Adding extenders to the concrete mix offers several important advantages.

The practice of adding ground-granulated blast-furnace slag, fly-ash or silica fume to the concrete mix offers important advantages, according to Bryan Perrie, managing director of The Concrete Institute.

Apart from cost savings that result from extenders generally being cheaper than Portland cement, extenders improve impermeability and durability of hardened concrete and often enhance the properties of concrete in its fresh and hardened state. As most extenders are secondary products from other processes, their use also plays a role in reducing greenhouse gas emissions.

Fine filler effect
“Cement extenders are less reactive than Portland cement,” notes Perrie. “This reduces the rate of early-age strength gain which affects the rate of heat development due to cementing reactions.

“Extremely fine extender particles act as nuclei for the formation of calcium silicate hydrate, which would otherwise form only on the cement grains. This 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,” he explains.

He adds that concrete which contains an extender produces heat at a slower rate than similar concrete made with only Portland cement, which reduces the likelihood of thermal cracking.

“However, 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.

The Concrete Institute
Tel: 011 315 0300
Website: www.theconcreteinstitute.org.za