Curing of concrete affected by cement extenders
It is important to consider the effect of cement extenders in a concrete mix when curing concrete, says Bryan Perrie, MD of The Cement Institute.
Perrie says the use of fillers and pozzolanic and hydraulic cement extenders is now commonplace in SA with a large number of available cements containing materials such as:
•Fly ash (FA), derived from the combustion of pulverised coal in thermal power plants;
•Ground granulated blastfurnace or corex slag (GGBS or GGCS), 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; and
•Finely ground but not chemically processed limestone.
“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.
“When these materials 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 readymix plants or on site.”
“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 from 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 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 concrete, positively affecting placeability, bleeding and long-term strength. Good-quality FA also has a fine-filler effect, stimulating hydration of the cement up to 28 days – an important consideration for concretes in which large proportions of cement are replaced by extenders.
“On the other hand, GGBS is considered to be a latent hydraulic material that will hydrate in the same way as cement when mixed with water. So if GGBS 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 hydration rate of GGBS is considerably slower than that of the cement.
It is thought that GGBS 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 GGBS.
“Since the major contribution of GGBS to the binding effect of the cement phase is through hydration, GGBS concretes are sensitive to inadequate curing at early ages because of this slow hydration rate. 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, resulting in an increased density of the cement paste phase. This boosts impermeability and 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.”
“When mixed with portland cement and water, finely ground limestone is chemically virtually inert (although there may be some minor reactions). Depending on its fineness, limestone may act as “fine filler” in fresh paste. The effect of limestone on the properties of concrete or mortar depends on the specific limestone, whether a grinding aid is used in production, and the fineness of the limestone.”
“In general, extended cements are more sensitive to poor, late or inadequate protection and curing and require more effective curing over longer periods to ensure optimal performance of the concrete.”
More information from Bryan Perrie, Tel: +27(0)113150300/www.theconcreteinstitute.org.za