Abstract: A system for forming a reaction product such as calcium silicate comprises an autoclave for receiving the reaction constituents and for reacting these constituents to form a reaction product, a holding vessel for receiving the reaction product and a flow passage connecting the autoclave to the holding vessel and for allowing the passage of the reaction product from the autoclave to the holding vessel. The flow passage includes a heat exchanger for transferring heat from the reaction product during its passage through the flow passage to another medium. The system includes an electronic control system for maintaining the pressure in the holding vessel a controlled amount beneath the pressure in the autoclave during the transfer of the reaction product from the autoclave to the holding vessel. This minimizes the structural damage to the reaction product during its transfer to the holding vessel. The holding vessel can, if desired, be used to mix additional material into the reaction product.