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This effect is due to the reorganization of the lattice network, corresponding to anhydrous calcium silicates with the low Ca/Si ratio. DSC thermograms of cement containing 15 % SF exhibit a typical exothermal effect for the presence of C-S-H (I) in pastes cured for 1, 3, 7, and 28 days, with and without superplasticizer. The general shift of the peak of this exotherm towards lower temperatures with hydration time suggests that a higher quantity of the CSH(1) with lower C/S is formed. It is generally thought that the product resulting from the pozzolanic reaction (Eq. 2) consists of C-S-H that has a Ca/Si ratio values between 0.9 and 1.3 [7]. Conclusion This paper investigates the effect of silica fume and polycarboxylic ether based superplasticizer on the hydration process of commercial Portland cement. Without superplasticizer, the quantities of Ca(OH)2 after 1 day of hydration of silica fume blended cement paste are lower than for pure cement paste, thus indicating pozzolanic reactivity of silica fume with Ca(OH)2. The superplasticizer significantly retards the nucleation and growth of hydration products. With superplasticizer, both cement hydration and pozzolanic reaction are retarded. The addition of SF acted as a strong accelerator, even in the case of superplasticizer addition. The high retardation of the hydration by small addition of Glenium superplasticizer was hypothetically explained by a steric hindrance of the long side chains linked to the polymer backbone providing a physical barrier on the reacting surface of cement and SF particles. On the other hand, the acceleration of hydration by SF particles may be attributed to a more permeable shell of hydration products forming around the dissolving clinker particles due to the extreme fineness of the SF particles that provides high surface for new preferential nucleation sites. DSC showed crystallization around 880 oC in the SF-modified cement pastes. This exothermic effect corresponds to anhydrous calcium silicates with a low Ca/Si ratio. The general shift of this peak towards lower temperatures with hydration time suggests that a higher quantity of the CSH with lower C/S is progressively formed. Acknowledgement Work was supported by Marie Curie Actions EU grant FP7-PEOPLE-2010-IEF-272653-DICEM. Refrences
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