Early hydration of C₄AF with silica fume and its role on katoite composition

C 4 AF is considered the least reactive main clinker phase, but its reactivity may be affected by adding supplementary cementitious materials (SCMs). Pure C 4 AF was synthesised in a laboratory furnace, and the role of silica fume without gypsum on its early hydration properties was monitored. Burning was carried out in four stages to achieve 99% purity of C 4 AF. Heat flow development was monitored by isothermal calorimetry over 7 days of hydration at 20°C and 40°C. The role of silica fume on hydrogarnet phase katoite (Ca 3 Al 2 (SiO 4 ) 3 – x (OH) 4 x x = 1.5–3) formation during early hydration was studied. Rapid dissolution of C 4 AF, formation of metastable C‐(A,F)‐H and its conversion to C 3 (A, F)H 6 was evidenced by isothermal calorimetry as a large exotherm. Changes in microstructure during early hydration were documented by SE micrographs, EDS point analyses, X‐ray mapping and line scans by SEM‐EDS. The phase composition was characterised by DTA‐TGA and QXRD after 7 days of hydration. The katoite diffraction pattern is similar for the reference sample and sample with silica fume, but substitution in its structure can be revealed by X‐ray microanalyses. The composition of katoite is variable due to the various extent of substitution of 4OH − by SiO 4 4− due to silica fume.