The impact of slag fineness on the reactivity of blended cements with high-volume non-ferrous metallurgy slag

Abstract

Non-ferrous metallurgy slags, when highly amorphous, have been shown to behave as supplementary cementitious materials, albeit typically with low reactivity. Due to this feature, high-volume replacement of ordinary Portland cement (OPC) with non-ferrous metallurgy slag is not encouraged. A classical method of increasing slag reactivity is by increasing the fineness of the slag, and this was investigated in the present work. To do so, a non-ferrous metallurgy slag was milled to specific surface areas of 4500 cm2/g and 6500 cm2/g, as determined by the Blaine permeability method. The influence of the fineness on the slag reactivity was investigated using the RILEM R3 isothermal calorimetry procedure. Results demonstrated that the reactivity of these slags was similar to that observed for siliceous fly ashes and increases with slag fineness. Moreover, isothermal calorimetry on blends of OPC with 30, 50 and 70 wt% slag of both finenesses was conducted to investigate the reaction kinetics. A retarding effect of the slag on the Portland clinker hydration was observed, which became more pronounced for higher slag fineness. Finally, mortar compressive strength was tested at 2, 7 and 28 days and compared with the calorimetric results and with thermogravimetric analysis, used to determine the bound water content, at 28 days. At late ages, the increased reactivity for higher slag fineness led to more bound water in hydration products and consequently higher compressive strengths. Moreover, strength gain at later ages due to higher slag fineness might compensate for strength loss due to higher replacement levels. However, at early ages, the strength remained low and increased slag fineness did not yield significant compressive strength enhancement; early strength enhancement likely requires different activation methods.