Effect of superplasticizer addition time and metakaolin source on the early-age hydration of limestone calcined clay cement (LC3)

It is known that superplasticizer (SP) presence and addition time play an important role in the hydration of Portland cement (PC). However, this is not clear yet for limestone calcined clay cements (LC 3 ). Besides, the role of metakaolin characteristics in the early hydration of LC 3 is not fully understood, especially in the presence of SP. In this work, PC and LC 3 pastes were produced without and with SP added directly in the mixing water or with a 10-min delay. LC 3 were produced with two different metakaolin sources. Isothermal calorimetry and in-situ X-ray diffraction (XRD) were conducted for 48 h. Results showed that LC 3 hydration was marginally affected during the first two days by SP incorporation, contrary to PC systems where SP delayed the hydration kinetics. Furthermore, direct SP addition delayed the conversion of the intermediate short-range ordered C-(A)-S-H into the known long-range ordered arrangement in both PC and LC 3 systems, while delayed SP addition prevented it. Calcined clays showed pozzolanic activity within the first two days of hydration in LC 3 samples, but SP incorporation delayed the pozzolanic reaction onset by around 9 h. SP hindered the crystallization of portlandite in both PC and LC 3 systems, which was more evident for PC.