Thermodynamic modeling of Portland cement – metakaolin – limestone blends: potential factors affecting the mechanical properties

Metakaolin is used in this study as a model compound for calcined clays, which are promising supplementary cementitious materials for future cements. A white Portland cement was replaced by 35 wt.% of metakaolin and limestone in four variable proportions (MK/(MK+LS) = 0.66, 0.75, 0.94 wt.%). These different metakaolin / limestone blends have been used to elaborate the so-called synergetic effect of calcined clays and limestone.
The phase assemblages were characterized by powder XRD, 27Al and 29Si MAS NMR after 1, 2, 7, 14, 28 and 182 days of hydration. The degrees of hydration for the principal phases, alite, belite, and metakaolin, based on 29Si MAS NMR, were utilized to model the hydration of the binders. An exponential function has been developed, which describes the hydration for each anhydrous phase, to model the hydration as a function of time. This approach was found to reduce the differences between equilibrium thermodynamics and experimental phase characterization.
The predicted and observed phase assemblages are used to identify other performance indicators. The predicted phase changes support the interpretation that ettringite is stabilized by the formation of monocarbonate, which reduces the predicted pore volume. In addition, larger amounts of calcium-silicate-hydrate (C-S-H) phases, with lower Ca/Si ratios, are predicted for increasing metakaolin levels. The presence of limestone suppresses the formation of straetlingite in the investigated binders for the studied time of hydration, when sulfate and carbonate anions are deficient, which otherwise stabilize ettringite and monocarbonate.

Author: W. Kunther, Z. Dai, J. Skibsted

Section: Science and production

Keywords: thermodynamic modeling, metakaolin, limestone, white Portland cement