Micro-Raman study of stable and metastable phases in metakaolin/Ca(OH)2 system cured at 60 °C

Abstract

The kinetics of hydration reactions between MK and Ca(OH)2 depends directly on the cementing matrix and curing temperature. For example, as the temperature increases in the metakaolin (MK)/calcium hydroxide system causes formation of the metastable hexagonal phases (C2ASH8 and C4AH13) that evolve into a thermodynamically stable cubic phase belonging to the hydrogarnet family (C3ASH6). This transformation will reduce the volume (approx. 9%) and a loss of durability of these cementing matrices. Although it is known that there are hydrated phases involved in the reaction, there is still an important scientific gap which occurs during the intermediate steps. For this reason, the micro-Raman spectroscopy is used for the first time as a suitable technique in order to study the conversion reaction in the MK/Ca(OH)2 system cured at 60°C. The results show that at short curing periods C–S–H gel as well as metastable phases with different AlO45− environments are present; this is the first time that these metastable phases have been observed by Raman spectroscopy. As hydration proceed C–S–H gel decreases, with a total disappearance at 30h. After longer hydration stages the metastable phases transform to stable hydrogarnets identified by two broad bands in the OH and AlO4 vibration region respectively (3670–3650cm−1 and 520–540cm−1) with no C–S–H gel formation.