Effects of metakaolin on autogenous shrinkage of cement pastes

The effects of partial replacements (5%, 10%, 15% and 20%) of Portland cement by high-purity metakaolin (MK) on the autogenous shrinkage of pastes (water/binder ratios 0.3 and 0.5) were investigated. In order to distinguish the effect of heterogeneous nucleation and pozzolanic activity from the dilution effect, some mixes were prepared using a coarse powder (Qref) instead of MK (10% and 20% for both w/b ratios). The hydration of cement–MK pastes was followed qualitatively by differential thermal analysis (DTA). DTA showed that C–S–H and gehlenite (C2ASH8) were the main compounds produced by MK pozzolanic reaction. Results showed that the long-term autogenous shrinkage of cement–MK pastes, for both w/b ratios, decreased as the cement replacement level by MK increased. No expansion was observed at early ages, contrary to the findings of other authors. With the elimination of the dilution effect, it was shown, at early ages, that the increase of autogenous shrinkage of the cement–MK pastes relative to cement–Qref mixes was due to heterogeneous nucleation. At later ages, autogenous shrinkage became lower for cement–MK pastes than for cement–Qref pastes, surely because the pozzolanic activity of MK became predominant. This behavior i.e. reduction of autogenous shrinkage, is one more benefit confirming the interest of using MK in concrete.