INVESTIGATIONS OF HYDRATION PRODUCTS IN HIGH-VOLUME FLY ASH BINDERS

Binder systems containing large amounts of fly ash have become interesting for the building material industry because of their relatively low cost, the high resistance against aggressive solutions, durability against alkali-silica reaction, low hydration heat and ecological friendliness. The hydration mechanism of these systems differs from that of neat portland cement pastes, mainly in type and amount of solid hydration products and the composition of pore solution. It was the aim of this work to compare the hydration reactions of binder systems containing fly ashes from brown coal combustion and portland cement in mixture proportions of 60% fly ash/40% portland cement with corresponding 70/30, 85/15 and zero fly ash mixtures. Investigations of the solid hydration products by high resolution solid 29Si MAS NMR spectroscopy, molybdate method, DTA/DSC and the chemical analysis of the pore solution showed that the pozzolanic reaction of the ash leads to a decrease of the hydroxide ion concentration in the pore fluid and the amount of solid calcium hydroxide, and increased formation of aluminosilicate hydrates. When all the Ca(OH)2 is consumed, the sulfate ion concentration in the pore solution begins to rise. The influence of the activity of fly ash, the mixture ratio of fly ash/portland cement, and the hydration time on the chemical structure of the C-S-H and C-S-A-H phases is discussed.