The effect of calcination temperature on the pozzolanic activity of corn straw leaf ash (CSLA) was evaluated. The CSLA samples calcined at temperatures of 500, 700, and 850 °C were mixed in a portlandite solution for 6 h, and residual samples were obtained. The CSLA and residual samples were analyzed using Fourier transform infrared spectroscopy, X-ray powder diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and a contact angle goniometer to determine the vibration bonds, minerals, phase composition, microstructure, Si 2p transformation behavior, and wetting behavior. The conductivity and loss of conductivity with mixing time of the CSLA-portlandite mixed solution was determined. The loss of conductivity of the CSLA prepared at 500 °C was high compared to that of the other calcination temperatures at the same mixing time, which was attributed to the higher amorphous SiO2 content in the CSLA at 500 °C. Calcium silicate hydrate was easily identified in the CSLA residual samples, and some dense small cubic and nearly spherical shaped calcium silicate hydrate particles were found in the CSLA residual samples at 500 °C. Based on the findings, it is recommended that CSLA be calcined at 500 °C using the cement system in view of higher pozzolanic activity but avoiding excessive agglomeration.