Cement extenders offer many benefits to concrete mixes but it is important to consider the effect of the extenders on the curing of concrete, says Bryan Perrie, managing director of The Concrete Institute.
Perrie says the use of pozzolanic and hydraulic cement extenders is now commonplace in South Africa with portland cement being blended with materials such as:
- Fly ash (FA), derived from the combustion of pulverised coal in thermal power plants;
- Ground granulated blast-furnace slag (GGBS), and ground granulated corex slag (GGCS), secondary waste products from the iron manufacturing process; and
- Condensed silica fume (CSF), a by-product of the smelting process used to produce silicon metal and ferro-silicon alloys.
"The use of these by-product or secondary materials has become popular - and encouraged - because of environmental and economic benefits. However, when added to portland cement concrete, these materials affect the physical and chemical properties of the cement paste phase and consequently respond differently to curing conditions.
"Dealing with the effect of extenders, briefly and generally, FA and CSF when mixed with cement and water undergo reactions with the calcium hydroxide released when cement hydrates in water. A pozzolanic reaction follows which produces a low-density Calcium-Silicate-Hydrate(CSH) gel, contributing to the binding action of the cement phase in the concrete.
"More importantly, the hydrates produced in the pozzolanic reaction are deposited in the spaces between the gel formed around the cement particles, thereby reducing the size and the degree of interconnection of the pores. This improves the strength and durability characteristics of the concrete.
"In addition to the pozzolanic reaction of FA in concrete, fly ash's particle shape and size aids the compaction and density of the resulting concrete with positive effects on placeability, bleeding and long- term strength. Good quality FA also has a fine filler effect in stimulating the hydration of the cement up to 28 days - an important consideration for concretes in which large proportions of the cement is replaced by a cement extender.
"On the other hand, GGBS/GGCS is considered a latent hydraulic material that will hydrate in the same way as cement when mixed with water. So, if GGBS/GGCS is added to a concrete mix together with the cement, the presence of calcium hydroxide and other alkalis accelerates the hydration of the GGBS although the rate of rate of hydration of the GGBS/GGCS is considerably slower than that of the cement. It is thought that GGBS/GGCS also participates in a pozzolanic reaction, although to a considerably lesser extent than FA and CSF. The extent of its involvement in the reaction appears to depend on the composition of the slag.
"Since the major contribution of GGBS/GGCS to the binding effect of the cement phase is through hydration, GGBS/GGCS concretes are sensitive to inadequate curing at early ages because of this slow rate of hydration. With normal replacement (about 50%), particular care should be taken in curing the concrete during the early period after casting.
"CSF, because of its fine particle size, can fill the spaces between cement particles, which results in an increase in the density of the cement paste phase. This boosts impermeability of the concrete and also the durability of the concrete.
"With regard to curing, at normal replacements levels - where about 7% of the cement is replaced by CSF - the resulting concrete may be treated in the same way as portland cement concrete, but if the replacement is based on achieving equivalent strength, one part of CSF can be used to replace between three and five parts of Portland cement, depending on the strength level. In this case, CSF concrete is considerably more sensitive to initial curing and care should be taken to avoid surface drying," Perrie adds.
