Carbonation Resistance Of Concrete Research Articles

The carbonation resistance of concrete on the basis of blended binders containing milled limestone

The current building industry is essentially changing due to implementation of new approach, which is predominantly motivated by the need to reduce its negative environmental impact. The paper is focused on the experimental evaluation of carbonation resistance in terms of accelerated tests performed on the concrete on the basis of blended binding system incorporating milled limestone. The used commercially produced cement was additionally modified by the limestone powder. The obtained results confirmed, that currently used design standards are convenient to real carbonation process, however further clinker replacement by the milled limestone causes significant loss of the durability of the hardened concrete.

Modeling carbonation depth of recycled aggregate concrete containing chlorinated salts

The durability of recycled concrete is susceptible to degradation from both chloride salt attack and carbonation when exposed in service environment. The depth of carbonation in recycled concrete subject to chloride salt attack was studied here. This investigation assessed the effects of mineral admixture, replacement rate of recycled coarse aggregate, and quality of recycled coarse aggregate on the carbonation depth. The carbonation depth Recycled concrete was studied through accelerated simulated carbonation in the laboratory for 7d, 14d and 28d. Based on the results, the correlation analysis was conducted to investigate the relationship between depth of carbonation, mineral admixture, recycled coarse aggregate replacement rate, recycled coarse aggregate quality and the presence of chloride ions. The objective of this study was mainly to investigate the effect of internal chloride ions on the depth of carbonation of recycled concrete subjected to chloride salt attack. The study referred to previous academic research on carbonation in recycled concrete that had not been affected by chloride salts, and conducted a comparative analysis to identify the carbonation depth pattern in recycled concrete influenced by chloride salt exposure. The study discovered a negative correlation between the replacement rate of mineral admixture and recycled coarse aggregate and the carbonation resistance of concrete. Moreover, enhancing the quality of recycled coarse aggregate positively impacted the carbonation resistance of concrete. An improved model was developed that considers the effect of internal chloride salts on the carbonation depth of recycled concrete, in reference to the empirical prediction model presently available. The model demonstrated a high level of agreement with experimental outcomes, displaying an error range of between −4.32% and 8.27%. It enables an in-depth comprehension of the carbonation behavior of recycled aggregate concrete in an environment exposed to chloride salt attack. Based on the model, the service life of recycled concrete structures can be more accurately forecasted, which can provide a theoretical direction for enhancing the durability of engineering structure.

Carbonation resistance of recycled fine aggregate concrete reinforced by calcium sulfate whiskers

This study investigates mechanical and carbonation resistance tests conducted on Recycled Fine Aggregate Concrete (RFAC) with three different Recycled Fine Aggregate (RFA) replacement rates and four calcium sulfate whisker (CSW) contents, and investigates the effects of RFA replacement rate and CSW content on the mechanical properties of RFAC and the carbonation resistance of RFAC with calcium sulfate whiskers at different ages of carbonation. The microstructure of CSW-modified RFAC was observed and analyzed by SEM electron microscopy to explore further enhancement and toughening mechanisms of CSW in RFAC, as well as the optimal content of CSW in RFAC. In addition, the carbonation depth of recycled concrete with calcium sulfate beard is modeled based on a quadratic fitting formulation. The test results show that the mechanical properties and carbonation resistance of RFAC would decrease with the increase of the RFA replacement rate. Incorporating CSW can effectively improve the mechanical properties and carbonation resistance of RFAC. At 60 % RFA substitution, CSW at 2 % content has the most significant effect on the mechanical properties and carbonation resistance of RFAC. Microscopic analysis shows that CSW has a bridging effect at the micro-cracks in the concrete matrix, preventing the creation and expansion of micro-cracks in the concrete matrix, thus improving RFAC mechanical properties of RFAC and resistance to carbonation. The predicted results of the carbonation depth model which are constructed from the test data coincide with the results. We can use them to predict and analyze the carbonation depth of the carbonation resistance of recycled concrete.

