Abstract: A method and system for determining the free cement content of concrete plant waste water held in a pit or tank P in which the temperature of the water in the pit or tank and the temperature of water in a reference pit or tank R.sub.1, is measured by thermocouples (T.sub.1, T.sub.2), the difference in temperature .DELTA.T determined and used to calculate an estimate of the free cement in the waste water whereby the reduced amount of cement to be used in a concrete batching process (without compromising the strength of the concrete product) can be determined.