Chemical and microstructural investigations on slag hydration products
Extensive literature reviews are presented in this thesis with respect to the hydration of slag, especially on the mechanism of hydration. The range of work in the thesis includes: 1) compressive strength testing on slag cement pastes and slag pastes with other alkaline activators; the effect of curing temperatures and slag compositions were specially discussed. 2) Analysis of pore fluid from slag/NaOH pastes. 3) Porosity and pore structure analysis of slag cement pastes as a function of age. 4) Phase development in hydrating slag or slag cement pastes. 5) Microstructural and microchemical development of slag hydration products. Electron microscopy has shown that several chemically and microstructurally distinct zones quickly develop in a hydrating slag cement paste. Initially, a dense layer of gel-like hydration product forms around slag grains. The microstructure and chemistry of the gel are not constant, but evolve with time. Microstructural evolution is manifested by the crystallization of the previously formed gel hydrate, with the formation of a hydrotalcite-like phase. This crystallization is accompanied by distinctive chemical evolution, in which Ca, Si and some A1 migrate into the outer matrix; however, Mg appears to be virtually immobile. The evolution results in the creation of pores in the in-situ slag hydration zones, and at the same time, the marked densification of the outer matrix. Mass balance calculations are used to support the microstructural observations and to generalize on them, so the extent of the densification potential can be assessed. The ability to calculate the potential for densification, at least in principle, is regarded as an important step forward in the design for durability. A theory, based on the microstructural observations, is proposed to account for the differences between the calculated and observed porosities.