Effect of composition,environmental factors and cement-lime mortar coating on concrete carbonation

The paper describes the physicochemical processes of concrete carbonation and presents a simple mathematical model for the evolution of carbonation in time, applicable under constant relative humidity higher than 50%. The model is based on fundamental principles of chemical reaction engineering, and uses as parameters the ambient concentration of CO₂, the molar concentratrations of the carbonatable constituents, Ca(OH)₂ and CSH, in the concrete volume, and the effective diffusivity of CO₂ in carbonated concrete. The latter is given by an empirical function of the porosity of hardened cement paste and of relative humidity, derived from laboratory diffusion tests. The validity of the model for OPC or pozzolanic cement concretes and mortars is demonstrated by comparison of its predictions with accelerated carbonation test results obtained in an environment of controlled CO₂ concentration, humidity and temperature. The mathematical model is extended to cover the case of carbonation of the coating-concrete system, for concrete coated with a cement-lime mortar finish, applied either almost immediately after the end of concrete curing or with a delay of a certain time. Parametric studies are performed to show how the evolution of carbonation depth with time is affected by cement and concrete composition (water/cement or aggregate/cement ratio, percentage OPC or aggregate replacement by a pozzolan), environmental factors (relative humidity, ambient concentration of CO₂), the presence and the time of application of a lime-cement mortar coating and its composition (water/cement, aggregate/cement and lime/cement ratios of the mortar, percentage OPC or aggregate replacement by a pozzolan).     

Accelerated carbonation and performance of concrete made with steel slag as binding materials and aggregates

Steel slag has been used as supplementary cementitious materials or aggregates in concrete. However, the substitution levels of steel slag for Portland cement or natural aggregates were limited due to its low hydraulic property or latent volume instability. In this study, 60% of steel slag powders containing high free-CaO content, 20% of Portland cement and up to 20% of reactive magnesia and lime were mixed to prepare the binding blends. The binding blends were then used to cast concrete, in which up to 100% of natural aggregates (limestone and river sands) were replaced with steel slag aggregates. The concrete was exposed to carbonation curing with a concentration of 99.9% CO₂ and a pressure of 0.10 MPa for different durations (1d, 3d, and 14d). The carbonation front, carbonate products, compressive strength, microstructure, and volume stability of the concrete were investigated. Results show that the compressive strength of the steel slag concrete after CO₂ curing was significantly increased. The compressive strengths of concrete subjected to CO₂ curing for 14d were up to five-fold greater than that of the corresponding concrete under conventional moist curing for 28d. This is attributed to the formation of calcium carbonates, leading to a microstructure densification of the concrete. Replacement of limestone and sand aggregates with steel slag aggregates also increased the compressive strengths of the concrete subjected to CO₂ curing. In addition, the concrete pre-exposed to CO₂ curing produced less expansion than the concrete pre-exposed to moist curing during the subsequent accelerated curing in 60 °C water. This study provides a potential approach to prepare concrete with low-carbon emissions via the accelerated carbonation of steel slag.     

Sulfate resistance and carbonation of plain and blended cements

Results of an experimental investigation on the sulfate resistance and carbonation of plain and blended cement mortars are reported in this paper. In the sulfate resistance test all the specimens were immersed in a 5% Na₂SO₄ solution for 24 months. Two different types of lignite fly ashes and two natural pozzolans were used for the production of 13 blended cements. An ordinary portland cement and a commercially available blended cement were also used for reference. The effect of mineral admixtures on the carbonation depth of mortars was also investigated. Results show that the addition of pozzolanic admixtures in most cases had a positive effect on the sulfate resistance. The carbonation depth in all blended mortars was greater than that in portland cement mortar. However the rate of carbonation of blended mortars was reduced as hydration progressed.     

