Accelerated Carbonation Conditions Research Articles

Report of RILEM TC 281-CCC: A critical review of the standardised testing methods to determine carbonation resistance of concrete

The chemical reaction between CO2 and a blended Portland cement concrete, referred to as carbonation, can lead to reduced performance, particularly when concrete is exposed to elevated levels of CO2 (i.e., accelerated carbonation conditions). When slight changes in concrete mix designs or testing conditions are adopted, conflicting carbonation results are often reported. The RILEM TC 281-CCC ‘Carbonation of Concrete with Supplementary Cementitious Materials’ has conducted a critical analysis of the standardised testing methodologies that are currently applied to determine carbonation resistance of concrete in different regions. There are at least 17 different standards or recommendations being actively used for this purpose, with significant differences in sample curing, pre-conditioning, carbonation exposure conditions, and methods used for determination of carbonation depth after exposure. These differences strongly influence the carbonation depths recorded and the carbonation coefficient values calculated. Considering the importance of accurately determining carbonation potential of concrete, not just for predicting their durability performance, but also for determining the amount of CO2 that concrete can re-absorb during or after its service life, it is imperative to recognise the applicability and limitations of the results obtained from different tests. This will enable researchers and practitioners to adopt the most appropriate testing methodologies to evaluate carbonation resistance, depending on the purpose of the conclusions derived from such testing (e. g. materials selection, service life prediction, CO2 capture potential).

Microstructural evolution and its impact on the mechanical strength of typical alkali-activated slag subjected to accelerated carbonation

This study aims to comprehensively investigate the evolution of microstructure, mechanical strength, and their correlation in alkali-activated slag (AAS) mortars, designed for application in the immobilization of liquid radioactive waste, under accelerated carbonation conditions (1% CO2, 20 °C and 60% RH). To gain insights into the underlying microstructural changes, CO2 uptake and decalcification of C-A-S-H were analyzed using TGA/DSC and EDS. The pore structure of AASs was systematically assessed across nano- to macro-scales, employing N2-adsorption, MIP, and SEM segmentation. Generally, carbonation led to a decrease in total porosity, primarily attributed to the reduction in meso-macropore volume. However, the pore size distribution of AAS exhibited a complex alteration over varying carbonation durations. Carbonation significantly reduced flexural strength, whereas its effect on compressive strength was comparatively milder. Notably, an evident linear correlation emerged between porosity and compressive strength in both reference and carbonated AASs.

Effect of hydration degrees on the accelerated carbonization (3 % CO2) behavior of natural hydraulic lime

Natural Hydraulic Lime (NHL), as a low-carbon binder, possesses a significantly higher carbon sequestration potential compared to cement-based materials. This study aims to elucidate the accelerated carbonation mechanism of NHL5 by investigating the evolution of phase composition and microstructure during the accelerated carbonation process, as well as the preferential reactivity of different carbonatable components. Different hydration levels of NHL5 paste were achieved by controlling various steam curing durations (with a water-to-binder ratio of 0.5); accelerated carbonation conditions included 3±1 % CO2, 25±3°C, and 70±5 % relative humidity. The results indicate that higher hydration levels positively impact the mechanical properties of samples after carbonation. The compressive strength of the sample with carbonation curing 28 after 7 days of steam curing (S7C28) reached 45.6 MPa, and the carbon sequestration per unit mass increased. Specifically, S7C28 achieved an increase in carbon sequestration from 71.8 g/kg to 364.3 g/kg after 28 days of incomplete carbonation, while the high degree of hydration also limited the diffusion of CO2. Qualitative and quantitative analyses confirm that the carbonation reactivity in NHL5 paste follows the order: Ca(OH)2>C-S-H gel>C2S. The continuous generation of carbonation by-products, such as calcite and silica gel, fills the pores of the samples, enhancing their strength. These results suggest that higher hydration levels in NHL5 significantly promote the effectiveness of carbonation curing at a 3 % CO2 concentration, leading to improvements in mechanical performance, carbon sequestration capacity, and microstructural characteristics.

