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This paper is a reply to the discussion by S.A. Walling and J.L. Provis on our papers “Impact of hydrated magnesium carbonate additives on the carbonation of reactive MgO cements” and “Enhancing the carbonation of MgO cement porous blocks through improved curing conditions”. Walling and Provis discuss the assignment of X-ray diffraction patterns, the use of acid digestion to quantify the degree of carbonation, the interpretation of thermal analysis data, and the stability of hydrated magnesium carbonates. In this reply, we confirm the conclusions made in the two papers based on the combined evidence of microstructural and mechanical performance results. [ABSTRACT FROM AUTHOR]

This paper is a discussion of two recent papers by Unluer & Al-Tabbaa which analysed accelerated carbonation of reactive MgO blocks. We suggest that the authors have incorrectly analysed key data, leading to overstated claims of MgO carbonation. Based on the reassignment of their X-ray diffraction data, it is proposed that little MgO carbonation occurred in the samples discussed in those papers, with CaCO 3 instead forming during accelerated carbonation. We also draw attention to the thermodynamic instability of nesquehonite under ambient conditions, which calls into question the long-term stability of these binders. [ABSTRACT FROM AUTHOR]

Determining the carbonation depth is of paramount importance in assessing the durability of cementitious materials as the carbonation process alters their near-surface physicochemical properties. This paper introduces a novel quantitative ultrasound (QUS) technique capable of nondestructively identifying the carbonation depth, regardless of the tortuosity of the carbonation front. A 2.5 MHz phased array transducer was employed to construct beamformed radio frequency images and measure two microstructure-dependent parameters – spectral slope (SS) and spectral intercept (SI) – based on backscatter and attenuation coefficients. The change in obtained SS and SI were then visualized as QUS images, which displayed the geometry of the carbonation front, correlating with images constructed using a phenolphthalein solution (1.6 mm difference in depth). Mineral compositions and porosities were analyzed to account for microstructural changes in each layer. This study opens an opportunity for nondestructive detection of carbonation depth, overcoming the limitations in conventional methods or regression-based approaches. • Quantitative ultrasound (QUS) technique is developed for detecting carbonation depth. • QUS images achieved based on scattering and attenuation phenomena confirm densification in carbonated layer. • Upon the QUS images, the measured carbonation depth shows great agreement with results from phenolphthalein solution. • Mineral compositions and porosities were evidenced by TG, MIP, XRD, and QXRD analyses. • The proposed method offers potential for visualizing ambiguous geometry of inclusions in cementitious materials. [ABSTRACT FROM AUTHOR]

Abstract Understanding the changes in the pore solution upon carbonation is crucial with respect to comprehending the mechanisms of reinforcement corrosion in carbonated mortar or concrete. In this paper we used Cold Water Extraction (CWE), a rapid leaching method, on profile ground powder from partially carbonated Portland cement and Portland-fly ash cement mortars to study changes in the pore solution composition. Carbonation decreased the free alkali metal content measured with CWE, which matched with the portlandite profiles determined by thermogravimetry and the carbonation depth detected with pH indicator. The alkali metal uptake by carbonation products was confirmed by SEM-EDS and thermodynamic modelling. Besides the decrease in the alkali metal concentration in the pore solution upon carbonation, we observed for both binders a considerable increase in chlorine and sulphur concentrations. This can further accelerate corrosion in carbonated concrete. [ABSTRACT FROM AUTHOR]

Abstract The long-term durability and their mechanisms of alkali-activated cement based materials have remained largely elusive. In this paper, carbonation of alkali-activated slag (AAS) mortars activated by NaOH and waterglass with different alkali dosages and silicate moduli has been investigated after exposure to 3 ± 0.2% (v/v) CO 2 at 20 ± 2 °C/65 ± 5% RH for 56 days. The results show that carbonation resistance of the AAS mortars increases with increase of not only alkali dosage but also silicate modulus. In addition to the higher pore solution alkalinity and slag reaction extent, the relatively higher carbonation resistance of the AAS mortars is attributed to the lower porosity and average pore size. The loss of compressive strength for the waterglass activated slag mortars after carbonation is due to decalcification of C-A-S-H phase, whereas the carbonation of katoite contributes to the increase of compressive strength of the NaOH activated slag mortars. [ABSTRACT FROM AUTHOR]

