| Summary: | Different binders affect the mechanical properties of the concrete differently. Previous results show that optimum 15% of thermally activated alum sludge (TAASA) replacing cement (C) increase the compressive strength of concrete, and reversed for fly ash (FA) at 45% and more. This paper investigates the optimum binders' ratio (total binder 25%) to produce highest compressive strength at 7, and 28 days. The ratios of FA: TAASA are S0-Control (0:0), S1-(4:0) S2-(3:1) S3-(2:2) S4-(1:3) and S5-(0:4). The cement used is a CEM II/B-L 32.5 N Portland composite. Results show that the slumps of all mixes decreased, with lowest for S1 (single binder FA). For the compressive strength (7 and 28 days), hybrid FA and TAASA with equal amount of 12.5% (S3) shows the highest, and all mixes' strengths are above the control (S0). The strength increase of S3 is 58%, that is, from 22 MPa at 7 days to 29 MPa at 28 days. The flexural and split tensile of S3 are also higher than control (S0) at both ages. Hybrid FA and TAASA binders can produce superior mechanical properties concrete when the correct ratios are used compared to single binder replacement or all-cement binder concrete. © Published under licence by IOP Publishing Ltd.
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