SCMs impart a wide range of exceptional properties to concrete, which are particularly sought after by construction professionals. While the advantages to concrete in its plastic state are quite numerous (workability, finishability, etc.), perhaps the greatest benefits imparted by SCMs can be seen in the hardened properties.
Enhanced strength—SCMs contribute to the strength gain of concrete. Typically, slag cement and fly ash will lower early strengths (1 to 14 days) but significantly improve long-term strengths (28 day and beyond), depending upon the proportions and materials used. For example, Class F fly ashes tend to have a slow strength gain curve contributing mainly to the strength beyond 28 days, whereas silica fume contributes primarily to strengths at 3 to 28 days. Both compressive and flexural strengths can increase markedly at 28 days and beyond with the addition of SCMs.
Reduced permeability—SCMs significantly extends the life of concrete by reducing permeability to chlorides and other aggressive agents, especially at later ages. Silica fume has a very profound effect on permeability, exhibiting as much as a five-fold reduction in permeability. In concrete structures, permeability is generally the critical factor affecting durability.
Resistance to alkali-silica reaction (ASR)—SCMs can prevent excessive expansion and cracking of concrete due to ASR. The amount of slag cement required depends on the nature of the slag cement, the reactivity of the aggregate, and the alkali loading of the concrete. In most cases, 50% slag cement is sufficient with highly reactive aggregates. The amount of fly ash required typically is in the range of 15 to 55%, depending on the chemical composition of the ash, reactivity of the aggregate, and the alkali loading of the concrete. Generally, Class F ashes are much more effective in controlling expansion due to ASR than Class C ashes. Silica fume can control ASR; however, the amount required generally results in poor constructability. Blends of slag cement and silica fume, as well as blends of fly ash and silica fume, have a synergistic effect in mitigating expansion due to ASR, while producing a very workable concrete.
Resistance to sulfate attack—Sulfates, which are present in seawater, wastewater, and some soils, can react with the alumina in portland cement, causing expansion. SCMs offer superior resistance to these attacks because they contain fewer of the compounds that react with sulfates and because their low permeability keeps sulfate-bearing waters out. Typically, slag cement, silica fume, and Class F fly ashes are very effective in improving sulfate resistance. The effectiveness of Class C fly ashes is dependent on the ash chemistry and the replacement level.
Resistance to thermal stress—If the temperature differential between the concrete’s surface and interior is too high, cracking and loss of structural integrity can result. High replacement levels of slag cement and/or fly ash in properly proportioned mixes can reduce the peak temperatures as well as the rate of heat generation. Reducing the heat of hydration of the mix can moderate the development of thermal stresses within the concrete and prevent cracking.
The environmental movement in general—and strong interest with LEED certification in particular—will continue to drive the demand for sustainable construction materials that lower the carbon footprint of our built environment. One proven solution for reducing greenhouse gases per ton of cement—without sacrificing performance—is portland limestone cement (PLC).