Slag or Flyash – What’s the difference?

In short, not a lot.

Either material can be used in combination with cement to produce blended and specialist cement products. Concrete producers can add either of them to improve specific concrete properties such as workability, sulphate resistance, heat of hydration or durability.

For the Western Australian market, slag is more commonly used due to the limited supply and varying quality of flyash in the region.

While the names vary depending on country of origin and local abbreviations, BGC Cement and many of its customers generally refer to slag as GGBFS (ground granulated blast furnace slag) or BMS (bulk milled slag).

GGBFS is a dried and finely ground by-product of the blast-furnace iron-making process, while flyash is the fine ash residue collected from the waste gases of power generating plants. What makes these products particularly valuable is that they both develop cementitious properties when they are activated by the reaction between the cement and water mix. The compounds formed by these reactions not only bind the concrete together, but also help block the pores in the concrete. This improves durability, making it more resistant to chemical attack.

Either material can be used as a cement replacement and whether slag and/or flyash is used depends mainly on the application and local availability. Higher addition rates of either slag or flyash usually increase initial setting times and reduce very early strength development when compared with straight cement mixes. However, strengths at 28 days and beyond are usually better when appropriate amounts of slag or flyash are added and the concrete is properly cured. While flyash is typically used at replacement rates up to 30%, GGBFS is often used at rates of up to 40% in General Blend (GB) products and 65% in Low Heat (LH) cement.

BGC Cement uses and supplies slag and provides a variety of pre-blended products for various applications.

GB is comprised of approximately 70% General Purpose (GP) cement and 30% slag. This blend gives additional benefits to plastic and hardened concrete. It is commonly used for road stabilisation and in the mining sectors for paste fill. GB has similar 28-day setting strengths to GP, but has a slightly longer final setting time.

LH is comprised of approximately 35% GP cement and 65% slag. This blend is ideal for use in mass concrete structures where rate of temperature rise and maximum temperature achieved is controlled to reduce the risk of thermal cracking. LH cement meets the requirements of AS3972 for classification as a Sulphate Resisting (Type SR) cement. Another popular use for LH cement is mining paste-fill, due to its slower setting profile and similar 28-day setting strengths to GP.