Effect of Temperature on the Hydration of White Portland Cement–Metakaolin Blends Studied by ²⁹Si and ²⁷Al MAS NMR

Metakaolin can be considered as a model compound for an aluminosilicate-rich supplementary cementitious material (SCM) because of its composition and highly pozzolanic properties. This work presents a systematic investigation of the hydration of white Portland cement (wPc)–metakaolin (MK) blends cured at different temperatures by ²⁹Si and ²⁷Al MAS NMR with the main focus on the structure and composition of the C-(A)-S-H phase. White Portland cement–MK paste samples with wPc replacement levels of 0, 10, 20, and 30 wt% have been prepared and hydrated for 1–8 weeks at five different temperatures (5, 20, 40, 60, and 80 °C). Information on the degree of reaction for alite, belite, and MK is derived from the ²⁹Si NMR spectra and shows that the hydration of the wPc–MK blends is accelerated with increasing temperature except for the blend with the highest MK level (30 wt%). The Al_IV/Si ratio of the C-(A)-S-H phase is found to be independent on the hydration time but increases slightly with curing temperature. On the other hand, a significant increase in the average aluminosilicate chain lengths is observed for blends with increasing temperature, reflecting a decrease in the Ca/Si ratio for the C-(A)-S-H phase. Strätlingite is present in the wPc–MK blends at 20 °C and the fraction of this phase decreases with increasing curing temperature.