Stability of the hydrate phase assemblage in Portland composite cements containing dolomite and metakaolin after leaching,carbonation, and chloride exposure

To reduce CO₂ emissions during the production of cement and to cope with increasing demands for concrete, and thereby cement, the cement industry needs to identify new supplementary cementitious materials. These new composite cements should provide, among others, a similar or improved durability of the concrete structures. This study investigated the hydrate phase assemblage in Portland cement pastes containing dolomite or a combination of dolomite and metakaolin after leaching, carbonation, and chloride exposure. The phase assemblage and phase compositions of the exposed samples and the unexposed reference samples were investigated using TGA, XRD, and SEM-EDS. The reaction of dolomite in the cement paste resulted in the formation of hydrotalcite. It was found that, unlike most other hydration phases, hydrotalcite can withstand high degrees of leaching and carbonation. When the samples were exposed to a chloride solution, the formation of a chloride-containing hydrotalcite was observed.     

Sulphate resistance of type V cements with limestone filler and natural pozzolana

Sulphate performance of concrete depends primarily on permeability. Under severe conditions of sulphate exposure, low-permeability concrete is prescribed and it must also be made with high sulphate resisting cement. For portland cement, the sulphate resistance depends on the C₃A content and the amount of CH produced at early stages of hydration. Some parameters that modify the quantity of early CH in the hardened cement paste are investigated in this paper. Two type V cements with quite different C₃S content and blended cements containing natural pozzolana or limestone filler were used. Expansion, flexural and compressive strength of mortar, immersed until 1 yr in sodium sulphate solution, with pH-controlled are presented. Results show that the sulphate performance of portland cement with high C₃S content is very poor compared with low C₃S portland cement. Addition of natural pozzolana provides the maximum sulphate resistance while the addition of 20% limestone filler declining sulphate performance of low C₃A cements. This behaviour can be attributed to the reaction between sulphate ions with CH into the paste that produces an alteration of the predominant mechanism of sulphate attack.     

Carbonation behaviour of recycled aggregate concrete

This paper reviews the effect of incorporating recycled aggregates, sourced from construction and demolition waste, on the carbonation behaviour of concrete. It identifies various influencing aspects related to the use of recycled aggregates, such as replacement level, size and origin, as well as the influence of curing conditions, use of chemical admixtures and additions, on carbonation over a long period of time. A statistical analysis on the effect of introducing increasing amounts of recycled aggregates on the carbonation depth and coefficient of accelerated carbonation is presented. This paper also presents the use of existing methodologies to estimate the required accelerated carbonation resistance of a reinforced recycled aggregate concrete exposed to natural carbonation conditions with the use of accelerated carbonation tests. Results show clear increasing carbonation depths with increasing replacement levels when recycled aggregate concrete mixes are made with a similar mix design to that of the control natural aggregate concrete. The relationship between the compressive strength and coefficients of accelerated carbonation is similar between the control concrete and the recycled aggregate concrete mixes.     

Enhancement of recycled aggregate properties by accelerated CO2 curing coupled with limewater soaking process

Strengthening the attached old cement mortar of recycled concrete aggregate (RCA) is a common approach to enhance the RCA properties. Accelerated CO₂ curing has been regarded as an alternative way to enhance the properties of RA. However, the improvement of the properties of RCA was limited by the shortage of reactive components in the old cement mortar available for the carbonation reactions. In this study, a CO₂ curing process associated with a limewater saturation method was performed cyclically on cement mortar samples, aiming to enhance the properties of cement mortars via artificially introducing additional calcium into the pores of the cement mortars. The results indicated that the adopted treatment method promoted the level of carbonation which was demonstrated by higher CO₂ uptake by the limewater saturated cement mortar when compared to that without limewater treatment. After 3-cycles of limewater-CO₂ treatment, the density of the cement mortar slightly increased by 5.7%, while the water absorption decreased by over a half. For mechanical properties, the compressive and flexural strength were increased by 22.8% and 42.4%, respectively. Compared to the untreated cement mortar samples, the total porosity of cement mortar was reduced by approximately 33% and the densified microstructure therefore resulted in a higher microhardness.     

