Mechanisms of water retention in cement pastes containing a self-curing agent

Self-cure concrete contains a chemical agent that reduces the evaporation of water from its surface, primarily by reducing the vapour pressure at the concrete pore solution surface. The self-curing agent developed at the Concrete Technology Unit, University of Dundee, also produces an alteration in cement hydration product microstructure, and it was considered that this may also contribute to the improved water retention properties. To investigate this, weight loss measurements were conducted on both self-cure and ordinary pastes exposed to controlled ambient conditions, whilst thermogravimetric analysis was carried out on identical specimens. It was found that, whilst the evolution of heat of hydration renders the early stages of drying very complex, it was possible to examine the diffusion dependent stage of drying. The diffusion coefficients observed for water vapour passing through the dry region of the self-cure paste surface were much lower than those observed for the control. This has been attributed to two mechanisms: the lower vapour pressure above the pore solution leading to a smaller difference across the dried portion of the paste and lower relative humidities in the cement pores, and the change in microstructure which reduces permeability.