Pozzolanic cements

Abstract

Natural and artificial pozzolanas have been used to obtain hydraulic binders for over a thousand years. Hardening of pozzolanic cement pastes can result from the reaction between pozzolana and the lime that is added to the mix as hydrated lime or is produced following hydration of portland cement silicates. The pozzolanic reaction does not alter cement clinker hydration; it complements and integrates the hydration process because it results in a lower portlandite content and an increase in calcium silicate hydrates. Besides reviewing the most recent investigations on pozzolana-containing cements, this paper shows that the behaviour of different types of pozzolana can be quite similar when they are blended and become hydrated along with portland cement clinker. Portland cement properties may undergo several qualitative modifications the extent of which substantially depends on the pozzolana/clinker ratio. So, a maximum is reached in pozzolanic cements. As in the case of pozzolanic cements, for which the current pozzolana content is about one third by weight of cement, the most outstanding variations induced in the behaviour of portland cement can be summarised as follows. Heat of hydration decreases whilst the rate of clinker hydration increases, paste porosity increases and permeability decreases, both portlandite content and Ca/Si ratio in C-S-H decrease and the C-S-H content increases. Chemical and physical properties of pozzolanic cements eventually affect engineering ones. Early strength of both pastes and concretes decreases while ultimate strength is often found to exceed that of the reference portland cement. If cements contain small amounts of very active pozzolana (silica fume, for example), both early and ultimate strengths may be higher than those of the substituted cement. Creep is found to increase definitely with increasing pozzolana content whereas shrinkage remains practically unaffected. Chemical and microstructural variations in the paste also influence resistance of concretes to environmental attacks. The low basicity and permeability resulting from the presence of pozzolana increase the concrete's resistance to lime leaching, sulphate and sea water attacks, and chloride penetration. Carbonation depth is practically unaffected. Pozzolana containing cements can help avoid expansion induced by alkali-silica reaction. Concrete resistance to freezing is not affected by the use of pozzolanic cement since it basically depends on the entrained air content. The results of a variety of studies introducing a comparison between pozzolana-containing cements and corresponding portland cements can be summarised as follows: cements with appreciable pozzolana contents perform better in the long term rather than at an early age. In most cases, however, the differences between the two types of cements are not so marked and as a consequence both cements are interchangeable especially for the most common building types.