Thermodynamic Modeling of Portland Cement—Metakaolin—Limestone Blends

Abstract

The partial replacement of Portland cement by different supplementary cementitious materials (SCM´s) has been investigated extensively in recent years with the aim of reducing the embodied CO₂ of blended Portland cements. In this work, we have utilized the maximum cement substitution of 35 wt%, according to the standard EN 197-1, and investigated the effect of changing the metakaolin/limestone ratio on the hydrating phase assemblages. Paste samples of the hydrated cement blends have been characterized by XRD, ²⁷Al and ²⁹Si MAS NMR spectroscopy and the results are compared with thermodynamic modeling. ²⁹Si MAS NMR is a very valuable technique for studies of hydrated cement blends, since it allows detection of amorphous and crystalline phases in an equal manner. The determined degrees of hydration have been implemented into thermodynamic modeling to improve the modeling approach and thereby the agreement between predicted and observed phase assemblages. A simple equation has been established for implementation of the hydration kinetics which employs only one mass and one dissolution-rate parameter to describe the hydration successfully. The agreement between the experimental and modeled phase assemblages improves significantly when the hydration kinetics for the anhydrous alite, belite, and amorphous MK phases are implemented. The phase assemblages of the hydrated blends change only for very high MK contents from a C(-A)-S-H, calcite, portlandite, monocarbonate and ettringite system to a phase assemblage that in addition contains strätlingite and other AFm phases.