ABSTRACT The formation of thaumasite from limestone cement in different sulfate concentration was investigated. The limestone cement consists of 70% clinker with very low sulfate content and 30% pure calcium carbonate. The pastes were cured for 4months in different concentrations of magnesium sulfate solution 1, 2, 4, and 6%. After specific times the reactions were terminated and the surfaces of samples scratched and analyzed by infra-red spectroscopy, x-ray diffraction and scanning electron microscopic analysis. Also the PH and sulfate concentration are measured with time. Analysis indicates that thaumasite can be obtained with low sulfate content at PH 10-10.5 and the rate of its formation increased with the increasing of sulfate concentration in solution where is formed after 60 days from curing in 4%.