Potential of weathered blast furnace slag for use as an addition in concrete

The paper investigates the potential for recovering granulated blast furnace slag after four to five years’ storage in a stockpile (weathered) for use as an addition in concrete. The initial research physically and chemically characterised fresh and weathered granulated slag. Thereafter, studies on ground materials in paste and mortar were carried out. The weathered granulated slag was similar to fresh slag in terms of particle size, shape and elemental composition. However, there was greater roughening of particle surfaces, with various weathering products forming. Following grinding, fresh slag comprised angular particles covering a range of sizes, while finer particles in weathered slag included fragmented reaction products. In cement paste, weathered slag gave reductions in chemically bound water. In mortar, this showed little difference in flow properties compared to fresh slag, with reductions in compressive strength and increases in porosity also noted. Further analyses suggest that, at equal Blaine fineness, weathered slag (a) is actually coarser than fresh material, affecting particle packing and giving larger capillary pores, and (b) has lower reactivity due to reduced surface area. The practical implications are examined and approaches to using weathered slag in concrete suggested.