Aqueous suspension containing Ca(OH)(2), Al(OH)(3) and gypsum (CaSO4. 2H(2)O) powders was subjected to ultrasound irradiation under atmospheric pressure to investigate its effect on synthesis of ettringite (3CaO . Al2O3. 3CaSO(4). 32H(2)O). irradiation of ultrasound to the suspension enables us to form ettringite in the product, and the sonochemical synthesis reaction reaches the equilibrium by about 120 min. Addition of excess amount of Ca(OH)(2) powder is effective in consuming the remained gypsum in the product, leading to the high yield of ettringite. Heating the suspension at about 80 degrees C, which is the same degree in the ultrasound irradiation, for 480 min produces tri-calcium aluminum monosulphate (CAM). The formation of CAM plays a significant role in delaying the synthesis reaction of ettringite in the suspension.