Quantitative evaluation of hydrated cement modified by silica fume using QXRD, 27Al MAS NMR, TG–DSC and selective dissolution techniques

The pozzolanic reaction (PR) and quantitative influence of silica fume (SF) on the evolution of all main mineral phases in the SF-containing composite cement system (CCS) with low water to binder ratios were investigated in detail. Multiple methods including quantitative X-ray diffraction (QXRD), 27Al MAS NMR, thermoanalysis (TG–DSC) and selective dissolution techniques, were used comprehensively for quantitative evaluation of CCS. QXRD results indicated that SF accelerated the early hydration of C3S, C3A and C4AF, but delayed that of C2S; the addition of SF decreased slightly the hydration degree of CCS at 90 days on the whole. The mass content of ettringite by QXRD in CCS was shown relatively stable because of SF, which was confirmed by the data of 27Al MAS NMR. Moreover, the PR of SF identified by TG–DSC started at 1 day but proceeded very slowly after 3 days, which was testified by the results of selective dissolution of CCS. Furthermore, the decline of portlandite (CH) content that was quantified by both QXRD and TG–DSC occurred far later than the starting time of PR, implying that it is not suitable to mark conventionally the drop of CH content as a start of PR.