Impact of high-volume steel slag with varying particle sizes on engineering properties of micro silica- modified concrete

The use of steel slag (SS) as partial replacement of natural fine and coarse aggregates and micro silica (MS) as partial replacement of cement was experimented with respect to engineering performance of concrete. In the present study, 0–70% of the natural sand and gravel, respectively, were replaced by fine steel slag (FSS) and coarse steel slag (CSS) as replacements on an absolute-volume basis. Also 10% MS was added to selected mixes in which 0%, 30%, 50%, and 70% SS was used. Workability, density, compressive strength, splitting tensile strength, flexural strength, sorptivity, ultrasonic pulse velocity (UPV) and performance in an adopted modified freeze–thaw exposure protocol were assessed. The SS-only mixtures showed the best compressive strength of 42.1 MPa which was obtained from the 40% SS mixture with no MS. However, when the SS replacement level increased, the compressive strength values decreased, even though the density of the concrete was increased, for example, to 70% SS, compressive strength was reduced to 31.8 MPa. The tested combinations of mixtures containing MS had 30% SS + 10% MS with the best overall combination of mechanical and durability related properties measured. The improvements observed herein are similar to those reported in literature for the filler and pozzolanic effects but were not directly ascertained in the present study by microstructural characterization, and thus specific changes in the interfacial transition zone, pore structure or C–S–H formation are not directly obtained. Likewise, the occurrence of possible volumetric instability and microcracking at high SS replacement levels are literature-supported interpretations, due to the absence of measurement of free CaO/free MgO contents and expansion. The results of the freeze-thaw test, based on the modified exposure protocol, are comparative in nature and do not represent standard performance. The results of a screening-level embodiment of carbon comparison showed that the 30% SS + 10% MS mixture has the highest calculated compressive-strength-to-embodied-carbon ratio of all the mixtures tested. The results show the engineering potential of moderate SS replacement and MS incorporation as well as the need for direct microstructural, expansion and life cycle evaluation in the future.