Optimizing Early Compressive Strength of Ultrafine POFA-Based Green HSC: An Experimental and RSM Approach with Silica Fume and Hybrid Fibers

<ns7:p> Recently, studies have focused on producing green, sustainable concrete with improved environmental performance, utilizing recycled waste materials. The early compressive strength of Green High Strength Concrete (GHSC) containing up to 60% ultrafine palm oil fuel ash (UPOFA) has been optimized utilizing hybrid materials, Silica Fume (SF), steel fiber (ST), and Polyethylene Terephthalate (PET) fiber. UPOFA was used as a substitute binder for cement in GHSC manufacture at 0%, 30%, and 60% replacement levels. SF was substituted for 0%, 10%, and 20% of the residual cement mass. Steel fiber and PET fiber were included at a rate of 1% of the total binder mass. The mix design parameters were optimized using the Response Surface Method (RSM) with a central composite design (CCD) approach. The experimental results showed that the GHSC containing 30% UPOFA, 20% SF, 1% ST, and 1% PET fiber reached greater strength at 3, 7, and 28 days. The incorporation of 20% SF, 1% ST, and 1% PET fibers enhanced the early ages strength reduction of 60% UPOFA-GHSC by 17%, 25%, and 18% at 3, 7, and 28 days, respectively, relative to the control mix. The constructed models showed significant correlation values (R <ns7:sup>2</ns7:sup> ) of 0.9697, 0.9546, and 0.9674 for compressive strength at 3, 7, and 28 days, respectively. Hence, RSM may be an efficient technique for improving mix design while lowering environmental impact by limiting waste volume and energy usage in cement manufacture, which contributes to the sustainability of concrete. </ns7:p>