Mechanisms of action of sulfates on concrete: factors of chemical and physical resistance.

This paper deals with physical and chemical factors that determine the intensity of the impact of sulfates on concrete. Samples - cylinders of cement paste and prismoidal specimens of the standard mortar with different binders and values of water-cement ratio were kept in solutions with different sodium sulfate concentrations at 8 and 20°C. After a certain time the samples of cement stone were studied over the entire depth by X-ray diffraction and 27Al and 29Si-NMR spectroscopy. This allowed a quantitative assessment of changes in crystalline and X-ray amorphous phases, in particular, C-(A)-S-H. It was established that ettringite was formed, and in the case of high sulfate concentrations - gypsum. Reducing the water cement ratio, or using fly ash make it possible to compact the structure and reduce the diffusion of sulfate ions in the tested samples so that the sulfate effect would not cause any damage. Ettringite and gypsum formation are considered the main cause of deformations, but no direct link between the amounts of newly formed ettringite and gypsum and the damage to the flat prisms was found. Porosity, phase stability and other factors do play an important role.

Section: Waste utilization

Keywords: concrete, sulfate, ettringite.