Literature study on the rate and mechanism of carbonation of lime in mortars / Literaturstudie über Mechanismus und Grad der Karbonatisierung von Kalkhydrat im Mörtel

The hardening kinetics of a lime based mortar is based on the uptake of carbon dioxide from the ambient air. The presence of watervapour is required in order to enable the reaction between the CO₂ and the lime (calcium hydroxide). Via this reaction the hardening of air lime is net uptaker of CO₂. An extensive literature study was made on the fundamentals of the carbonation process in mortars with different compositions. The results of the study indicate that carbonation ranges from 80 % up to 90 %. It is clear that the mechanism and the kinetics of the carbonation depend strongly on the mineralogy, texture of mortars, type of additive used, the lime use for the mortar, the width of the walls, thickness of the mortar (less carbonation when mortar depth increases) as well as the timeframe allowing for the carbonation process to take place. Under natural conditions, actual building practice and depending on the thickness of the mortar/plaster, carbonation takes between a few weeks and several years. The results of this study were used for the environmental footprint study in order to calculate the capture of CO₂ that occurs progressively during the hardening of a building materials containing lime.     

Empirical modelling of CO<Subscript>2</Subscript> uptake by recycled concrete aggregates under accelerated carbonation conditions

In order to assess the potential CO₂ capture ability of recycled concrete aggregates (RCAs) subjected to accelerated carbonation, an empirical prediction model has been developed in relation to carbonation conditions and the characteristics of RCAs. In this study, two sources of RCAs were used: RCAs from a designed concrete mixture and RCAs obtained from crushing of old laboratory concrete cubes. Two types of carbonation approaches were employed: (A) pressurized carbonation in a chamber with 100% CO₂ concentration and (B) flow-through carbonation at ambient pressure with different CO₂ concentrations. Four groups of RCAs particles with sizes of 20–10, 5–10, 2.36–5 and <2.36 mm were then tested and evaluated. It was found that a moderate relative humidity, a CO₂ concentration higher than 10%, a slight positive pressure or a gas flow rate of >5 L/min were optimal to accelerate the RCAs carbonation. Moreover, the CO₂ uptake of fine RCAs particles was faster than that of large RCAs particles. The developed model was able to predict the CO₂ uptake in relation to relative humidity, particle size, carbonation duration and cement content of the RCA under the tested carbonation conditions.     

Corrosion behaviour of rebars in fly ash mortar exposed to carbon dioxide and chlorides

Addition of fly ash has beneficial effects on some mechanical properties of concrete, as well as on the corrosion process induced by the chloride ion. The aim of this study was to investigate the effect of fly ash addition on the corrosion process occurring in reinforced concrete exposed simultaneously to carbon dioxide and chloride. The corrosion process of steel rebars embedded in mortar with 15% and 30% of fly ash was tested under carbon dioxide and sodium chloride contamination. Monitoring of open circuit potential and electrochemical impedance spectroscopy (EIS) were used to follow the corrosion process. Results have shown that under accelerated carbonation fly ash mortar shows higher corrosion rates. The chloride content in mortar exposed to accelerated carbonation increases with the amount of fly ash. However, under natural carbonation it decreases with the addition of fly ash.     

Preliminary investigations into the supercritical carbonation of cement pastes

Interactions between supercritical carbon dioxide (scCO₂) and hydrated cement pastes, of various water/cement ratio, have been investigated. The carbonation process was greatly accelerated in the scCO₂ compared to that in natural or CO₂ enriched environments. The nature of the reactions was dependent on the amount of water present in the paste. Thus carbonation of samples dried prior to treatment resulted in the reaction of all the unhydrated C₃S and C₂S, but little conversion of calcium hydroxide to calcium carbonate. In contrast, carbonation of samples containing moisture resulted in the conversion of most of the calcium hydroxide whilst the amounts of C₃S and C₂S reacted increased as the water/cement ratio increased. During the carbonation treatment, the pore structure of the cement pastes was altered and substantial reductions in porosity were achieved. The process may be used to improve the durability of glass fibre reinforced cement by lowering the alkalinity and calcium hydroxide content of the matrix.     

