formation of hydrotalcite-like compounds and retarded the decomposition of C-S-H gel, thereby reducing the carbonation degree of concrete. [ABSTRACT FROM AUTHOR]

Carbonation curing has demonstrated potential of improving concrete performance while facilitating carbon dioxide utilization. However, reinforcement corrosion behavior in carbonation-cured concrete has not been documented. This paper presents a study on chloride-induced corrosion in reinforced concrete subjected to carbonation curing. A special carbonation curing process was developed for precast non-prestressed applications. Performance of carbonation curing was evaluated by concrete compressive strength, pH value, and carbon dioxide uptake, while corrosion resistance of the carbonation-cured concrete was assessed by reinforcing bar mass loss and concrete chloride content. To understand the mechanism, concrete and cement paste were further characterized using mercury intrusion porosimetry, absorption, and electrical resistivity tests. Micromorphology was assessed by scanning electron microscopy. It was found that apart from rapid early-age strength gain, carbonation curing could significantly reduce chloride permeation in concrete concerning both total and free chloride contents. It was attributed to the reduced pore size and pore volume by calcium carbonate precipitation. With subsequent 28-day hydration, the carbonation-cured concrete displayed a pH over 12.0 at the surface of steel reinforcing bars and a micromorphology similar to the non-carbonated reference. The direct corrosion tests showed that the corrosion-induced mass loss of steel reinforcing bar was lessened by 50% in concrete subjected to carbonation curing. [ABSTRACT FROM AUTHOR]

This paper addresses whether carbonation in existing concrete structures affects the compressive strength estimated using the CAPO-TEST, a post-installed, pullout test conforming to ASTM C900 and EN 12504-3. Fifteen bridges, ranging from 25 to 52 years of age at the time of testing, were investigated. For each bridge, average values of core strengths and CAPO pullout strengths were obtained. Carbonation depth, which varied from 2 to 35 mm (0.08 to 1.4 in.), was measured using chemical staining methods. It was anticipated that, as the depth of carbonation increased, the pullout strength would increase for the same underlying concrete strength. Thus, the in-place compressive strength estimated on the basis of the manufacturer's general correlation would be expected to systematically exceed the strength measured by the cores. It was found that, on average, the compressive strength estimated from the CAPO-TEST and the general correlation was only 2.8% greater than the measured core strength. More importantly, there was no correlation between depth of carbonation and the relative error of the estimated strength based on the CAPO-TEST. [ABSTRACT FROM AUTHOR]

High-volume fly ash (HVFA) concrete, which typically has 50 to 60% fly ash as the total cementitious material content, is widely used to achieve sustainable development in the concrete industry. Strength development and carbonation are critical research topics for using HVFA concrete. This paper presents a numerical procedure to evaluate the strength development and carbonation depth of HVFA concrete. This numerical procedure consists of a hydration model and a carbonation reaction model. The hydration model analyzes the fly ash dilution effect and the pozzolanic reaction. The amount of carbonatable materials, such as calcium hydroxide (CH) and calcium silicate hydrate (CSH), are calculated using reaction degrees of cement and fly ash. The compressive strength development of cement-fly ash blends are evaluated from CSH contents. The calculation results from the hydration model, such as the amount of carbonatable materials and the porosity, are used as input parameters for the carbonation reaction model. By considering the effects of material properties and environmental conditions, the carbonation reaction model analyzes the diffusivity of carbon dioxide and the carbonation depth of HVFA concrete with different curing conditions, different fly ash contents, and different water-binder (w/b) ratios. [ABSTRACT FROM AUTHOR]

Carbonatable calcium silicate cement (CSC) is a promising approach to reducing the carbon footprint associated with concrete production. Carbonatable CSC gains strength by reacting with carbon dioxide (CO2). While the concept of carbonation is well known, more information on the curing process is needed. This study focuses on studying the impact of drying time, carbonation duration, and degree of saturation (DOS) on the carbonation reaction of CSC mortar. Samples were exposed to different drying durations at controlled environmental conditions to reach various DOSs ranging from 100 to 0%. The samples were then exposed to carbonation under the same environmental conditions for different durations. Neutron radiography (NR) was performed on the samples during drying to determine the DOS corresponding to various drying durations. NR was also used during the carbonation period to determine the degree of carbonation (DOC) in real time. The impact of carbonation on the diffusivity of water vapor (Dh) and pore size distribution of CSC-based samples was examined using dynamic vapor sorption (DVS). It was concluded that the carbonation reaction increased as the DOS decreased from 100 to 40%. The carbonation reaction ceased for samples with DOS values less than 6% DOS. It was also concluded that as the DOC increased, the pore structure was refined, which led to a decrease in the Dh of the CSC mortar samples. [ABSTRACT FROM AUTHOR]

