Microstructure Evolution Mechanism of Quaternary Phase Paste Containing Metakaolin and Silica Fume

Abstract

To promote the sustainable development of high-performance calcium aluminate cement (CAC)-based material and the efficient utilization of supplementary cementitious materials (SCMs) in CAC, the evolution mechanism of the microstructure of quaternary phase (Q phase, Ca20Al26Mg3Si3O68) paste containing metakaolin (MK) and silica fume (SF) was investigated. The strength of SF-blended paste decreases with time when cured at 40 °C, while that of MK-blended paste shows sustained strength gain. The microstructure of outer products (OPs) for SF-blended paste suffers from significant damage when the age increases from 3 to 28 d, whereas that for MK-blended paste exhibits no significant difference, showing a dense morphology. The microstructure evolution of OP is mainly controlled by the reactions between SCMs (i.e., MK and SF) and early metastable hydrates (i.e., CAH10 and C2AH8). The dissolved silica from SF reacts with CAH10 to form C2ASH8 but cannot completely hinder its conversion to C3AH6, while the preferentially dissolved alumina from MK can stabilize CAH10 due to the common-ion effect. As the rate of conversion of C2AH8 to C3AH6 is much faster than that of CAH10, the rapid conversion of C2AH8 cannot be effectively suppressed by MK or SF.