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

C₄AF is considered the least reactive main clinker phase, but its reactivity may be affected by adding supplementary cementitious materials (SCMs). Pure C₄AF 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 C₄AF. 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 (Ca₃Al₂(SiO₄)_{3 – x}(OH)_4x x = 1.5–3) formation during early hydration was studied. Rapid dissolution of C₄AF, formation of metastable C-(A,F)-H and its conversion to C₃(A, F)H₆ 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 SiO₄⁴⁻ due to silica fume.