Evaluation of the technical properties of reactive-MgO cements produced by solar calcination of magnesite in a fluidized bed reactor

Magnesium oxide (MgO)-based cements display very interesting technical properties and environmentally-friendly features. The novel idea investigated in this study is to synthesize MgO cements, using as raw material natural magnesite calcined in fluidized bed heated by concentrated solar energy. Calcination was performed in a lab-scale system equipped with a concentrated solar simulator, operated under different process conditions. The most reactive MgO was mixed with 3% by weight of MgCO3 (nucleation agent) and four different solutions containing magnesium acetate or chloride. The binders were hydrated in air or 20% CO2 atmosphere (accelerated carbonation conditions) until 28 days. X-ray diffraction, differential-thermal and mercury intrusion porosimetry analyses, and compressive mechanical strength tests, were performed on the hydrated systems. Solar calcination produced a highly reactive MgO. The performance of the cement pastes improved at higher curing times, and when using magnesium acetate as hydration agent, as also witnessed by the application of a kinetic model. Accelerated carbonation conditions further enhanced the mechanical properties of the cements thanks to the formation of nesquehonite, allowing to reach a mechanical strength comparable to that of ordinary Portland cements class 32.5. The achieved outcomes encourage the production of low-CO2 magnesite cements from solar calcined magnesite, boosting the green aspect of the entire process.

Resistance of ordinary and low-carbon cements to carbonation: Microstructural and mineralogical alteration

The rising demand for green construction materials has led to tremendous research efforts towards the development of low-carbon cements, although their carbonation resistance is expected to be lower than that of ordinary Portland cement (OPC). This study aims to assess the carbonation resistance of various cements by thoroughly investigating the changes in the microstructure and mineralogy under accelerated carbonation conditions, which have not been comprehensively described in the literature, especially for blended cements. Two types of materials were investigated: cement pastes made from type CEM I and III cements and mortars with the same water/cement ratio of 0.55. Various analytical techniques have been used to examine microstructural and mineralogical alterations as a function of carbonation duration. It was demonstrated that CEM III cement pastes exhibited the fastest carbonation rate, whereas CEM I cement pastes had the largest CO2 uptake owing to carbonation. The mass change of carbonated materials and the carbonation depth were found to be well correlated. Overall, carbonation refines the microstructure, resulting in a significant decrease in water sorptivity, although an increase in the specific surface area and micropore volume was observed, indicating the opening of gel pores due to carbonation, which is more pronounced for CEM III materials. Additionally, this study allows a qualitative assessment of the extent in alteration of C-(A)-S-H gel structure and bound water of CEM I and CEM III materials.

Methodology for Evaluating the CO2 Sequestration Capacity of Waste Ashes.

The concentration of CO2 in the atmosphere is constantly increasing, leading to an increase in the average global temperature and, thus, affecting climate change. Hence, various initiatives have been proposed to mitigate this process, among which CO2 sequestration is a technically simple and efficient approach. The spontaneous carbonation of ashes with atmospheric CO2 is very slow, and this is why accelerated carbonation is encouraged. However, not all ashes are equally suitable for this process, so a methodology to evaluate their potential should be developed. Such a methodology involves a combination of techniques, from theoretical calculations to XRF, XRD, DTA-TG, and the calcimetric determination of the CaCO3 content. The present study followed the approach of exposing ashes to accelerated carbonation conditions (4% v/v CO2, 50-55% and 80-85% RH, 20 °C) in a closed carbonation chamber for different periods of time until the maximum CO2 uptake is reached. The amount of sequestered CO2 was quantified by thermogravimetry. The results show that the highest CO2 sequestration capacity (33.8%) and carbonation efficiency (67.9%) were obtained for wood biomass bottom ash. This method was applied to eight combustion ashes and could serve to evaluate other ashes or comparable carbon storage materials.

Hydration, carbonation and strength development of reactive MgO cement blended with lime (CaO) under different curing conditions

This paper investigated the effect of lime added as substitution (by up to 30 wt%) of magnesia on the performance, phase composition and CO2 sequestration capacity of blended reactive MgO cement (RMC) under two different curing conditions. A relationship between the strength development and formation of hydration and carbonation phases was also established. The results showed that the incorporation of lime promote the setting and initial heat generation of RMC. More lime replacement led to higher CO2 absorption capability under both ambient and accelerated carbonation conditions, mainly due to the improved calcite. The presence of uncarbonated portlandite was found to have negative impact on strength development of blended RMC before and after water exposure. Carbonation curing enhanced the conversion of portlandite and brucite into calcite and nesquehonite, respectively, which effectively reduced the porosity and improved the denseness of blended RMC. The improvements in carbonates content, morphologies and microstructure in samples containing 20% lime resulted in ∼30% higher 28-day compressive strength than the plain samples.

