The impact of Al2O3 amount on the synthesis of CASH samples and their influence on the early stage hydration of calcium aluminate cement

Abstract In this work the impact of Al2O3 amount on the synthesis (200 °C; 4–8 h) of calcium aluminium silicate hydrates (CSAH) samples and their influence on the early stage hydration of calcium aluminate cement (CAC) was examined. It was found that the amount of Al2O3 plays an important role in the formation of calcium aluminate hydrates (CAH) because in the mixtures with 2.7% Al2O3 only calcium silicate hydrates (CSH) intercalated with Al3+ ions were formed. While in the mixtures with a higher amount of Al2O3 (5.3–15.4%), calcium aluminate hydrate – C3AH6, is formed under all experimental conditions. It is worth noting that the largest quantity of mentioned compound was obtained after 4 h of hydrothermal treatment, in the mixtures with 15.4% of Al2O3. It was proved that synthesized C3AH6 remain stable up to 300 °C and at higher temperature (945 °C) recrystallized to mayenite (Ca12Al14O33), which reacted with the rest part of CaO and amorphous structure compound, resulting in the formation of gehlenite (Ca2Al2SiO7). Moreover, the synthesized C3AH6 addition induced the early stage of CAC hydration. Besides, in the samples with an addition, the induction period was effectively shortened: in a case of pure CAC (G70) paste, hydration takes about 6–6.5 h, while with addition – only 2–2.5 h. The synthesized and calcinated compounds was characterized by using XRD and STA analysis.