Mechanical Characteristics of Fresh and Hardened Gluminous Cement Pastes

Abstract

In this research the rheological properties of fresh aluminous cement pastes were investigated mainly by means of a coaxial rotational viscometer. The effect of conversion on the pore structure of the hardened high alumina cement (HAC) pastes was monitored using a mercury intrusion porosimeter. A scanning electron microscope / KEVEX energy dispersive X-ray analysis combination, and in some instances a differential thermal analyser, were used to study the morphological changes in the interfacial zone between HAC pastes and various substrates. A brief study was made of the feasibility of using by-product phosphogypsum as a cheap additive to eliminate conversion effects in HAC pastes. The consistency of HAC fresh pastes was highly dependent on mixing time and temperature. The development of a thixotropic structure on mixing involves a thermally activated process for which the activation energy was estimated to be 43+ 1 KJ/mole. The sensitivity of HAC pastes to mixing and storage conditions made it essential to develop stringent procedures for specimen preparation, curing, and testing. Mineralogical and pore size distribution studies on hardened HAC pastes have revealed a number of relationships between these two factors and the strength of the material. In addition, the shape of the pore size distribution curves was found to be directly related to the mechanical and mineralogical conditions of the paste. Analytical and morphological studies of the interfacial zone developed between HAC pastes and various substrates indicated that this zone is seriously affected by the conversion process. In general, the paste side of the interface, which was initially largely made up of small platelets of probbable composition C2 (AF)H8,  was converted to gibbsite crystals of high variability of size and Shape. Crude phosphogypsum was found to be an unsuitable additive in HAC because of its acidity. The purified form, however, was comparable to natural gypsum in producing a cementitious material of slow setting properties at a relatively low ambient temperature, but which has proved to be reliable on subsequent exposure to a higher temperature.