Limestone is the major raw material for cement production. Limestone needs to satisfy certain specifications regarding the composition, as laid down by the Bureau of Indian Mines, to qualify for cement production. The presence of impurities in limestone beyond the specified limits causes the rejection of enormous quantity of limestone in cement manufacturing companies. This limestone, commonly known as low-grade limestone, may also have the potential to replace clinker partially in cement. Using ground limestone powder of high purity to replace cement has proved very promising. The addition of finely ground limestone in cement accelerates early hydration by providing extra sites for nucleation. In addition, it helps to form new hydration products known as carboaluminates by chemically combining with reactive alumina present in the binder. The seminar discusses the results of a study conducted to understand the hydration behaviour of cementitious blends containing finely ground low-grade limestone. Limestone samples collected from various mines in India were used in the study. Hydration studies were conducted for multiple replacement levels of limestone. The effect of the fineness of limestone powder on the hydration of cement also was studied.