Carbon capture and storage potential of biochar-enriched cementitious systems

A promising solution to nullify the net embodied greenhouse gas emissions of civil infrastructure is the use of carbon-negative materials for concrete manufacturing. Carbon-neutral coal ash and agriculture/forestry by-products, such as biochar, exhibit a high carbon dioxide (CO2) uptake potential. The aim of the study is to explore the viability of using biochar as a carbon sink and to develop carbon-neutral concrete with improved performance. Experimental findings suggest that the optimal amount of biochar (1%) slightly improves hydration and mechanical properties, but the combination with mineral additives significantly enhances the performance. Thermogravimetric analysis (TGA) revealed that compared to OPC, the addition of 1% biochar contributes to a 42% increase in CO2 uptake, while the combination of biochar with 10% class C fly ash further increases CO2 capture capacity of the mix by 92%. Under accelerated carbonation conditions, the biochar-enriched mortars exhibit a 20% higher modulus of elasticity indicating an effectively increased stiffness over the reference carbonated OPC. The carbonated biochar mortars also exhibit up to 64% increased toughness indices indicating the material's great resistance to crack coalescence and propagation at the strain softening stage. Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy (SEM-EDS) results validated our prediction that the porous morphology of biochar promoted enhanced CO2 absorption and in-situ mineralization of calcium carbonate, resulting in a denser and stronger cement matrix.

Performance of bacterial mediated mineralization in concrete under carbonation and chloride induced corrosion

Prevention of concrete structures from early deterioration under aggressive environments is a major challenge. The present study investigated the performance of bacterial-treated concrete structures under harsh conditions such as carbonation and chloride exposure. Improvement in the mechanical strength and reduction in penetration properties was recorded in bacterial-treated specimens. The maximum carbonation depth of 18.27 mm and reduction of pH (8.6) were recorded in control specimens during carbonation exposure (5% CO2) for 120 days, whereas significant resistance against CO2 diffusion and much-reduced carbonation depth was observed in BAC specimen (9.28 mm) and BSC (11.13 mm) specimens. Due to accelerated carbonation conditions, further increment in mechanical strength and reduced penetration was observed in control and bacterial treated specimens due to microstructure densification caused by carbonation reaction products. During chloride exposure (5% NaCl) for 120 days, the control specimen showed the initiation of rebar corrosion with Ecorr and Icorr values of −522.2 mV/SCE and 0.34 μA/cm2, respectively. However, bacterial-treated reinforced specimens (BARC and BSRC) significantly resisted the penetration of chloride ions. At the end of chloride exposure, BARC and BSRC specimens showed lower Ecorr values of −203.4 mV/SCE and −301.4 mV/SCE and Icorr values of 0.03 μA/cm2 and 0.02 μA/cm2, respectively. These results suggested that the biogenic treated concrete specimens resisted from the deterioration effects of chloride and carbonation exposure and improved their durability properties.

Experimental Evaluation Method of Carbonation in Concrete Structure Using Nonlinear Resonant Ultrasonic Spectroscopy

Carbonation is a typical type of concrete deterioration that can cause various physicochemical changes and adversely affects the performance of reinforced concrete structures, particularly in offshore structures. Nonlinear resonant ultrasonic spectroscopy (NRUS), which is a non-destructive test technique, is applied in this study to experimentally evaluate the degree of concrete carbonation without damaging the structure. Nonlinearity parameters of test specimens exposed to accelerated carbonation conditions are measured via NRUS. The measured nonlinearity parameters indicate lower values for longer carbonation durations. This is because internal microcracks are reduced by CaCO<sub>3</sub>, which is a carbonation product. The consistent trend of the experimental results shows that NRUS can be effectively applied to evaluate concrete carbonation.

Nutrient-doped synthetic silicates for enhanced weathering, remineralization and fertilization on agricultural lands of global cold regions- A perspective on the research ahead.

There is now a dire demand for negative emissions technologies (which sequester CO2 from the atmosphere) that can be rapidly deployed, are scalable, and are demonstrably safe and effective. Enhanced weathering of silicate minerals has demonstrated a significant potential for CO2 capture and sequestration by the formation of pedogenic carbonates in soils, subsoils, and sediments. This technique has also been shown to deliver fruitful results in terms of improving soil health, and in turn plant health, through remineralization. The silicate minerals that possess the highest weathering rates (e.g., wollastonite), are relatively rare in nature, whereas the abundant ones (e.g., anorthite and forsterite) have a slower pace of weathering, especially in colder and drier climates such as found in the extensive agricultural lands of Western Canada and the Western United States. Herein, we offer a perspective on the opportunities for computational studies targeting atomic-scale interaction of CO2 with silicates and synthesis of fast-weathering silicates (such as larnite and bredigite), whose composition can be tuned to also support soil fertilization and remineralization, and whose production must be integrated with green and carbon-neutral technologies to ensure net-negative life cycle emissions.

