Carbonation of C-S-H gel and its Effects on the Carbonation Rate in Blended Cement Paste - An Experimental and Modelling Study

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.