Phase assemblage of cement pastes with SCM at different ages

Abstract

For economical and ecological reasons, supplementary cementitious materials (SCM) are increasingly used in concrete. Although having a lot of advantages, concretes with SCM have different cement matrices and do not have the same evolution as a function of time as CEM I concretes. In this paper, the microstructure of various cement pastes with/without SCM (slag GGBS, fly ash FA and metakaolin MK), has been investigated as a function of the curing time.The microstructure was characterized not only by usual techniques such as XRD and TGA-DTA, but also by 29Si and 27Al NMR spectroscopy. The use of a combination of techniques for microstructural characterization allowed to quantify the proportion of each cementitious phase. This quantification showed the evolution of the cementitious matrices as a function of the binder content and of the curing time. For example, the alumina content in the C-S-H and the average length of C-S-H chains are higher for cement pastes with SCM than for OPC ones. Hydrated alumina phases proportions (AFt, AFm and TAH) are also higher in SCM cement paste. For MK and FA cement pastes, in addition to pozzolanic reactions, it seems that a small part of calcium from clinker hydration is used to form these aluminate phases.