Numerical simulation of the PGNA signal from chlorine diffusion gradients in concrete.

A nondestructive test method for detecting chlorides in concrete has been developed based on prompt gamma neutron activation (PGNA). Its performance has been modeled using a hybrid MCNP/optical ray tracing approach. Since the chlorides often come from de-icing salts applied to the concrete surface, the Cl concentration has a non-linear depth profile which is typically modeled by the erfc function. The signals from this distribution have been simulated for several significant Cl capture peaks to estimate the erfc function parameters.

Hydration of tricalcium and dicalcium silicate mixtures studied using quasielastic neutron scattering.

Quasielastic neutron scattering was used to study the hydration reaction of tricalcium and dicalcium silicate mixtures by following the fixation of hydrogen into the reaction products, and by applying hydration models to the data. The reaction kinetics were well-described by an Avrami-derived model for the nucleation and growth regime during early hydration times and a diffusion-limited model for later periods. This study showed that the hydration reaction is not a simple linear combination of the reactions for the individual components. Compressive strength tests correlated with the neutron scattering data, suggesting that the details of the interaction affect the microstructure and therefore the strength of the product. Results suggest that favorable reaction mechanics provide optimal strength when an 80-95% tricalcium silicate and 20-5% dicalcium silicate mixture is used.