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