To understand how slag cement compares with standard Portland cement, one needs to understand the composition of Portland cement. Portland Cement is made up of four main crystals; Tricalcium Silicate, Dicalcium Silicate, Tricalcium Aluminate, and Tetracalcium Aluminoferrite (more commonly known as C3S, C2S, C3A and C4AF respectively). The C4AF contributes little or no strength to the mixture and serves as more of a flux in the cement manufacturing process.

The C3S and C3A crystals provide concrete with its early strength as the two crystals react with water. The C3A crystals contribute strength from the concrete’s initial set to 3 days. The C3S crystals provide strength to the concrete from 1 to 7 days after the initial set. Concrete gets its long-term strength from the C2S crystals, which provide strengths at 14 days and beyond. This is why cements high in C2S are often used in dams and other large structures where higher long-term strengths are desired.

Cements with higher percentages of C3S and C3A achieve higher strengths within the first week after placement, but have lower ultimate strengths. On the other hand, cements with higher percentages of C2S will be slower to gain strength, but will ultimately gain more strength in the long run and will provide the concrete with much better durability.

Slag Cement is similar to the C2S phase of Portland cement. It gains strength at a slower rate preliminarily, but continues to gain strength for a much longer period of time. Consequently, when a portion of the Portland cement is replaced by slag cement, the concrete is able to achieve a higher ultimate strength and better long-term durability.

Fly Ash is also used in Metro Ready Mix’s industry-leading concrete mixes. Fly Ash is a byproduct of coal-fired electric power generation. It is formed as inorganic particles in the coal are captured in the exhaust and condense into microscopic glassy spheres. Fly Ash is an effective supplement to Portland Cement in that its physical shape, when added to concrete mixes, acts like a lubricant to the cement paste, giving the concrete mixture better workability. While Fly Ash is not a hydraulic cement (it cannot react directly with water the way Portland Cement and Slag Cement do), it can react with the intermediate reaction products from Portland and Slag Cements. For this reason, the addition of Fly Ash to concrete is generally limited to 20 to 30% of the total mixture. As long as Fly Ash is used judiciously, the final product can be stronger, more durable, less expensive and more environmentally friendly.

For more information on slag cement please visit the Slag Cement Association at www.slagcement.org