The paper addresses the carbonation of CSA based binders from the mechanical, chemical and mineralogical point of view. Three different binders have been investigated, revealing that many hydrated phases can protect ettringite against carbonation. In particular, the presence of AFm phases gives a significant contribution to buffer the pH in alkaline range and to preserve the protective film of steel reinforcement bars. In a carbonated environment AFm phases act as ionic exchangers: carbonate ions replace sulfates in the interlayer and in these conditions the released sulfate ions contribute to the formation of new ettringite, with a beneficial effect on compressive strength performances. Experimental data are compared with models and carbonation mechanisms are described. [ABSTRACT FROM AUTHOR]

Steel fibre reinforced concrete (SFRC) is increasingly being used in the construction of civil infrastructure. However, there are inconsistencies among international standards and guidelines regarding the consideration of carbon-steel fibres for the structural verification of SFRC exposed to corrosive environments. This paper presents a review of the published research regarding carbonation- and chloride-induced corrosion of SFRC, and proposes a deterioration theory for cracked SFRC exposed to chlorides and carbonation, based on the damage at the fibre-matrix interface. The review confirms an overall agreement among academics and regulators regarding the durability of uncracked SFRC exposed to chlorides and carbonation. Contrariwise, the durability of cracked SFRC is under discussion at the technical and scientific level, as there is a large dispersion on the experimental results and some of the mechanisms governing the corrosion of carbon-steel fibres in cracks and its effects on the fracture behaviour of SFRC are not fully understood. [ABSTRACT FROM AUTHOR]

Numerous researches have reported that there is a tendency chloride content first climbs to the maximum then declines with depth increasing in the surface layer of concrete under cyclic drying-wetting environments, which is temporally called ‘maximum phenomenon’ in this paper. This research focuses on the impact of different carbonation conditions on this phenomenon for both pastes and mortars. The distribution of chloride suggests that coupled effect of carbonation and capillary suction/moisture evaporation is more likely to lead to the formation of maximum phenomenon than the merely effect of capillary suction/moisture evaporation. Furthermore, analysis of pore structure and phase composition reveals that this particular phenomenon is directly related to the release of bound chloride fixed in Friedel's salt triggered by carbonation. In addition, the forming process of maximum phenomenon is proposed in the end based on Friedel's salt decomposition under cyclic drying and wetting condition. [ABSTRACT FROM AUTHOR]

Sorptivity is broadly used for characterising the pore connectivity of cementitious materials, with applications in design for durability. A water sorptivity coefficient (WSC) is typically obtained from the ratio between the amount of absorbed water and t 0.5 . This relationship is however not linear for cementitious materials, and conventions are needed for the computation. Variable criteria in the literature complicate the comparison of WSCs. This paper proposes a new approach for describing the entire absorption process. We substantiate the hygroscopicity of calcium silicate hydrates and the effect of swelling during the process as the main causes for the anomalous capillary absorption by cementitious materials. We present a theoretical model with a single descriptive coefficient of capillary absorption progressing linearly with t 0.25 . The model fits remarkably well to experimental data, and it solves the problem of lack of linearity with t 0.5 . A full description of the transport process is then offered. [ABSTRACT FROM AUTHOR]

This paper presents an experimental investigation on carbonation in fatigue-damaged concrete. Uniaxial and eccentric fatigue loads were exerted on prism specimens to obtain concrete with uniform compressive, gradient compressive, and gradient tensile damage patterns. Carbonation tests were subsequently conducted on these fatigue-damaged concrete specimens. Fatigue tests showed that while elastic modulus decreased and residual strain increased with loading cycles, stresses redistributed in eccentrically fatigued specimens, but strains maintained linear distribution at all times. Carbonation tests showed that the widely accepted proportional relationships between carbonation depths and square roots of exposure durations also applied to carbonation in fatigue-damaged concrete. Also, carbonation rates evolved linearly with residual strains in concrete with low damage gradients, and a 75% increase could be observed at a residual strain of 0.002. In addition, fatigue damage with low gradients did not significantly sway the ways that relative humidity, temperature, and carbon dioxide concentration affected carbonation of concrete. [ABSTRACT FROM AUTHOR]

This paper investigates the long-term performance of structural concretes in China southeast coastal area, the city of Shenzhen, exposed to atmosphere and chlorides. The structural concretes, incorporating large quantity of supplementary cementitious materials (SCM), were among the first massive application of such mixtures in construction in China. In both cases, in-field sampling and tests have been conducted during more than 20 years for the resistance of concretes to carbonation and chloride ingress respectively. The synthetic analysis of these results shows that (1) the concretes have substantial strength growth in atmospheric environment with exposure age, but the carbonation rate does not all obey the square root law of exposure time; (2) the protection of silane impregnation of high-performance concrete against chloride ingress maintains its hydrophobic effect after nearly 20 years; (3) Notable "skin" effect of surface concrete was observed for both atmospheric and marine exposures. [ABSTRACT FROM AUTHOR]

