Composition of C - S - H in pastes with 1 increasing levels of silica fume addition

10 New results show that the microstructure development of cement-silica fume blends is very different 11 from plain cement. portlandite (CH) tends to precipitate as platelets and even around clinker grains as 12 “CH rims” and is consumed by pozzolanic reaction with silica fume. The Ca/Si ratio in the inner 13 product (IP) C-S-H decreases as CH is consumed to reach Ca/Si ≈ 1.40-1.50 at the point when CH has 14 disappeared, and then drops down to 1.00 in absence of CH. At later ages, the IP C-S-H is often 15 composed of two distinct regions. The outermost (formed first) consists of originally high Ca/Si C-S-H, 16 which Ca/Si slowly decreases. The second (formed later) forms only once CH is no longer present and 17 has a lower Ca/Si. Between 10 and 38°C, the main effect of increasing the temperature is to accelerate 18 the reaction of cement and increase the reactivity of silica fume. The changes in Ca and Si in the pore 19 solution of similar systems suggest that the composition of the solution and the solids reciprocally 20 influence each other. 21