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

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 degrees 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 C3AH(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.