This research deals with improvement in the mechanical strength of Pavement Quality Concrete (PQC) when admixed with Rice Straw Ash (RSA) and Microsilica (MS). Nine mixes were prepared by partially substituting Ordinary Portland Cement (OPC) by MS (2.5%, 5%, 7.5%, and 10%), RSA (10%) and RSA-MS composite (5%–5%, 5%–7.5%, 10%–5% and 10%–7.5%). Maximum improvement was found when OPC was partially replaced by 7.5% in the case of MS and 5%–7.5% in case of RSA-MS composite. All the mix showed increased strength, w.r.t the control mix. X-Ray powder diffraction (XRD) and Scanning Electron Microscope (SEM) techniques were employed for characterization of the selected samples. Power regression equations were established to predict split tensile and flexural strength from compressive strength. They were compared with universally accepted equations and were found to be more accurate for RSA and MS admixed concrete. Mix R1M3 (5% RSA, 7.5% MS) is recommended based on the findings. © 2019 Elsevier Ltd