Evaluation of Concrete Carbonation Based on a Fiber Bragg Grating Sensor.

The carbonation of concrete greatly affects its service life. In this paper, fiber Bragg grating (FBG) sensors were used to investigate the relationship between concrete carbonation and its mechanical properties. A T130 High Sensitivity Strain Cable Sensor with a good linearity was used to monitor the internal strain in concrete, to investigate the variation in the elastic modulus of concrete with carbonation time. A mathematical model of elastic modulus and carbonation time of concrete based on FBG was established. At the same time, the authors explored the relationship between the carbonation depth and compressive strength of concrete and the carbonation time using a phenolphthalein solution test and a compressive strength test, respectively. The experimental results indicate that the carbonation depth, compressive strength, and elastic modulus of concrete increase with carbonation time. In the early stage of carbonation, these three parameters increase rapidly, while they grow slowly in the later stage of carbonation. The varying trend of the elastic modulus of concrete is consistent with the compressive strength, which shows a binomial relationship. Therefore, the elastic modulus, measured using FBG sensors, is used as an indicator of the characterization of the carbonation resistance of concrete. This work provides a new approach for concrete carbonation detection and assessment.

Service Life Design of Concrete Structures Made of High-Volume Limestone Powder Concrete—Case of the Carbonation-Induced Corrosion

One of the paths to CO2 emissions reduction in the concrete industry is to use low-clinker cements, providing at the same time the performance of concrete that is adequate for application in concrete structures. This paper explores the impact of the clinker replacement with high amounts of limestone powder (21–70% in the powder phase) on concrete carbonation resistance. To quantify this impact, the empirical relationship between the carbonation resistance and the compressive strength of the high-volume limestone powder concrete (HVLPC) was established. For that purpose, the regression analysis was applied on the experimental results collected from the published research. The service life analysis based on the full probabilistic approach was performed using the fib Model Code 2010 prediction model and proposed empirical relationship. The first-order reliability method (FORM) was applied to solve the limit state function of reinforcement depassivation with a reliability index equal to 1.3. The obtained minimum concrete cover depths were 40–110% higher compared to those prescribed in the current European standard EN 1992-1-1:2004 for indicative strength classes. Based on the full probabilistic analysis, recommended cover depths are given for all carbonation exposure classes, commonly applied concrete strength classes, and service lives of 50 and 100 years.

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.

Enhancing the carbonation and chloride resistance of concrete by nano-modified eco-friendly water-based organic coatings

To effectively improve the carbonation and chloride resistance of concrete with eco-friendly waterborne organic coatings, two nanomaterials (i.e., nano-SiO2 and nano-TiO2) were utilized to modify the water-based polyurethane and epoxy resin paints at dosages of 1 wt%, 2 wt%, and 3 wt%, and the coated concrete specimens were fabricated. Then, accelerated aging experiments of ultraviolet irradiation were conducted on the coated concrete specimens, followed by accelerated carbonation or Coulomb electric flux experiments. In addition, partial coating film samples were analyzed by scanning electron microscopy and Fourier transform infrared spectroscopy. Results show that the application of water-based organic coatings can substantially improve the carbonation and chloride resistance of concrete. The two-component water-based epoxy resin coating exhibited a higher quality than the one-component water-based polyurethane coating, thus resulting in better improvement of the carbonation and chloride resistance of concrete. However, the anti-aging capability of water-based epoxy resin coating is lower than that of water-based polyurethane coating, resulting in the rapid decline of the carbonation or chloride resistance of coated concrete after ultraviolet aging. The addition of nano-SiO2 or nano-TiO2 to the water-based organic coating can reduce the defects in the coating, effectively improve the density of the coating, and further improve the carbonation and chloride resistance of the coated concrete, with average improvement ranges of 51–74 % and 24–27 %, respectively. Meanwhile, the addition of nano-SiO2 or nano-TiO2 can help reduce the damage of water-based coatings caused by ultraviolet irradiation, thereby improving the long-term carbonation and chloride resistance of coated concrete, among which the improvement effect of nano-TiO2 is higher than that of nano-SiO2.

