Ground granulated blast furnace slag (GGBFS) is designated in ASTM C 989 and consists mainly of silicates and aluminosilicates of calcium. GGBFS is divided into three classifications based on its activity index. Grade 80 has a low activity index and is used primarily in mass structures because it generates less heat than portland cement. Grade 100 has a moderate activity index, is most similar to portland cement with respect to cementitious behavior, and is readily available. Grade 120 has a high activity index and is more cementitious than portland cement. To be used in cement, GGBFS must have the chemical requirements listed in Table 3.

Table 3. GGBFS Chemical Composition
Maximum Requirements (ASTM 989), %
Sulfide sulfur (S)
Sulfate ion reported as SO3

GGBFS can be used as a portland cement replacement ranging from 35%-70% by mass.


When used in portland cement, GGBFS offers the following advantages over unmodified portland cement:


Before using GGBFS as a portland cement replacement, several precautions must be realized. Since the granulated blast furnace slag is ground, it is not round. An admixture is needed to increase workability rather than increasing the water content. GGBFS only provides increased sulfate resistance or resistance to ASR if > 35% by mass is added. The time of set is slightly delayed by approximately 20 minutes, and early compressive strengths are decreased. GGBFS also causes increased drying shrinkage. When using this or any other alternative cementing material with portland cement, it is necessary to create trial mixtures to ensure proper proportioning for the desired properties.