New frontiers in sustainable cements: Improving the performance of carbonated reactive MgO concrete via microbial carbonation process

The low conversion of reactive MgO cement (RMC) into hydrated magnesium hydroxy carbonates (HMHCs) due to limited CO2 diffusion towards the sample core leads to inefficient RMC use and low strength development in RMC concrete. This study proposed a novel technique to enhance hydration and carbonation at both the exterior and core sections of RMC concrete via a synergistic combination of microbial carbonation process (MCP) with nucleation seeding (S). The production of CO32− ions via the introduction of urease-producing bacteria to catalyze the hydrolysis of urea enabled self-carbonation of RMC, leading to the formation of HMHCs under ambient conditions. Improvements in CO2 dissolution and formation of brucite with a low crystallinity further stimulated HMHC formation under accelerated carbonation conditions. The simultaneous use of MCP and S resulted in dense microstructures composed of HMHCs with improved morphologies, translating into strengths that were >3 times of the control (62 vs 20 MPa).

Determination of carbonation resistance of concrete through a combination of cement content and tortuosity

The underlying motivation of the study is to understand the effect of CO2 buffering capacity and pore structure characteristics on the rate of carbonation of concrete. An investigation on the influence of different parameters of concrete on the rate of carbonation of 11 concrete mixes made using PC (Portland cement) and six different SCMs (Supplementary cementitious materials) is presented in this work. Compressive strength and various transport properties (oxygen permeability, porosity, resistivity and sorptivity) of concrete are measured after 28 days of curing. Concrete samples are subjected to accelerated carbonation conditions (2.5% CO2, 60% R.H. and 20 °C temperature) for 60 days after the end of the curing and preconditioning period. No notable correlation is observed between the rate of carbonation and measured parameters of concrete. A pragmatic approach to measure the carbonation resistance of concrete is proposed by means of a carbonation resistance factor derived using the tortuosity and cement content of concrete. The carbonation resistance factor could be effectively used to compare concrete mixes containing different SCMs for their carbonation resistance providing an alternative to tedious and time-consuming carbonation tests.

Evaluations of frost and scaling resistance of fly ash concrete in terms of changes in water absorption and pore structure under the accelerated carbonation conditions

In this study, the frost and scaling resistance of concrete with various fly ash (FA) replacement ratios exposure to air and accelerated carbonation conditions were investigated. The changes in water absorption were measured by RILEM / CIF test and in pore structure were measured by the Archimedes method and the mercury intrusion porosimetry method due to carbonation. Results show that the influence of FA and carbonation on the frost and scaling resistance of concrete can be ignored due to air bubbles by air entraining (AE) admixture in case of AE FA concrete. For Non-AE FA concrete, carbonation of FA concrete does not change the gel porosity/capillary porosity ratio, which is the reason for the frost resistance is not influenced by carbonation of FA concrete. Additionally, it is noteworthy to note that the scaling resistance is strongly connected to the pore volume above 75 nm, and that when exposed to carbonation, the scaling resistance tends to rise as a result of the increase in pore volume above 75 nm that occurs as a result of carbonation. Because of the addition of FA and carbonation, the frost and scaling resistance are far more reliant on the changes in pore structure than they are on the water absorption.

Early-Age Properties of Mortars Exposed to Varied Accelerated Carbonation Conditions

Early-Age Properties of Mortars Exposed to Varied Accelerated Carbonation Conditions

Effect of the molar ratio of calcium sulfate over ye'elimite on the reaction of CSA cement/slag blends under an accelerated carbonation condition

The present study endeavored to elucidate the effect of molar ratio of calcium sulfate over ye'elimite (m-value) on the reaction of calcium sulfoaluminate (CSA) cement/blast furnace slag blends under an accelerated carbonation condition. CSA cement was replaced with slag at a level of 50%. Paste samples were exposed to the accelerated carbonation condition for 28 days after a sufficient precuring period. The phase evolution, local Al and Si frameworks, physical properties, and CO2 uptake aspects of the samples were explored. The test results indicated that the phase composition of the samples was not altered regardless of the m-value. Meanwhile, an increase in the m-value was revealed to enhance the reaction degree of Si in the slag to form C-(A)-S-H after carbonation. The CO2 uptake level of the sample with the highest m-value (2.0) was calculated to be the lowest, signifying that the pore structures and strength development upon carbonation were influenced more by the occurrence of C-(A)-S-H formation rather than by the packing effect of calcium carbonates.