Effects of supplementary cementitious material(SCM) on carbonation resistance of concrete

Using fly ash (FA) and ground granulated blast furnace slag (GGBS) as representatives of supplementary cementitious material (SCM), the effects of the amount and combination of SCM addition on the carbonation resistance of concrete were systematically analyzed through rapid carbonation tests. Combined with XRD chemical analysis and DSC-TG thermogravimetric analysis, the influence mechanism of SCM on the carbonation resistance of concrete was discussed. The research results indicate that the addition of SCM increases the carbonation depth of concrete.
 When the single addition of FA exceeds 40%, the carbonation depth of concrete increases very quickly. Under the premise of the same total addition amount, the carbonation resistance performance of the composite FA and GGBS groups of concrete is better than that of the single FA group. Among the different combinations of FA and GGBS, the concrete with S95 grade GGBS+ grade I FA has the best carbonation resistance. The impact of FA on the carbonation resistance of concrete is manifested as a positive and negative effect.

Study on carbonation of construction joints through field tests on a 30-year-old bridge and accelerated carbonation tests

Construction joints accelerate concrete carbonation, which is one critical durability concern of concrete infrastructures. However, due to the lack of experimental data, it is still challenging to quantify its negative impact on concrete carbonation resistance. In this study, the carbonation depth of construction joints in the girder of Jinshan Bridge, which was exposed to an urban atmospheric condition in a metropolitan city for 30 years, were examined. Moreover, accelerated carbonation tests were conducted to investigate the effect of the water-cement ratio, compressive stress level, carbonation time, and the surface treatment of the joint on the concrete carbonation. The results showed that the joint influence factor, defined as the ratio between the depth at the joint section and the depth of monolithic concrete, ranged from 1.34 to 3.99 with a mean value of 2.26, and a lognormal distribution represented the random nature of the factor well. Furthermore, a function for predicting the distribution curve of carbonation depth for concrete with a construction joint was proposed, and methods to improve the carbonation resistance at the construction joint were suggested: reducing concrete’s water-cement ratio and applying silane impregnation on both sides of the joint within the range of 2.0 times of cover thickness.

Steel slag's effect on concrete mechanical properties and durability

The utilization of steel slag as a mineral additive in concrete has been the subject of research aimed at assessing its impact on concrete's properties such as compressive strength, chloride penetrability, drying shrinkage, and resistance to carbonation. In this study, two different conditions were considered: constant water-binder ratio and compressive strength after 28 days. The results showed that increasing the amount of steel slag in concrete decreased its compressive strength, particularly its early strength, permeability, and ability to resist carbonation under constant W/B conditions. At lower water-binder ratios, however, the adverse effects of steel slag on the strength of compression, penetrability, and the carbonation resistance of concrete were less pronounced. When the W/B was high early in the drying process, steel slag accelerated shrinkage, but it had no impact on the final shrinkage at 90 days. The impact on concrete drying shrinkage was minimal at low W/B. Under continuous 28-day compressive strength conditions, it was also discovered that concrete containing steel slag had a lesser early strength and a higher late performance than pure cement concrete. It was found that the penetrability, carbonation resistance, and drying shrinkage of steel slag concrete were comparable to those of regular and pure cement concrete. This research highlights the potential of steel slag as a mineral additive in concrete production, but its use must be carefully considered to ensure optimal results.

