The surface area of cement paste as measured by neutron scattering: evidence for two C-S-H morphologies

Small-angle neutron scattering was used to measure the effect of water-to-cement ratio (w/c) and cement batch variation on the surface area of ordinary Portland cement (OPC) paste after hydration for 28 days at room temperature, and to measure the development of surface area over the first 3 days of hydration at 30°C. The 28-day surface area was found to increase with w/c ratio in proportion to the volume of original water-filled space available for reaction product to form. The surface areas of different batches of type I OPC were quite similar, while that of a type II OPC was some 15% lower. Early surface area development at 30°C followed the heat evolution for the first 24 h of hydration and then leveled off, suggesting that further heat evolution was associated with reaction product, which added little to the surface area. These results support the theory that two different morphologies of the calcium-silicate-hydrate gel reaction product form during cement hydration.