Assessment of the properties and environmental impact of carbonated reactive magnesia containing industrial waste

To achieve the goal of carbon neutrality, carbon capture and storage (CCS) is considered to be an effective approach. This study investigated the microstructural development of reactive magnesia cement-industrial waste (i.e., pulverized fly ash and ground granulated blast-furnace slag) formulations under accelerated carbonation conditions. The density and isothermal calorimetry analyses were supported with microstructural analysis performed. Results showed that pulverized fly ash and ground granulated blast-furnace slag could be activated by reactive magnesia cement, resulting in the formation of phases such as magnesia silicate hydrate, hydrotalcite and hydromagnesite, whose formation was enhanced in the presence of accelerated carbonation. Associated with a low initial pH, samples with pulverized fly ash outperformed samples with ground granulated blast-furnace slag counterparts in terms of their strength development. The study led to the determination of a formulation containing the reactive magnesia cement and pulverized fly ash with a higher mechanical performance than the control group, also highlighting the need for the revision of the adopted carbon footprint calculation to incorporate several critical factors.

Influence of carbonation on chloride binding of mortars made with simulated marine sand

This study investigated the effects of carbonation on the chloride binding of mortars containing simulated marine sand. Various admixtures were added, and metakaolin (MK), fly ash (FA), nano-Al2O3 (NA), and aluminum (A) were considered as supplementary cementitious materials (SCMs) for use in mortars. Changes in free chloride content, total chloride content, bound chloride content, chloride binding rates, and pH values as a function of mortar depth were analyzed, and the effects of carbonation on the hydration products were also assessed. The results indicated that in the place of cement, SCMs could significantly increase the carbonation zone depth in mortar. The chloride ions migrated outward and inward under accelerated carbonation conditions in the mortar, generating two peak values for the chloride content in the mortars. Consequently, the incorporation of SCMs was beneficial for chloride ion migration in mortars under carbonation. Additionally, the bound chloride content and chloride binding rates were initially extremely low, then gradually increased, and finally stabilized with increased depth in the mortars under carbonation. Furthermore, the incorporation of SCMs resulted in the decreased chloride binding capacity of the mortars under accelerated carbonation, and Friedel’s salt gradually decomposed with carbonation.

Description of the concrete carbonation process with adjusted depth-resolved thermogravimetric analysis

AbstractThe thermal gravimetric analysis (TG) is a common method for the examination of the carbonation progress of cement-based materials. Unfortunately, the thermal properties of some components complicate the evaluation of TG results. Various hydrate phases, such as ettringite (AFt), C–S–H and AFm, decompose almost simultaneously in the temperature range up to 200 °C. Additionally, physically bound water is released in the same temperature range. In the temperature range between 450 °C and 600 °C, the decomposition of calcium hydroxide and amorphous or weakly bound carbonates takes place simultaneously. Carbonates, like calcite, from limestone powder or other additives may be already contained in the noncarbonated sample material. For this research, an attempt was made to minimise the influence of these effects. Therefore, differential curves from DTG results of noncarbonated areas and areas with various states of carbonation of the same sample material were calculated and evaluated. Concretes based on three different types of cement were produced and stored under accelerated carbonation conditions (1% CO2 in air). The required sample material was obtained by cutting slices from various depths of previously CO2-treated specimen and subsequent grinding. During the sample preparation, a special attention was paid that no additional carbonation processes took place. As reference method for the determination of the carbonation depth, the sprayed application of phenolphthalein solution was carried out. Microscopic analysis was examined to confirm the assumptions made previously. Furthermore, the observed effect of encapsulation of calcium hydroxide by carbonates caused by the accelerated carbonation conditions was examined more closely.

Physical and Chemical Relationships in Accelerated Carbonation Conditions of Alkali-Activated Cement Based on Type of Binder and Alkali Activator.

Alkali-activated cements prepared from aluminosilicate powders, such as blast furnace slag and fly ash, are rapidly attracting attention as alternatives to cement because they can significantly reduce CO2 emissions compared to conventional cement concrete. In this study, we investigated the relationship between the physical and chemical changes by accelerated carbonation conditions of alkali-activated cements. Alkali-activated cements were prepared from binders composed of blast furnace slag and fly ash as well as alkali activators sodium silicate and sodium hydroxide. Physical changes were analyzed from compressive strength, pH, and neutralization depth, and chemical changes were analyzed from XRD, TG-DTG, and 29Si MAS NMR. The C–(N)–A–S–H structure is noted to change via carbonation, and the compressive strength is observed to decrease. However, in the case of Na-rich specimens, the compressive strength does not decrease by accelerated carbonation. This work is expected to contribute to the field of alkali-activated cements in the future.