A New Type of Mineral Admixture and Its Impact on the Carbonation Resistance of EPS Concrete

In this study, the effect of microbead dosages (0%, 5%, 10%, 15%, and 20%) on the carbonation resistance of expanded polystyrene (EPS) concrete was investigated. Five groups of EPS concrete specimens were produced and underwent rapid carbonation testing. The carbonation depth and strength after carbonation of the specimens were measured at different carbonation ages (7 days, 14 days, and 28 days) and analyzed to determine the effect of microbead dosages and compressive strength on carbonation resistance. Results indicated that the carbonation depth increased with the progression of carbonation time. The introduction of microbeads was found to significantly improve the carbonation resistance of EPS concrete, leading to a reduction in carbonation depth of over 50% after 28 days and an increase in strength after carbonation by 18–56%. A relative compressive strength model for EPS concrete after carbonation was developed, which could accurately characterize the growth of compressive strength. Based on the analysis of EPS concrete carbonation depth data, a prediction model for the carbonation depth of EPS concrete with microbead dosage was established through fitting, providing improved accuracy in predicting carbonation resistance. The microstructure of EPS concrete was also examined using scanning electron microscopy to uncover the underlying mechanisms of microbead enhancement on carbonation resistance. These findings have potential implications for future research and engineering applications in the carbonation resistance of EPS concrete.

Experimental and Prediction Model Study of Carbonation Resistance of High Performance Concrete under Influence of Curing Conditions

The carbonation resistance of high performance concrete is closely related to the service life of concrete structure, and is an important parameter to reflect its durability. The carbonation resistance of concrete is influenced on selection of curing conditions greatly. In the paper, based on the common curing conditions, the carbonation tests of high performance concrete under different curing methods and curing time were carried out, and the influence mechanism of curing conditions on the carbonation resistance of materials was qualitatively analyzed. The exponential function expression of curing time and carbonation depth was fitted when curing by wet straws. The results show that: in terms of carbonation resistance, the order is wet straw curtain curing, plastic film curing and curing agent curing, and should be cured in advance. The carbonation depth is effectively reduced by short-term curing (3d) and medium-term curing (7d), but the reduction rate slows down, and 7d curing is suggested. The fitted prediction model is calculated the carbonation depth of concrete materials, and the test results are provided data for the selection of curing conditions and time of engineering.

Research on Influences of Ultrasonic Vibration Agitation Stirring on Carbonation Resistance of Cement-Based Materials after Absorption of CO2

To disclose influences of ultrasonic vibration agitation on the carbonation resistance of cement-based materials after absorption of CO2, the variation laws in internal carbonization zone were explored by the testing carbonization depth and carbonization range (pH variation range) of cement mortar after CO2 absorption at different ages. Results demonstrated that when CO2 absorption volumes of the cement mortar before carbonization were 0.44%, 0.88%, 1.32%, 1.76%, and 2.20% (28 d), the carbonization depth under ultrasonic vibration decreased by 5.5%, 12.3%, 21.7%, 20.7%, and 26.7% compared to those under mechanical stirring, respectively. When the ultimate CO2 absorption volume increased to 2.2% of cement mass, the extended degree of cement mortar was 103.23 mm, which decreased by 5.4% compared to that before CO2 absorption. pH variation values of the carbonization range under ultrasonic vibration presented a rising trend with the increase of CO2 absorption volume of cement mortar before carbonation. This indicated that, with the increase of CO2 absorption volume of cement mortar before carbonation increases under ultrasonic vibration, the carbonization process of the hardened body of cement mortar might be decelerated to some extent. Additionally, changes in internal composition and physical images of cement-based materials after absorption of CO2 were analyzed through microtest means like SEM and XRD. A carbonation resistance model was constructed, thus enabling disclosure of the variation mechanism of carbonation resistance of cement-based materials after absorption of CO2 under mechanical stirring and ultrasonic vibration. Results demonstrated that the higher CO2 absorption volume of fresh slurry generated more “nano-level” CaCO3 crystal nucleus. Accordingly, it could improve the porous structure of the cement mortar, decrease the quantity of capillary tubes significantly, improve the compaction degree of cement-based materials effectively, and lower the diffusion rate of CO2 in the cement paste base, thus improving the carbonation resistance. Research conclusions have important significance to decrease CO2 emissions and improve carbonation resistance of concrete.

