Extenders can affect concrete curing

The effect of cement extenders is a key consideration when curing concrete to achieve optimum results. Bryan Perrie, CEO of Cement & Concrete SA (CCSA), expands on technical points to note.

Bryan Perrie, CEO, Cement & Concrete SA

The use of fillers, as well as pozzolanic and hydraulic cement extenders, is now commonplace in South Africa, with cements containing materials such as: - fly ash, derived from the combustion of pulverised coal in thermal power plants - ground granulated blast-furnace slag

(GGBS), a secondary product from the iron manufacturing process - condensed silica fume (CSF), a by-product of the smelting process, used to produce silicon metal and ferro-silicon alloys - finely ground, but not chemically processed, limestone. “The use of these secondary materials has become popular – in fact, encouraged – because of environmental and economic benefits. However, when added to concrete made with Portland cement, these materials can affect the physical and chemical properties of the cement paste phase and consequently respond differently to curing conditions,” Perrie states.

He says when the extenders are included in the cement at the cement factory, the performance of the cement can be adjusted and is therefore different to cases where extenders are added at a readymix plant or on-site.

“Dealing with the effect of extenders and fillers briefly and generally, fly ash and condensed silica fume – when mixed with cement and water – react with the calcium hydroxide released when cement hydrates in water. A pozzolanic reaction follows, which produces a low-density calcium-silicate-hydrate gel that enhances the binding action of the cement phase in the concrete,” he explains.

Strength and durability

More importantly, however, 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.

Perrie says that, in addition to the pozzolanic reaction of fly ash in concrete, fly ash’s particle shape and size aid

Concrete curing