Supplementary cementitious materials (SCMs), like fly ash (FA) and blast furnace slag (BFS), are widely used in concrete to partially replace the cement clinker in the production of blended cement or directly replace part of Ordinary Portland cement (OPC) in the concrete. No matter how the SCMs are introduced, they can reduce the cost of concrete and CO2 emissions during clinker production. From both economic and sustainable development point of view, it is necessary to incorporate more SCMs into concrete. However, the addition of SCMs will affect the cement hydration and bring challenges to the durability of concrete, especially the resistance to carbonation. Carbonation happens when atmospheric CO2 penetrates inside the concrete and reacts with calcium-bearing phases of the cement paste. The main calcium-bearing phases involving in carbonation of concrete are portlandite (CH) and calcium silicate hydrate (C-S-H). With the addition of SCMs, the amounts of both calcium-bearing phases are reduced due to so-called dilution effects. Moreover, the portlandite is consumed by the pozzolanic reactions of SCMs, to produce C-S-H phases with less Ca/Si ratio. Apparently, the C-S-H phase is now the dominant calcium-bearing phase and is expected to play the key role in the durability of concrete under carbonation. Therefore, it is important to investigate the carbonation mechanisms of C-S-H phases and their effects on the carbonation development in blended cement concrete. This study aims at a better understanding of the carbonation mechanisms of different types of C-S-H and their effects on the chemistry of the reaction products and microstructure development of cement paste blended with SCMs during carbonation. @en

Many (inter)national standards exist to evaluate the resistance of mortar and concrete to carbonation. When a carbonation coefficient is used for performance comparison of mixtures or service life prediction, the applied boundary conditions during curing, preconditioning and carbonation play a crucial role, specifically when using latent hydraulic or pozzolanic supplementary cementitious materials (SCMs). An extensive interlaboratory test (ILT) with twenty two participating laboratories was set up in the framework of RILEM TC 281-CCC ‘Carbonation of Concrete with SCMs’. The carbonation depths and coefficients determined by following several (inter)national standards for three cement types (CEM I, CEM II/B-V, CEM III/B) both on mortar and concrete scale were statistically compared. The outcomes of this study showed that the carbonation rate based on the carbonation depths after 91 days exposure, compared to 56 days or less exposure duration, best approximates the slope of the linear regression and those 91 days carbonation depths can therefore be considered as a good estimate of the potential resistance to carbonation. All standards evaluated in this study ranked the three cement types in the same order of carbonation resistance. Unfortunately, large variations within and between laboratories complicate to draw clear conclusions regarding the effect of sample pre-conditioning and carbonation exposure conditions on the carbonation performance of the specimens tested. Nevertheless, it was identified that fresh and hardened state properties alone cannot be used to infer carbonation resistance of the mortars or concretes tested. It was also found that sealed curing results in larger carbonation depths compared to water curing. However, when water curing was reduced from 28 to 3 or 7 days, higher carbonation depths compared to sealed curing were observed. This increase is more pronounced for CEM I compared to CEM III mixes. The variation between laboratories is larger than the potential effect of raising the CO₂ concentration from 1 to 4%. Finally, concrete, for which the aggregate-to-cement factor was increased by 1.79 in comparison with mortar, had a carbonation coefficient 1.18 times the one of mortar.

@en

This study aims to predict the autogenous shrinkage of alkali-activated concrete (AAC) based on slag and fly ash. A variety of analytical and numerical models are available for the prediction of autogenous shrinkage of ordinary Portland cement (OPC) concrete, but these models are found to show dramatic discrepancies when applied for AAC due to the different behaviours of these two systems. In this study, a new numerical approach is developed to predict the autogenous shrinkage of alkali-activated slag (AAS) and alkali-activated slag-fly ash (AASF) concrete from the experimental results on corresponding paste. In this approach, the creep of AAS and AASF and the restraining effect of the aggregate are particularly considered. By this approach, a fairly good prediction is obtained. Moreover, the microcracking in paste caused by restraining aggregates is evaluated. The results indicate that AAC is subjected to high tendency of development of microcracking.