On the role of compaction in disputes over the quality of the supplied concrete

This paper evaluates the effect of poor compaction on the compressive strength, sorptivity, gas permeability and accelerated carbonation resistance of bulk concrete in structures, framed in the scenario of disputes over the quality of the supplied concrete. For that purpose, several real-scale elements (beams, columns and slabs) were cast with concrete incorporating Portland-limestone cement and fly ash addition, covering a wide range of slumps (35–123 mm) and compressive strengths (25–65 MPa). The elements were then subjected to different compaction conditions (standard vibration and no vibration), and later cored for concrete testing. A new compaction indicator that overcomes the main practical limitations associated with the determination of the excess voidage of concrete is proposed, as well as new factors for correcting the effects of on-site compaction in the estimation of the potential carbonation resistance and the compressive strength of concrete. It is also shown that vibration no longer improves the quality of bulk concrete in structures for slumps above ~ 140 mm, which suggests that, in cases of dispute where greater slumps are specified, no correction for compaction should be needed in the estimation of the potential properties of concrete.

Carbonation Resistance of Marine Concrete Containing Nano-SiO2 under the Action of Bending Load

In order to study the influence of nanomaterials on the carbonation resistance of marine concrete under bending loads, an appropriate amount of nano-SiO2 was added to plain concrete, and a self-developed carbonation box and bending loading device were used to conduct a coupling test. Four different stress levels were set to measure the carbonation depth of nano-concrete at different ages. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to analyze the concrete interfacial transition zone. The carbonation depth was used as the test index to evaluate the durability of nano-SiO2-based concrete under the combined action of bending load and carbonation. The test results showed that the compressive and flexural strengths of concrete remarkably improved when the nano-SiO2 concentration was 2%. Compared with regular concrete, the compressive and flexural strengths of nano-SiO2 based concrete improved by 15.5% and 15.3%, respectively. When the stress level was 0.15 and 0.6, the carbonation depths of NS20 were 20.5 and 18.4% lower than those of PC in the tensile zone and 18.9 and 23.7% lower than those of PC in the compression zone. The carbonation depth of the NS20 tensile zone was lower by 31 and 18.4% at 3 and 28 days than that of PC. Compared with PC, the carbonation depth in the compression zone of NS20 decreased by 50 and 23.7%, and the carbonation depth of nano-concrete was significantly lower than that of conventional concrete under the same stress level and age. When the stress level is constant, the carbonation depth of the tension zone and compression zone increases gradually with the increase in age, and the carbonation depth of the concrete in the first 7 d was 50% that at 28 days. Under the same age, the carbonation depth in the tension zone increased with increasing stress levels, while the carbonation depth in the compression zone decreased with increasing stress levels. When the stress level was 0.3–0.45, the slope of the carbonation depth curve significantly increased. SEM and XRD analysis results revealed that nano-SiO2 significantly improved the internal structure of concrete by reducing the width of the microcracks, the number of pores, and the number of microcracks. The number of C3S/C2S and CaCO3 crystals in nano-SiO2 based concrete was significantly less than that in plain concrete, and the amount of C-S-H gel was more than that in plain concrete. Under bending loads, the nano-SiO2 significantly improved the carbonation resistance of concrete. When the dosage of nano-SiO2 was 2%, its improvement effect was the most significant.

