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, Ca 20 Al 26 Mg 3 Si 3 O 68 ) 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 MKblended 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., CAH 10 and C 2 AH 8 ). The dissolved silica from SF reacts with CAH 10 to form C 2 ASH 8 but cannot completely hinder its conversion to C 3 AH 6 , while the preferentially dissolved alumina from MK can stabilize CAH 10 due to the common-ion effect. As the rate of conversion of C 2 AH 8 to C 3 AH 6 is much faster than that of CAH 10 , the rapid conversion of C 2 AH 8 cannot be effectively suppressed by MK or SF.