Low-cost embedded lead zirconate titanate (PZT) piezoelectric transducers, fabricated with piezoelectric ceramic discs and mixtures of cement powder, epoxy resin and curing agent, has been developed and used as sensors for continuously monitoring early hydration of cementitious materials in situ under different temperatures. The influence of slag incorporation and curing temperature on the hydration process of cementitious materials is investigated based on the ultrasonic wave transmission velocity obtained from the experiments. It is found that, unlike ordinary Portland cement paste, the hydration process of slag blended cement paste could be divided into five typical stages. With the increase of the curing temperature, the fourth stage, which is related to the pozzolanic reaction of slag, becomes more evident. The initial setting time obtained from Vicat needle test corresponds well to the first inflection point of the ultrasonic pulse velocity (UPV) curve, while final setting time is corresponding to the point of maximum rate of velocity change. A good exponential relationship between UPV and compressive strength of cementitious paste is found when the pastes are cured at 20 °C and 30 °C. However, linear relationship between UPV and compressive strength is found when the curing temperature is 50 °C.