Behaviour and Properties of Lime-Pozzolan Pastes

Abstract

Lime pozzolan mortars have been used by numerous ancient civilizations of builders, and are now regaining popularity as environmental sustainable alternatives to cements for masonry and concrete applications. This paper studies the relationship between the composition and physical characteristics of nine pozzolans and their corresponding reactivity, water demand, setting times and mechanical properties in lime/pozzolan pastes. A diverse selection of artificial pozzolans were investigated including Ground Granulated Blastfurnace Slag (GGBS); Leca; Pulverised Fuel Ash (PFA); Calcined Clay (MetaStar); Microsilica (MS); Rice Husk Ash (RHA); Red Brick Dust (RBD); Tile and Yellow Brick Dust (YBD). Each pozzolan was found to have a particular water demand for a given workabililty that was related to the particle surface area. It was concluded that the pozzolan surface area has a much greater influence on the water demand of the paste than its particle size or the lime:pozzolan ratio. It appears from this research, that the effect of the water content of the paste on the setting times surpasses the influence of the reactivity of the pozzolans. It was also demonstrated that the particle size of the pozzolan critically affects the set of the paste, both the coarsest and the finest pozzolans set the fastest; and there is a critical particle size, ranging between 63 and 250um, for which the set slows down significantly. Finally, it was also noted that pozzolanic reaction takes place at variable speed for different pozzolans, and that there can be instances when hydration can be slower than carbonation.