Carbonation of Calcium-Silicate-Hydrate Gel: Elucidation of Atomic Structure Mechanisms and Reaction Kinetics Using Pair Distribution Function Analysis

Calcium-silicate-hydrate (C-S-H) gel is the main binder phase in Portland cement-based concrete, and therefore the chemical degradation of this phase is an important factor controlling the material’s long-term durability. This is especially relevant for blended cements containing supplementary cementitious materials (SCMs), where additional C-S-H gel precipitates at the expense of portlandite. Furthermore, aluminum-substituted C-S-H gels (known as C-A-S-H gels), which exist in certain blended cements and alkali-activated materials, possess similar local bonding environments and nano/micron morphologies to C-S-H gel, and therefore the structural changes and reaction kinetics pertaining to C-S-H gel degradation is highly relevant to other cement-based systems. Here, carbonation of C-S-H gel has been studied using in situ X-ray pair distribution function (PDF) analysis, revealing the exact atom-atom correlations present in this gel together with the structural changes that occur during decalcification and formation of the amorphous silica gel. The results reveal that calcite formation in carbonation of C-S-H gel is the rate-limiting factor, with excess dissolved calcium and carbonate species in solution (local supersaturation) leading to the formation of vaterite. Due to the existence of non-crystalline phases during the carbonation reaction (C-S-H gel and amorphous silica gel), quantification of the phase evolution together with reaction kinetics using reciprocal-space Rietveld analysis leads to an overestimation of vaterite formation during the initial stages of reaction, and at later stages of reaction the additional calcite formation occurs via vaterite dissolution. On the other hand, real-space PDF analysis, which does not require assumptions to be made regarding the structure of the non-crystalline phases, reveals that during the later stages of reaction additional calcite formation occurs at the expense of the continually decalcifying C-S-H gel. Hence, this investigation highlights the advantages of PDF analysis for investigating complex reactions such as carbonation of C-S-H gel, with particular emphasis on determination of the local structural changes and associated reaction kinetics.