This paper reports on the effect of the temperature used to activate kaolinite-based paper industry waste on the hydration products obtained when the activated waste was mixed with a lime solution. After activation at temperatures ranging from 500° to 700°C, clay waste exhibited high pozzolanic activity. Analysis with instrumental techniques such as X-ray diffraction, differential thermal analysis, infrared spectroscopy, and scanning electron microscopy showed that higher temperatures favored the formation of calcium aluminate hydrates, hydrotalcite-like compounds, and strätlingite, while at lower temperatures, CSH gels constituted the majority hydrated phase. According to these findings, reaction kinetics differed substantially from the results obtained at temperatures of 700°–800°C, particularly in terms of phase quantity and timing of appearance. This study shows that metakaolinite can be obtained from clay waste at temperatures of under 700°C. The hydrated phases forming under these conditions are the same as more observed at higher activation temperatures (>700°C). The main differences found were the sequence of phase formation and the quantity of hydrates detected.