Steel slag retards the early-age hydration of cement. However, the key component which plays a major role in steel slag is still unclear. In this paper, it was identified that C 12 A 7 in steel slag plays a major role in retarding the early-age hydration of cement by selective treatment methods such as sieving, calcination, water washing and carbonation. The pure C 12 A 7 was mixed with cement to confirm this retarding effect. Results show that the early-age hydration of C 3 S is significantly retarded with the increase of C 12 A 7 content. In most of cement-steel slag composite binders, the C 12 A 7 content is usually at a low level, and the retarding effect of C 12 A 7 on the early-age hydration of C 3 S leads to prolonged setting time. When the C 12 A 7 content is high enough, it leads to fast setting due to a large number of products with large specific surface areas produced by the rapid reaction of C 12 A 7. [ABSTRACT FROM AUTHOR]

This paper presents a large collection of microstructural studies on concretes exposed to marine environments for periods between 2 and 34 years. The phase changes were studied using optical as well as scanning electron microscopy. A total of 21 concretes, taken from 9 different locations along the Norwegian and Danish coastlines, were investigated. Chemical zonation and mineralogical zonation were observed in the surface regions of marine-exposed concrete. Three zones were found irrespective of the age, location or binder composition: a magnesium-rich zone, a sulfur-rich zone, and a chlorine-rich zone. The absence of major damage indicates that the observed phase changes led only to minor scaling. This suggests that, rather than sulfate attack, sea water merely causes sulfur enrichment. [ABSTRACT FROM AUTHOR]

This paper presents a modeling strategy to simulate the corrosion of steel reinforcement in atmospheric environment due to carbonation of concrete. Its principal objectives are to analyze the effects of the progressive formation of corrosion products at the steel/concrete interface on concrete cover cracking. The approach is based on modeling studies carried out independently on carbonation, corrosion and creep. These models are coupled and integrated into a numerical 3D simulation procedure for investigating the behavior of concrete mesostructures. A viscodamage model is used to reproduce both creep and damage behaviors of mortar, and the approach is applied to the simulation of a 3D reinforced concrete mesostructure including explicitly the coarse aggregates. The numerical results highlight the influence of aggregates and the effects of creep on crack initiation and propagation. [ABSTRACT FROM AUTHOR]

The carbonation rate of concrete has implications for the lifecycle carbon emissions of concrete. This paper describes the reported effect of several variables on the rate of concrete carbonation and collates a data set of measurements published in the literature. Many studies producing predictive models for the carbonation rate constant, K, use only small data sets. 1999 measurements of carbonation depth as a function of time and other variables were collected for analysis. Models in the form ln (K) = a + bI1 + cI2 + … have been produced by which the rate of carbonation can be predicted. Hierarchical Models were used to combine different authors' data and introduces a new explanatory variable called ‘origin’, which indicates whether the concrete was taken from a working structure or cast specifically for experiments. Two models of the carbonation rate using concrete properties have been produced, allowing prediction of K over a range of conditions and compositions. [ABSTRACT FROM AUTHOR]

In this paper, a mathematical model is developed to simulate the physical–chemical coupling process of supercritical carbonation in cement-based materials. This model takes into account the rate of chemical reaction, mass conservation for gas–liquid two phase flow, diffusion and dispersion of CO 2 in water, energy conservation for porous medium and the solubility of CO 2 in water. Numerical results are obtained and compared with experimental results. The degree of carbonation, temperature, gaseous pressure, moisture content and saturation of water within the material are predicted and presented. The influence of material saturation, temperature and pressure of supercritical CO 2 on carbonation depth is investigated through parametric studies. The comparisons with test results suggest that the coupled model can be used to predict carbonation process of cement-based materials under supercritical conditions. [ABSTRACT FROM AUTHOR]

Cement is a huge carbon dioxide producer. Supplementary cementitious materials can help reduce this outcome. However, carbonation of these blended cements remains an active subject of research. Accelerated carbonation tests (10% CO 2 , 25 °C and 62% RH) are performed on fly ash blended cement pastes. Experiments are performed at varying ages of carbonation (1 to 16 weeks) to measure the evolution of the carbonation depth over time and to quantify key parameters: thermogravimetric analysis (TGA), mercury intrusion porosimetry (MIP) and gamma ray attenuation method (GRAM). The total porosity decreases with a rearrangement of the microstructure due to carbonation and the creation of big capillary pores for the paste with the highest contents of fly ash (60 vol.%). The C-S-H molar volume evolution during fly ash-blended cement carbonation is calculated using a method combining MIP, TGA and GRAM formerly successfully applied to OPC paste in a paper published in the same journal. [ABSTRACT FROM AUTHOR]

