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. LAY DESCRIPTION The paper discusses the role of silica fume on katoite formation during early hydration of C4AF. Rapid dissolution of C4AF, formation of metastable C‐(A,F)‐H and its conversion to C3(A, F)H6 was evidenced by isothermal calorimetry. Changes in microstructure during early hydration were documented by SE micrographs and X‐ray mapping and line scans by SEM‐EDS. The phase composition after 7 days of hydration was characterised by DTA‐TGA and QXRD. The high mobility of Al and Ca and the low mobility of Fe in the relic microstructure is discussed in the paper. Although katoite has the similar diffraction pattern for reference sample and sample with silica fume, different degrees of substitution in its structure can be seen in the silica fume system using SEM X‐ray microanalyses.