In the present work, recycled aggregate from construction and demolition waste including both coarse fractions and fines was used to produce structural concrete. A reduction of concrete strength and elastic modulus, in particular using fines and of the carbonation resistance of concrete under accelerated conditions, was observed with respect to using natural aggregate. The mechanism of water absorption of recycled aggregate in concrete was also studied. For the recycled aggregate, even including fines, similar levels of water absorbed to obtain the same strength as natural aggregate in concrete testing were observed. Lastly, it was shown that the water absorption of the recycled aggregate can be reduced by preliminarily carbonating the recycled aggregate. The Los Angeles coefficient was also increased by carbonation. [ABSTRACT FROM AUTHOR]

Once concrete is set and hardened, it is very hard to evaluate its primitive composition once again. Currently, the conditions required for testing of original cement content in the hardened concrete are severe; the test is complicated, and the test precision is hard to be guaranteed. To explore a convenient and easy method to calculate the cement content in the hardened concrete, in this test, concrete of different mixture proportions was prepared and the carbonation depth of concrete at different curing ages was tested. Based on formulas in references and experimental results, a method to determinate the cement content in hardened concrete via carbonization depth was put forward, in which many influences such as CO2 concentration are taken into consideration. The result shows that the feasibility of this method is good, which can provide a certain reference for practical engineering. [ABSTRACT FROM AUTHOR]

Proponents of water-soluble chloride testing argue that only chlorides in the pore solution contribute to corrosion and that this testing is more representative of free chlorides and therefore should be required. Proponents of acid-soluble chloride testing argue that although water-soluble testing may be more representative of free chlorides in the pore solution at early ages, bound chlorides can become unbound with time, making the water-soluble test unconservative for predicting later-age free chlorides. However, watersoluble testing likely unbinds some admixed chlorides during testing. If the number of chlorides released as part of the watersoluble test exceeds the number of chlorides released at later ages (that is, from carbonation), the water-soluble test should be sufficiently conservative. This research quantifies the release of admixed chlorides as a result of testing and carbonation. Results indicate that water-soluble testing is sufficiently conservative in most cases for assessing admixed chloride contents in various cementitious systems. [ABSTRACT FROM AUTHOR]

Major cities worldwide are densely built with concrete structures. Moreover, urban infrastructures partly cause, and are in return adversely impacted by, the urban heat island (UHI) effect that elevates localized temperature, further to the rise caused by the climate change-induced (CCI) impact of CO2 emissions. While the influence of temperature on carbonation is generally well established based on experimental research, there are hardly any analytical engineering methods specifically for evaluating temperature effect on natural carbonation. In the present study, an equation referred to as the temperature correction factor (TCF) submodel, capable of accounting for temperature effect on natural carbonation, was nested into the natural carbonation prediction (NCP) model, then used to conduct the evaluation. Major cities worldwide are densely built with concrete structures. Moreover, urban infrastructures partly cause, and are in return adversely impacted by, the urban heat island (UHI) effect that elevates localized temperature, further to the rise caused by the climate change-induced (CCI) impact of CO2 emissions. While the influence of temperature on carbonation is generally well established based on experimental research, there are hardly any analytical engineering methods specifically for evaluating temperature effect on natural carbonation. In the present study, an equation referred to as the temperature correction factor (TCF) submodel, capable of accounting for temperature effect on natural carbonation, was nested into the natural carbonation prediction (NCP) model, then used to conduct the evaluation. [ABSTRACT FROM AUTHOR]

Carbonation of concrete systems can result in corrosion of the steel reinforcement. The International Union of Laboratories and Experts in Construction Materials, Systems and Structures (RILEM) Concrete Permanent Committee-18 (CPC-18) test method is commonly used to determine the carbonation depth of ordinary portland cement (OPC) systems. However, the standard does not clearly state whether this test is appropriate for specialty cementitious systems such as calcium aluminate cement (CAC) and calcium sulfoaluminate cement (CSA). Thermogravimetric analysis (TGA), on the other hand, is a proven test method to determine the carbonation front of all cementitious systems. This study assesses the reliability of CPC-18 by comparing CPC-18 test results with TGA results for carbonated OPC, CAC, and CSA systems. Results indicate that CAC and CSA systems carbonate faster than OPC systems, and the CPC-18 method can be used for testing the carbonation depth of CSA systems. However, the CPC-18 test method produces a less distinct color in CAC, which could lead to higher variability in results. [ABSTRACT FROM AUTHOR]

