15233 Blast Furnace Slag

Blast-furnace slag (see Figure 15.4) is a glassy material that is made by rapidly quenching molten blastfurnace slag and grinding the resulting material into a fine powder. It is composed essentially of silicates and aluminosilicates of calcium. When it is ground to cement fineness, it is referred to as ground granulated blast-furnace slag (GGBFS), and it is commonly used in HPC mixtures. GGBFS is classified by ASTM C 989 (AASHTO M 302) according to its level of reactivity. Depending on the desired properties, the amount of GGBFS can be as high as 80% of the total cementitious materials content.

The use of GGBFS reduces the permeability of the mature concrete. It is believed that this improvement is a result of the reaction of the GGBFS with the calcium hydroxide and alkalis released during hydration of the Portland cement. The reaction products fill the pore spaces in the paste and result in a denser microstructure. In addition to reducing the permeability of concrete, GGBFS also improves resistance to sulfate attack because of the low calcium hydroxide content. Like fly ash, GGBFS is also used to reduce the temperature rise in mass concrete. GGBFS improves the workability of fresh concrete. It is believed that the smooth, dense surfaces of the slag particles (Figure 15.4) result in very little water absorption during the mixing process.

FIGURE 15.4 Ground granulated blast-furnace slag particles.

In North America, slag is not as widely available as in Europe and Asia, and hence there is not much information available as to its performance in high-strength concrete. However, the indications are that, as with fly ash, slags that perform well in ordinary concrete are suitable for use in high-strength concrete, at dosage rates between 15 and 30%. The lower dosage rates should be used in the winter, so that the concrete develops strength rapidly enough for efficient form removal. For very high strengths in excess of 96 MPa, it will likely be necessary to use the slag in conjunction with silica fume.

The chemical composition of slag does not generally vary very much. Therefore, routine quality control is generally confined to Blaine specific surface area tests, and x-ray diffraction studies to check on the degree of crystallinity (which should be low).

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