Wheat straw ash, a potential pozzolanic material, can contribute to the reduction of CO2 emissions by lowering the demand for Portland cement. The particle size of pozzolanic admixtures has a significant effect on the microstructure of the interfacial transition zone of concrete. In this study, concrete specimens were made with water-binder ratios of 0.35 and 0.55 and three sizes of wheat straw ash particles obtained by mechanical processing for various durations. The particle size of the wheat straw ash decreased as the processing duration increased. Specimens with an equivalent amount of silica fume were also made in order to make a comparison. The influence of the particle size of the ash and the water-binder ratio on the microstructure of the interfacial transition zone was investigated via compressive strength and microhardness tests, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The results show that increasing water-binder ratio caused a reduction in microhardness values. This effect was minimized with the addition of fine size wheat straw ash and silica fume particles. Additionally, incorporating fine ash particles prominently enhanced the microstructure of the interfacial transition zone. (C) 2019 Elsevier B.V. All rights reserved.