High volume waste foundry sand self-compacting concrete – Transitioning industrial symbiosis

In light of massive industrialization, Industrial symbiosis has gained traction by most countries to employ wastes of one industry as a resource to another. Waste foundry sand obtained from the metal casting industry is a threat to the environment and the health of living beings due to the presence of organic and inorganic materials. However, this waste can be a resource for the construction industry. Despite its broad application scope, usage of waste foundry sand is highly limited due to inadequate understating of its performance in concrete. It becomes essential to understand the behaviour of waste foundry sand in concrete, more importantly, for accessing a strength-efficient and durable structure. The concrete samples were examined by conducting a number of experiments to investigate ties between strength, durability and microscopic insights. It has been observed that the inclusion of waste foundry sand can improve strength properties at later ages. Particular attention is paid to study the detailed microstructural investigations conducted on the concretes having up to 50% waste foundry sand. In addition, a leaching analysis was performed that revealed an increase in heavy metal concentration with the increasing WFS content. However, its concentration was below the acceptable limit, as evidenced by USEPA. Mineral and compositional characteristics monitored the effect of calcium aluminosilicate in the concrete matrix from early to later ages by X-ray diffraction and energy dispersive spectrometer analysis for the solid constituents of the concrete specimen, respectively. In light of the above, this study reveals that the strength properties of blended SCC were improving at later ages; significant improvement was discovered after 91 days of curing time. The compressive strength results were observed to increase by 16.6% for the specimens containing 40% waste foundry sand relative to control concrete at 91 days. The highest strength properties were achieved at 365 days, with 50% replacement ratios of waste foundry sand. The values for UPV were observed to improve as the curing age increases with the reduction in the rate of carbonation resistance of concrete. The carbonation depth was observed to be less than 1 mm for specimens containing 40% and 50% waste foundry sand for accelerated and atmospheric carbonation at 56 days and 2 years, respectively. The originality of the study relies on the possibility of developing waste foundry sand-self compacting concrete with the maximum utilization of waste foundry sand, achieving the targeted results. The study showed a good agreement between the waste foundry sand and concrete that creates a ground for the enhanced application of industrial symbiosis.

Carbonation of limestone powder concrete: State-of-the-art overview

Recently, research into finding long-term solutions to CO2 mitigation in the cement and concrete sectors was initiated since commonly used supplementary cementitious materials are not globally available in sufficient amounts. One of the possible solutions to that problem is to develop concrete with a higher percentage of limestone in the powder phase. This work presents a critical overview of the state-of-the-art in the field of the carbonation resistance of limestone powder concrete. Experimental research performed so far has shown that if the simple replacement of cement with limestone powder is applied in the standard mix design, a maximum of 10-15% of cement (clinker) could be replaced to maintain similar carbonation resistance. It has also been proven that the dilution effect of weakly inert limestone powder can be compensated for with several measures in the mix design of concrete. By adjusting the cement, limestone, and water content and their particle size distribution, it is feasible to design concrete formulations with up to 50% limestone in the powder phase that are comparable to referent Portland cement concrete in terms of rheological and mechanical performance and carbonation resistance. This is an environmentally significant reduction in the clinker content, but it comes at the cost of a larger superplasticizer content. Prediction models for the carbonation resistance of the limestone powder concrete are poorly developed. Amongst them, the fib MC 2010 prediction model is considered relatively simple and robust; however, it has not yet been proven that the model is applicable to concrete with a higher content of limestone powder (>15-20% of the powder phase). The future research should be oriented towards further optimization of the concrete mix design and implementing this model on the middle-and high-content limestone concrete carbonation.

An investigation on optimizing the carbonation resistance of coal bottom ash concrete with its carbon footprints and eco-costs

Energy from coal-fed thermal power plants is provided at the cost of the generation of coal ash. Coal bottom ash (CBA) is an ash generated in coal-fed power plants which is landfilled. Existing literature reports the potential of CBA as a replacement for Portland cement (PC) and natural fine aggregates (NFA) in concrete. Carbonation is an important durability parameter of concrete having fatal consequences at later ages if not estimated and controlled as it leads to corrosion in reinforcement. In the present study, experimental, microstructural, and statistical analysis along with life cycle assessment was performed to investigate the combined effect of two-hour grinded CBA (GCBA) as PC (10-30%) and raw CBA as NFA replacement (0-50%) on compressive strength and carbonation resistance. Accelerated carbonation tests were performed at an exposure of four weeks after 28 and 90 days of curing. Among CBA-based concrete mixes, concrete with 20% GCBA and 25% CBA (G20C25) reported higher compressive strength and carbonation depth owing to pozzolanic reactivity and filler effect of fine CBA particles. However, G20C25 resulted in comparable performance in comparison with the control mix in terms of strength and carbonation resistance. The findings of X-ray diffraction spectroscopy, scanning electron microscopy and Fourier transform infrared spectroscopy also validate the trends. The mathematical models derived for the carbonation resistance and strength were well-fitted. Multi-objective optimization recommended 21.5% GCBA and 29.8% CBA as the optimum amount that resulted in 20.08% and 19.40% lower carbon footprints and eco-costs compared to control mix.

