Summary This paper presents a study of a cement-based solidification/stabilization process of storm water runoff solid residuals by non-conventional differential thermal analysis (NCDTA). The study was used to investigate the early hydration stages of a type I Portland cement containing the raw residual, two fractions of the residuals (coarse and fine), and two additives (quicklime and sodium bentonite). During these stages the fine fraction of the residuals retards the hydration reactions more than the coarse one, and both fractions have components that are reactive during the hydration process. When sodium bentonite is present in the pastes, the higher the initial cement content of the pastes, the lesser is the reactivity of the residuals. The presence of quicklime, which undergoes simultaneous highly exothermal hydration, accelerates the cement hydration reactions as well as those due to the presence of the residual solids. In these quicklime-containing compositions, the effect of sodium bentonite is similar to that when no quicklime is added, except when the whole residuals are used.