Crushed rock aggregates may lead to unhealthy concentrations of radon gas in concrete buildings. The radon exhalation rate is dependent not only on the concrete mixture but also on its microstructure. Carbonation changes the microstructure, and it also influences the radon exhalation rate. With the guidance of the standard carbonation test, EN 12390- 3:2019, and radon exhalation rate tests as specified in ISO-EN 1165-7, it is found that carbonation has a significant effect on the radon exhalation rate, being reduced by approximately 25% for ordinary portland cement (OPC) after carbonation. In concretes with a substitution of supplementary cementitious materials (SCMs), the radon exhalation rate increased by approximately 30% after carbonation. Conclusively, concretes containing OPC and concretes containing SCMs (fly ash and slag) showed opposite trends as a result of increased carbonation ingress into the concrete mixtures. Diffusion measurements of OPC concrete and concrete containing slag (SCM) support this hypothesis. [ABSTRACT FROM AUTHOR]

The effect of a fixed ratio of different additives on the carbonation behavior of ground-granulated blast-furnace slag (shortened as slag) activated with a fixed concentration of Na2SO4 was investigated. Slag was activated by 1% (Na2O-equivalent) Na2SO4 (M0) and partially replaced with 10%, by weight, of one of the following additives: limestone powder (LS10), fly ash (FA10), portland cement (PC10), silica fume (SF10), metakaolin (MK10), and hydrated lime (HL10). The compressive strength values were measured and compared with those activated with the traditional common activators. After 28 days of curing, the pastes were exposed to 5% concentration of CO2 coupled with 20 ± 1°C and 65% surrounding temperature and relative humidity, respectively, for different durations of 2, 4, and 8 weeks. Compressive strength, pH value, and carbonation depth of carbonated specimens were determined and compared with noncarbonated ones exposed to the same conditions but at a natural CO2 concentration. The results were analyzed with special tools to determine the different phases. The results revealed that it is possible to increase the carbonation resistance of slag activated with Na2SO4 by using some additives. The specimens of LS10 exhibited the highest carbonation depth, while SF10 specimens exhibited the lowest carbonation depth. The remaining additives showed intermediate results between LS10 and SF10. [ABSTRACT FROM AUTHOR]

Mortar cubes containing different addition levels (0, 1.5, 4, 8, 42, and 95%, by water mass replacement concentration) of Opuntia ficus-indica (OFI) mucilage were exposed for a 14-year (5110-day) period in a natural CO2-laden environment. Physical characterization tests were performed on the mortar cubes, such as saturated electrical resistivity (ρS), percent total void content (%TV), water capillary absorption (εEFF), and compressive strength (fc). Changes in pH due to carbonation were also determined, and carbonation rates (KCO2) were recorded. Findings suggest that the addition of OFI mucilage concentrations between 4 and 8% (by water mass replacement) may be suitable for durability-enhancing applications in cement-based mortar exposed to carbonation-induced environments. [ABSTRACT FROM AUTHOR]

Carbonation of concrete can cause the uptake of CO2 and alleviate the CO2 emissions burden of the concrete industry. This study shows a procedure for evaluating the CO2 uptake of fly ash (FA)-blended high-strength concrete, considering both the service and recycling stages. First, a blended cement hydration model is proposed to evaluate the contents of carbonizable substances, porosity, and CO2 diffusivity. In the service stage, a one-dimensional carbonation model is proposed for evaluating carbonation depth. In the recycling stage, an unreacted core model is proposed for evaluating the carbonation process of spherical crushed concrete. Second, CO2 uptake models are proposed for the service and recycling stages, considering concrete materials, structural elements, and environmental exposure. The total CO2 uptake ratio is determined as the sum of CO2 uptake in the service and recycling stages. The calculation results show that, as the FA replacement ratio increases from 0 to 40%, the CO2 uptake ratio in the service stage increases from 2.78 to 4.72%, and the total CO2 uptake ratio increases from 18.9 to 20.8%. As the surface to volume ratio of the structural element increases, or the size of particles of crushed concrete decreases, the rate of CO2 uptake increases but the total CO2 uptake ratio does not change. [ABSTRACT FROM AUTHOR]