Corrosion behaviour of steel in high alumina cement mortar cured at 5,25 and 55 °C: chemical and physical factors

The corrosion behaviour of embedded steel was related to the composition of the pore phase in equilibrium with the hydrated phases and the porosity of the high alumina cement mortars subsequent to curing at 5,25 and 55 °C. The corrosion of reinforcements was evaluated by electrochemical techniques. The effect on corrosion of 3% by weight of cement of NaCl, added during the mixing process, and of the accelerated carbonation of mortars in CO₂ atmosphere were also determined. The pH value and the chemical composition of pore fluid of plain high alumina cement (HAC) mortar cured at all three temperatures suggested that the embedded steel was in a passivated state. The resistance of HAC to carbonation and its greater potential for chloride binding by chloroaluminate formation are believed to make HAC inherently more protective to steel, relative to normal Portland cement, during ingress of chloride from external sources. High corrosion rates reported in literature for steel embedded in HAC may be attributable to bad practice, not to lack of passivity.     

Carbonation of calcium sulfoaluminate mortars

This study investigated potential physical and chemical parameters that could govern the carbonation rate of calcium sulfoaluminate (CSA) mortars and endeavored to elucidate the microstructural and chemical factors that govern CSA cement's carbonation rate. Experiments included: water absorption, oxygen diffusion, mercury intrusion porosimetry, quantitative X-ray diffraction, thermogravimetric analysis, accelerated carbonation, compression and flexure tests. Additionally, the carbonation process was investigated using thermodynamic modeling. The results show that CSA mortars carbonate much faster than Portland cement mortars and at approximately the same rate as calcium aluminate cement mortars. Additionally, CSA mortars carbonate slower with decreasing w/c, and the anhydrite content of the CSA mortars strongly affects the ye'elimite reaction kinetics which plays an important role in imparting carbonation resistance in CSA mortars. Finally, calcium sulfate additions to CSA clinker to produce CSA cement dilutes the clinker content and reduces the amount of CO₂ that the CSA cement can ultimately bind.     

Evaluation of the combined deterioration by freeze–thaw and carbonation of mortar incorporating BFS,limestone powder and calcium sulfate

The durability performance of cementitious material is traditionally based on assessing the effect of a single degradation process. However, this study investigates the coupled deterioration properties of mortar incorporating industrial solid waste—ground granulated blast furnace slag (BFS) and different mineral admixtures, such as calcium sulfate (CS) and limestone powder (LSP). The combined deterioration properties caused by carbonation and frost damage in the mortar sample were experimentally investigated with respect to accelerated carbonation and freeze–thaw tests. Different degrees of deterioration, i.e. after subjected to 12, 30 and 60 freeze–thaw cycles, were induced in the freeze–thaw tests. The experimental investigation of single degradation revealed that the compressive strength, frost resistance and carbonation resistance decrease as the BFS replacement ratio increases by weight from 0 to 45%. The less amount of CH in the BFS cement leads to the carbonation progress more easily. Moreover, to achieve the same strength as ordinary Portland cement, 2 wt% CS and 4 wt% LSP in the BFS mortar are required. However, the data shows that incorporating LSP into the BFS mortar produces a lower frost resistance. The combined damage tests revealed that different deterioration degrees resulting from 12, 30 and 60 freeze–thaw cycles slightly decreased the carbonation resistance, which is related to the decrease in the inkbottle pore volume due to its water retention characteristics. Simultaneously, the pre-carbonation deterioration could effectively decrease the surface mass scaling of the freeze–thaw and the pore structure undergoes densification due to pre-carbonation.     

Use of tomography to understand the influence of preconditioning on carbonation tests in cement-based materials

Recognition of the rising amount of atmospheric CO₂ has brought renewed interest in understanding the effects of carbonation on reinforced concrete performance. In laboratory testing, the specimens must be preconditioned to effectively study carbonation. This paper studied the influence of several preconditioning schemes on the carbonation profiles of cement paste specimens subjected to accelerated carbonation tests. The evolution of microstructure and moisture during carbonation were investigated accordingly. Bulk of the work was based on an extended X-ray attenuation method (XRAM), which relied on X-ray computed tomography (CT). A novel method was introduced to evaluate the extent of damage due to drying. Based on extent of damage, the paper recommends standard-cured specimens for carbonation tests as compared to water-cured specimens. Also, when comparing between oven drying and mass balancing, the latter was shown to be more suitable, as the inner moisture distribution becomes more uniform after this drying protocol, and less fluctuation of humidity will occur during carbonation.     