@en

Blended cements, where Portland cement clinker is partially replaced by supplementary cementitious materials (SCMs), provide the most feasible route for reducing carbon dioxide emissions associated with concrete production. However, lowering the clinker content can lead to an increasing risk of neutralisation of the concrete pore solution and potential reinforcement corrosion due to carbonation. carbonation of concrete with SCMs differs from carbonation of concrete solely based on Portland cement (PC). This is a consequence of the differences in the hydrate phase assemblage and pore solution chemistry, as well as the pore structure and transport properties, when varying the binder composition, age and curing conditions of the concretes. The carbonation mechanism and kinetics also depend on the saturation degree of the concrete and CO₂ partial pressure which in turn depends on exposure conditions (e.g. relative humidity, volume, and duration of water in contact with the concrete surface and temperature conditions). This in turn influence the microstructural changes identified upon carbonation. This literature review, prepared by members of RILEM technical committee 281-CCC carbonation of concrete with supplementary cementitious materials, working groups 1 and 2, elucidates the effect of numerous SCM characteristics, exposure environments and curing conditions on the carbonation mechanism, kinetics and structural alterations in cementitious systems containing SCMs.

@en

Supplementary cementitious materials (SCMs) like fly ash (FA) and blast furnace slag (BFS) are broadly used in concrete to replace part of the Ordinary Portland Cement (OPC) because of both economic and environmental issues. In concrete blended with SCMs, C-S-H with different C/S ratios, formed from the hydration and pozzolanic reactions of blended cement, is the major calcium-bearing phases which reacts with CO2 during carbonation. Therefore, it is important to study the carbonation rate of different C-S-H phases. In this paper, the C-S-H phases (C/S ratio: 0.66 to 2.0) were synthesized and used for accelerated carbonation testing. Synthetic C-S-H phases with different C/S ratios were identified by X-ray diffraction and 29Si nuclear magnetic resonance (NMR). Carbonation rate and products of different C-S-H phases are also determined. The results show that C-S-H (I) phases with different target C/S ratio (lower than 1.40) were synthesized in the mix solution of lime and fume silica. The portlandite appears in the products when the designed C/S ratio is higher than 1.40 under this synthetic condition. C-S-H with lower C/S ratio is decomposed faster than that with a higher C/S ratio. After exposition to the accelerated carbonation condition for three days, in this research, the C-S-H phases with different C/S ratio were all fully decomposed to CaCO3 and silica gel.

@en

C-S-H with different Ca/Si ratios, formed from the hydration and pozzolanic reactions of blended cement paste, are the major calcium-bearing phases which react with CO2 during the carbonation. Therefore, it’s important to study the carbonation rate of different C-S-H phases. In this paper, the C-S-H phases (Ca/Si ratio: 0.66 to 2.0) were synthesized and used for the accelerated carbonation test with the CO2 concentration of 3%±0.2. Synthesized C-S-H phases with different Ca/Si ratio were identified by 29Si nuclear magnetic resonance (NMR). Carbonation rate and products of different C-S-H phases were studied by NMR and Fouriertransform infrared spectroscopy (FTIR). The results show that C-S-H phases with different Ca/Si ratio (lower than 1.40) was synthesized. C-S-H with lower Ca/Si ratio is decomposed faster than that with a higher Ca/Si ratio. After three days’ accelerated carbonation, the C-S-H phases with different Ca/Si ratio were all fully decomposed to CaCO3 and silica gel.@en

Because of environmental and economic benefits, a fraction of cement is increasingly replaced by limestone fillers raising a question on to what extent limestone fillers affect the durability of cementitious materials. This work aims at understanding the effects of water/powder (w/p) ratio and limestone filler replacement on water permeability of cement pastes. A newly proposed technique using a controlled constant flow concept was applied to measure permeability of hardened cement paste samples following a factorial experimental design. It was observed that both limestone filler and w/p ratio significantly influence the water permeability. At a given w/p ratio, adding limestone filler made the microstructure coarser, especially for high w/p ratio. Nevertheless, if the comparison is based on a given water/cement (w/c) ratio instead of w/p ratio, the limestone filler replacement refined the microstructure in terms of capillary porosity and pore size distribution, resulting in permeability decreases of cement pastes. Furthermore, a modified Carmen-Kozeny relation was established which enables prediction of the permeability from capillary porosity and the critical pore diameter. @en