How much slag cement do I include in a concrete mixture?
The amount of slag cement added to a concrete mixture, as a percentage of cementitious material by weight, normally ranges from 20 to 80 percent. The amount for a specific project depends on several factors including application, early and later age strength requirements, durability requirements and ambient temperature to name a few. For information on proportioning and a table of suggested percentages for various applications and end results, see SCIC #2, Concrete Proportioning.
Most general or structural concrete applications (flatwork, paving, foundations, walls, columns, floors, etc.) typically use between 25 and 50% slag cement. Optimum slag cement percentage for maximum strength development is generally between 40 and 50 percent. A specification based on concrete strength at 28 days may be able to use less total cementitious material (portland + slag cement) than a similar plain portland mixture is mixture strength is optimized.
If durability parameters are specified (e.g. permeability, sulfate resistance, alkali-silica reaction (ASR) resistance) up to 70 percent slag cement may be required. For instance, one combination of portland and slag cement, in combination with a highly reactive aggregate may need as much as 70 percent slag cement to mitigate ASR, while a less reactive aggregate, combined with a lower alkali cement and slag cement may only need 25 percent.
Precast and prestressed concrete is an excellent application for slag cement at levels between 20 and 50 percent. If heat curing is used (or ambient curing temperatures relatively high) higher levels within this range are common. Using higher levels of slag cement in precast, provides the additional benefits of a more flowable mixture with smoother surface finish (fewer bugholes), and a whiter concrete appearance, often favored by designers and architects.
Mass concrete utilizes the highest potential levels of slag percentage to provide effective heat mitigation and reduced thermal stress. Mass applications use between 50 and 80 percent slag cement, with the thickest, most massive placements normally in the 65 to 80 percent range. Some slag cements in massive placements may not provide sufficient heat mitigation at the 50 to 65 percent levels because slag cement is activated by heat, and may react faster than desired because of the heat generated by portland (and slag) hydration.