The acceleration mechanism of nano-C-S-H particles on OPC hydration

In recent decades, a novel concept of accelerating hydration process of cement has been proposed by adding external nuclei into a hydrating cement system, which is called seeding technology. Nano-sized C-S-H particles are prepared and characterized by means of dynamic light scattering (DLS), TEM, XRD, FT-IR and TGA. It has been realized that polycarboxylate superplasticizer (PCE) often leads to lower early strength of hardened cement paste (hcp) due to its retardation action, which is undesired for pre-fabricated concrete and repair mortars industry. The mechanism of nano-C-S-H seeds accelerating cement hydration containing PCE is investigated by isothermal calorimetry. The mortar strength growth with inclusion of seeds is investigated at every curing age of 1 d, 3 d and 28 d. Results show that nano-C-S-H particles significantly promote the early strength development for mortar containing PCE without obvious reduction of 28 d strength. The threshold pore size of hcp is also significantly decreased. The addition of nano-sized seeds decreases [Ca] in the pore solution, which is needed by C-S-H nucleation. More importantly, this paper brings a direct evidence that nano-C-S-H seeds provide more nucleation sites for further generated CS-H precipitate by separated hydration method. C-S-H gel is identified as the substance growing on nano-C-S-H seeds by FT-IR, XRD and TGA. The separated hydration result firmly indicates that homogeneous nucleation is irrelevant in the blank hydrating cement pastes and the C-S-H hydration product is able to grow on the surface of the added foreign C-S-H seeds. (C) 2020 Elsevier Ltd. All rights reserved.