This paper aims at giving new insights into the impacts of the particle sizes of diatomite waste on the hydration properties of Portland cement from the perspective of quantitative analysis using Rietveld refinements method. XRD identification coupled with Rietveld quantitative phase analysis, as well as isothermal calorimetry and SEM observation, was performed. The results show that the cement hydration during the induction period does not show any significant differences with the decrease in the sizes of the diatomite waste particles; however, hydration during the acceleration period was significantly enhanced. Finer diatomite waste particles (d50 < 1070.9 nm) result in a significant increase in the amounts of ettringite (AFt) and calcium hydroxide (CH) during acceleration period of the cement. Moreover, diatomite waste incorporation contributes to accelerate the early formation of monosulfate (AFm); however, the formation of the AFm was not favorable as the sizes of the diatomite waste particles decrease. The improvement on the hydration of the Portland cement was largely affected by the particle sizes effects of the diatomite waste at the very early stage, and then by the reactivity of diatomite waste at the later stage.