Company Announcement - Including extenders to the concrete mix offers several important advantages, says Bryan Perrie, managing director of The Concrete Institute. Among the benefits derived from the now widespread practice of adding ground granulated blast-furnace or corex slag, fly ash, or silica fume, to the concrete mix are :

•  Cost savings: Extenders are generally cheaper than Portland cement;
•  Technical benefits: Extenders improve impermeability and durability of the hardened concrete and often improve the properties of concrete in the fresh state.
• Significant benefits to reducing greenhouse gas emissions as most extenders are secondary products from other processes.

"Portland cement extenders are all less reactive than portland cement. This can affect the rate of early-age strength gain, causes a 'fine-filler' effect, which affects the rate of heat development due to cementing reactions. Substituting a Portland cement extender for part of the cement in a concrete mix,  may reduce the rate of strength gain at early ages. The extent of the reduction increases with increasing substitution level. "Extremely fine extender particles act as nuclei for the formation of calcium silicate hydrate which would otherwise form only on the cement grains. This fine-filler effect brings about a denser and more homogeneous microstructure of the hardened cement paste and the aggregate-paste interfacial zones, resulting in improved strength and impermeability. "The extent of the fine filler effect depends on the content of extremely fine particles in the extender. Fine particles of filler materials, such as limestone, can also exhibit the fine-filler effect," Perrie explains.

He says concrete in which part of the Portland cement is replaced by an extender produces heat at a rate slower than that of a similar concrete made with only Portland cement. The slower the rate of heat development, the lower the temperature rise and therefore the smaller the likelihood of thermal cracking.

"However, it should be remembered that improvements to the properties of hardened concrete brought about by the use of extenders, can be fully realised only if the concrete is properly cured," Perrie cautions.