Many studies have been conducted on the proportional design of fly-ash-blended low-CO2 concrete, but those studies did not consider the limitation of its carbonation durability. Especially due to climate change, the carbonation reaction is accelerated and the necessity for carbonation durability is enhanced. This study presents a computational program to design fly ash-blended concrete considering CO2 emission, strength, and carbonation under different climate-change scenarios. First, CO2 emission is calculated from concrete mixtures. Compressive strength and carbonation depth are evaluated using an integrated hydration- strength-durability model. Regarding the carbonation durability issue, two exposure conditions combined with three climate-change scenarios are considered. Second, a genetic algorithm (GA) is used to find the optimal concrete mixture. CO2 emission is set as the fitness function, and compressive strength and carbonation depth are set as the constraint conditions of the GA. The optimal mixture has the minimum CO2 emission and can meet various constraints. Based on case studies of concrete mixtures under different exposure conditions and climate change scenarios, the effect of climate change on the proportional design of fly ash-blended concrete is clarified. To meet the challenges of climate change, a richer mixture of fly ash-blended concrete is necessary. As the compressive strength of fly ash-blended concrete increases, the CO2 emissions also increase. [ABSTRACT FROM AUTHOR]

Ettringite-based binders are used in niche applications that require a high compressive strength in a very short period of time to minimize construction times and disruption to the traveling public or user. High early strength is achieved due to the formation of ettringite within the first few hours of hydration, which results in a non-expansive system capable of reaching strengths of approximately 20 MPa (2900 psi) in the first 3 hours of hydration. Ettringitebased binders also carbonate at a faster rate than ordinary portland cement (PC)-based systems as a result of the limited amount or absence of portlandite (CH). Carbonation results in the conversion of ettringite into products that occupy less space, resulting in a loss of strength and increased porosity. The formation of ettringite is achieved through the use of systems composed of calcium aluminate cement (CAC, main phase CA) and calcium sulfate (CS), or calcium sulfoaluminate cement (CSA, main phase C4A3S) interground with calcium sulfate. In many cases, ettringite-based binders are used to accelerate ordinary PC-based systems to achieve a relatively high compressive strength and a suitable working time while still maintaining the hydration characteristics of PC. This study compared the performance of carbonated and noncarbonated concrete in terms of the resistance of the near-surface concrete to deicer-salt scaling and chloride ion penetration. Chloride binding tests were also conducted on the carbonated and noncarbonated cement paste samples and mechanical properties conducted on mortar specimens. The results show that CSA-based binders carbonate at a much faster rate than both CAC and PC based systems as a result of the increased ettringite content within the system. A decrease in the mechanical properties of carbonated ettringite-based binders is also observed as a result of the conversion of ettringite. [ABSTRACT FROM AUTHOR]

The article discusses a study which examined the combined effect of carbonation and chlorides, specifically the influence of carbonation on the presence of water and acid-soluble chlorides. It described the exposure regimes used to test carbonation depths of specimens with the assumption of using concrete cylinders. Also cited are the chemical compounds used to determine the values of carbonation depth including sodium chloride (NaCl) and carbon dioxide.

To study the effects of concrete strength, joint type, and load condition on the carbonation resistance of segmental joints, two series of specimens were prepared and subjected to accelerated carbonation. Series I, comprising 18 monolithic beams made of two strength grades of concrete and loaded at various strain levels, was used to quantify the effect of the load conditions in terms of strain. Series II, comprising 14 specimens, made of two strength grades of concrete with direct wet joints, roughened wet joints, dry joints, and epoxied joints, was used to study the carbonation resistance of segmental joints at zero strain and at a specific level of compressive strain. Based on the carbonation depth determined by phenolphthalein, and the carbonation coefficients fitted according to Fick's first law, a number of conclusions can be drawn. In the case of the specimens made of concrete C60 with cement grade as high as 52.5 MPa (7614 psi), the carbonation resistance was satisfactory, and the carbonation depth of both the monolithic and jointed specimens could be neglected. In the case of the specimens made of concrete C40 with cement grade as low as 32.5 MPa (4713 psi), the monolithic concrete exhibited optimal resistance to carbonation, superior to that of the specimens with direct wet joints, roughened wet joints, and dry joints. However, there was variation in the carbonation coefficients of the specimens with epoxied joints under different strain states. [ABSTRACT FROM AUTHOR]