Experimental and Theoretical Study on Carbonization Coefficient Model of NS/SAP Concrete

Carbonization coefficient research has great significance in concrete carbonization evaluation. Nano-silica (NS) can reduce the content of Ca(OH)2, which is generated during the hydration of concrete, resulting in improved carbonization resistance and compressive strength of concrete. This paper investigates the carbonization effects of concrete with internal curing, such as Super Absorbent Polymer (SAP). The research shows that SAP can promote hydration of the internal concrete but form tiny pores after releasing the water completely, which may cause a reduction of carbonation resistance of concrete. The concrete was modified by adding SAP, ranging from 0 to 0.24%, to ascertain the optimal content of SAP. The addition of NS changed the concrete from 0 to 1.5% to confirm the optimal range of NS. To establish a reasonable suitable theoretical model of NS/SAP concrete, the influence factors of the carbonization coefficient of concrete were analyzed first. Later, the accelerated carbonization test was carried out on 100 mm × 100 mm × 100 mm cube specimens with different carbonization time to obtain the compressive strength and carbonization depth to establish the carbonization coefficient model of NS/SAP concrete. Before analyzing experimental data, the specimens were randomly divided into fitting and validation groups. Based on the regression analysis of the fitting group, the carbonization coefficient model was established, which embodied the influence of various parameters on concrete carbonization, including SAP content, NS content, water–cement ratio, CO2 concentration, temperature, relative humidity, and compressive strength. According to the validation analysis of the verification group, the mean relative error of the model is 5.04%, and the residual mean square error is 0.1751. Compared with the literature models, this study’s carbonization model can accurately predict the carbonization depth of NS/SAP concrete.

Carbonation rate of alkali-activated concretes and high-volume SCM concretes: a literature data analysis by RILEM TC 281-CCC

The current understanding of the carbonation and the prediction of the carbonation rate of alkali-activated concretes is complicated inter alia by the wide range of binder chemistries used and testing conditions adopted. To overcome some of the limitations of individual studies and to identify general correlations between mix design parameters and carbonation resistance, the RILEM TC 281-CCC ‘Carbonation of Concrete with Supplementary Cementitious Materials’ Working Group 6 compiled and analysed carbonation data for alkali-activated concretes and mortars from the literature. For comparison purposes, data for blended Portland cement-based concretes with a high percentage of SCMs (≥ 66% of the binder) were also included in the database. The analysis indicates that water/CaO ratio and water/binder ratio exert an influence on the carbonation resistance of alkali-activated concretes; however, these parameters are not good indicators of the carbonation resistance when considered individually. A better indicator of the carbonation resistance of alkali-activated concretes under conditions approximating natural carbonation appears to be their water/(CaO + MgOeq + Na2Oeq + K2Oeq) ratio, where the subscript ‘eq’ indicates an equivalent amount based on molar masses. Nevertheless, this ratio can serve as approximate indicator at best, as other parameters also affect the carbonation resistance of alkali-activated concretes. In addition, the analysis of the database points to peculiarities of accelerated tests using elevated CO2 concentrations for low-Ca alkali-activated concretes, indicating that even at the relatively modest concentration of 1% CO2, accelerated testing may lead to inaccurate predictions of the carbonation resistance under natural exposure conditions.