Effectiveness of using CO2 pressure to enhance the carbonation of Portland cement-fly ash-MgO mortars

Acceleration on the carbonation of reactive MgO cement is essential for its widespread application. There is currently a dearth of published reports on the effect and sensitivity of using pressurized CO₂ on the properties and performance of reactive MgO cement blends. This study is motivated by improving the understanding of the effectiveness of accelerating the carbonation process. Pressurized CO₂ (up to 1.0 MPa) was employed to enhance the carbonation of mortar blends consisting of Portland cement, fly ash and reactive MgO. Results revealed that the carbonation front and mechanical properties of the mortars were developed quickly owing to the effectively accelerated carbonation under pressurized CO₂. In comparison to the 0.1 MPa pressure, the relatively higher pressure (0.55 and 1.0 MPa) were much more effective in achieving stronger mechanical properties within 1 day. However, an increasing curing duration from 1d to 14d under the lower CO₂ pressure of 0.1 MPa caused a 1.8–2.9 times increase in compressive strength. This indicates that either increases in pressure or curing duration under pressurized CO₂ enhances the carbonation and mechanical properties of the mortars.     

A maturity approach to estimate compressive strength development of CO2-cured concrete blocks

An alternative CO₂ curing method for precast concrete products has been proposed in order to achieve rapid strength development at early age, as well as to capture and store greenhouse gas (CO₂). In this paper, an experimental study for the development of a maturity approach is presented to estimate the strength development of carbonated concrete blocks. In order to promote the use of industrial flue gas containing CO₂, a flow-through CO₂ curing regime at ambient pressure and temperature was employed using different atmospheric conditions, such as various CO₂ concentrations, RH values and gas flow rates. The experimental results showed that the compressive strength or maturity of the carbonated concrete blocks was affected by two factors: accelerated cement hydration and carbonation extent. A high CO₂ concentration, a fast gas flow rate and a moderate relative humidity were essential for enhancing the maturity and the strength development. The developed model based on the maturity approach may accurately predict the strength development of the carbonated concrete blocks.     

Durability of recycled aggregate concrete designed with equivalent mortar volume method

Results of a comprehensive investigation about the durability of structural-grade concrete made with recycled concrete aggregate (RCA) are presented. The RCA-concrete mixes were proportioned using a new concrete mix design method, termed the equivalent mortar volume (EMV) method. The EMV method is based on the hypothesis that RCA is a composite material comprising mortar and natural aggregate; therefore, when proportioning a concrete mixture containing RCA, one must account for the relative amount and properties of each the two components and adjust both the fresh coarse aggregate and fresh paste content of the mix accordingly. Tests were conducted to study the freeze–thaw, chloride penetration and carbonation resistances of the mixes proportioned by the EMV method and by the conventional method. Results of the test showed that RCA-concrete mixes proportioned by the EMV method have higher resistance to freeze–thaw action, chloride penetration and carbonation than those designed with the conventional method, and they satisfy the current requirements for concrete exposed to severe environments.     

Carbonation durability of blended cement pastes used for waste encapsulation

Blended cement pastes are currently used for encapsulation of low level and intermediate level nuclear waste in the UK. However, there is still little information on the long-term durability of those mixes to some chemical attacks. Accelerated testing may predict the long-term durability or at least help the selection of more durable formulations. In this work, blended blastfurnace slag (BFS)/Portland cement (OPC) pastes containing 60, 75 and 90% BFS and pulverised fuel ash (PFA)/OPC pastes with 40, 55 and 75% PFA were cured at 20 and 60°C for 90 days then submitted to natural and accelerated carbonation (5% CO₂). The effects of the curing temperature as well as the OPC replacement level on the carbonation ratio are presented. Results showed a good correlation between natural and accelerated carbonation for the pastes studied. Carbonation was found to be governed by the amount of calcium hydroxide available in the mixes before the process started.