Long-time monitoring of the microstructural change in hardening cement paste by SANS

Hydrating cement paste is an essential part of concrete because its structure largely determines the physical properties of concrete (e.g. its strength and durability). Small-angle neutron scattering (SANS) investigations were used to obtain a non-destructive description of statistically representative microstructures (nanometre and micrometre scale). The results obtained illustrate the possibilities of using SANS to monitor the time-dependent microstructural changes that occur during the hydration process. All samples were made from ordinary Portland cement (OPC) with a water/cement ratio of 0.38. By controlling the volume ratio of D2O in the hydration water (C-dw), a variation in contrast is achieved. In the special case of C-dw = 0.38 the scattering length densities of the hydration products are nearly homogeneous and the contrast between them is drastically reduced. In order to study the influence of the hydration conditions, the sample thickness, temperature, the size fraction of the cement clinker and C-dw were varied. The hydration products and inner surfaces formed by the separated microstructural objects were found to show a fractal behaviour. The fractal dimension obtained from the scattering curves reflects the microstructural changes that occur during the hydration process within the size range of a few nanometres up to a few micrometres, depending on the chosen hydration parameters. In this size range the SANS effect is dominated by the hydration products.