Early-age carbonation curing of concrete products results in improved strength, increased surface hardness, and reduced surface permeability to water, as well as the reduction of efflores-cence. Carbonation reactions between carbon dioxide and calcium compounds result in permanent fixture of the carbon dioxide in thermodynamic stable calcium carbonate. The moisture content, relative humidity, and temperature profile of the hydrated system have considerable and important influence on the rate and ultimate extent of carbonation. During carbonation, CO2 penetrates the surface of concrete and reacts with cement hydration products- namely, calcium hydroxide and calcium silicate hydrates-to form carbonates. This study quantifies carbon sequestration levels in concrete masonry units using various curing methodologies. The test results of a dynamic pressurized CO2 curing chamber and normal ambient CO2 pressure at various concentrations levels are compared to traditional kiln curing procedures. Early compressive strength profiles for 30% CO2 cured concrete masonry units (CMUs) are equivalent to 100% CO2 cured CMUs and exceed the traditional kiln-cured compressive strengths. Carbon sequestration reduced water requirements by 20% for optimum strength performance and provided water conservation opportunities. [ABSTRACT FROM AUTHOR]

Several methods were compared for measuring depth of carbonation in 50.8 x 101.6 mm (2 x 4 in.) mortar cylinders that had been prepared from ordinary portland cement, with 0 to 40% Type C coal fly ash (FA) substitution, and reacted for up to 28 days in an atmosphere of 50% CO2 at 62% relative humidity (RH). Freshly split sections were analyzed by traditional phenolphthalein staining, digital image analysis (stained and unstained), pH profiling of aqueous digests, and Fourier-transform infrared spectrophotometry. Advancement of the carbonation front was modeled using the diffusion-limited unreacted core model, illustrating that partial substitution of portland cement by FA increased the rate of sample carbonation. The traditional phenolphthalein method significantly underestimated the advancement of the carbonation front's leading edge. The other techniques were all in reasonable agreement with one another. However, of these, digital image analysis was the fastest and least expensive method to carry out. [ABSTRACT FROM AUTHOR]

Previous studies were confined mostly to the deterioration of concrete structures under a single deteriorating factor, such as chloride ingress only or carbonation only, although the real environment may be a combination of such factors. Therefore, the purpose of this study is to explore the influence of carbonation on chloride penetration in concrete structures. Several series of concrete specimens were tested. The test results indicate that the chloride penetration is more pronounced when the carbonation process is combined with the chloride ingress. It is also shown that the ratio of water-soluble chloride to the acid-soluble chloride content is higher for the case of the carbonated test series than the case of the normal noncarbonated test series. This may cause a more vulnerable situation to the corrosion of steel bars when chloride ingress is combined with carbonation. This study allows for a more realistic assessment of durability for concrete structures that are subjected to a combined environment of chlorides and carbonation. [ABSTRACT FROM AUTHOR]

The effect of initial curing on carbonation curing of lightweight concrete masonry units (CMUs) was examined. Initial curing was performed from 4 to 18 hours at a relative humidity (RH) of 50% and temperature of 25°C (77°F). Based on cement content, 4-hour carbonation curing allowed concretes to uptake 22 to 24% CO2 with initial curing and 8.5% without initial curing, while prolonged 4-day carbonation recorded an uptake of 35%. Carbonated CMUs exhibited higher early strength but lower 28-day strength due to water loss by initial curing in comparison to hydrated and steam-cured references. A water-spray mechanism was thus devised to compensate the water loss and, ultimately, made the late strength of carbonated CMUs comparable to references. Carbonation curing can replace steam in CMU production to accelerate hydration, improve durability, and recycle cement kiln CO2 in a beneficial manner. [ABSTRACT FROM AUTHOR]

A theoretical model for predicting the carbonation depth of concrete is presented. The proposed, experimentally determined model describes the carbonation depth as a function of the time of exposure, the water-binder ratio, and early-age curing conditions. The result is a new mathematical formulation of carbonation depth as a hyperbolic function of time, which assumes that the depth of the carbonation is ultimately limited as the concrete pores fill with carbonation products. Tests were carried out for 6 years in a natural atmospheric concentration of CO2; in an urban-industrial environment. The results showing the carbonation depth for concrete using different types of cement are used to derive equations to predict carbonation depth. It is expected that the resulting equations can be applied to the prediction of the concrete cover thickness for reinforcement in concrete structures. [ABSTRACT FROM AUTHOR]