Abstract: The use of reactive magnesia (MgO) as the binder in porous blocks demonstrated significant advantages due to its low production temperatures and ability to carbonate, leading to significant strengths. This paper investigates the enhancement of the carbonation process through different curing conditions: water to cement ratio (0.6–0.9), CO2 concentration (5–20%), curing duration (1–7days), relative humidity (55–98%), and wet/dry cycling frequency (every 0–3days), improving the carbonation potential through increased amounts of CO2 absorbed and enhanced mechanical performance. UCS results were supported with SEM, XRD, and HCl acid digestion analyses. The results show that CO2 concentrations as low as 5% can produce the required strengths after only 1day. Drier mixes perform better in shorter curing durations, whereas larger w/c ratios are needed for continuous carbonation. Mixes subjected to 78% RH outperformed all the others, also highlighting the benefits of incorporating wet/dry cycling to induce carbonation. [Copyright &y& Elsevier]

Abstract: In this paper, an analysis of the accelerated carbonation test according to the French Standard XP P18-458 is presented. It aims at a better understanding of the carbonation kinetics in the early days of this test. The influence of preconditioning by oven-drying was studied by means of carbonation depths monitoring (phenolphthalein spraying and thermogravimetric analysis) and measurements of relative humidity (RH) inside concrete samples. The main results question the relevance of the preconditioning because it does not establish a hydric balance between the tested samples and the carbonation chamber. Moreover, an absence of carbonation close to the sample surface was revealed during the early days of testing. This is explained by the very low water content resulted from oven-drying. This gives finally an explanation of the whole carbonation kinetics recorded during this test. [Copyright &y& Elsevier]

Abstract: Reactive magnesia (MgO) cements have emerged as a potentially more sustainable and technically superior alternative to Portland cement due to their lower production temperature and ability to sequester significant quantities of CO2. Porous blocks containing MgO were found to achieve higher strength values than PC blocks. A number of variables are investigated to achieve maximum carbonation and associated high strengths. This paper focuses on the impact of four different hydrated magnesium carbonates (HMCs) as cement replacements of either 20 or 50%. Accelerated carbonation (20°C, 70–90% RH, 20% CO2) is compared with natural curing (20°C, 60–70% RH, ambient CO2). SEM, TG/DTA, XRD, and HCl acid digestion are utilized to provide a thorough understanding of the performance of MgO-cement porous blocks. The presence of HMCs resulted in the formation of larger size carbonation products with a different morphology than those in the control mix, leading to significantly enhanced carbonation and strength. [Copyright &y& Elsevier]

Reinforcement spacers are a critical component of concrete structures. Their presence affects microstructure and transport properties of concrete cover though this is not widely appreciated. This paper presents the first study to determine whether the negative effects of spacers can be mitigated through the use of supplementary cementitious materials such as silica fume, fly ash and blast-furnace slag. Concrete samples (>200) with different spacers, binders, curing and drying regimes were prepared and tested for diffusion, permeation, absorption, electrical conductivity, carbonation and microstructure. It was found that spacers increase all transport properties, the extent depending on type of spacer, drying regime and transport mechanism. The spacer-concrete interface is weak, porous and micro-cracked, and this lowers the resistance of concrete to ingress of aggressive agents. The beneficial effects of SCMs (strength enhancement and densification) and prolonged curing (120-day) are insufficient to overcome the negative effects of spacers. Implications for durability are discussed. [ABSTRACT FROM AUTHOR]

This paper presents a comparative study on the corrosion rate and microstructural characteristics of Portland cement (PC) and calcium aluminate cement (CAC) -based concrete mixtures subjected to microbially-induced sulphuric acid (biogenic H 2 SO 4) attack for 120 months in an outfall sewer. The higher amount of the amorphous AH x within the CAC-based concrete matrix provides a higher neutralisation capacity to the attacking acid, thus giving it better performance than PC-based concrete mixtures during biogenic H 2 SO 4 attack. In addition the results show that the biogenic corrosion rate of PC-based concrete mixtures increases with the cement content while that of CAC-based concrete mixtures decreases with the cement content. [ABSTRACT FROM AUTHOR]