Heat conduction calorimetry is a convenient method to detect cement-admixture incompatibility. However, such instruments tend to be used mainly in chemical laboratories. This paper presents the design of a simple unthermostated heat conduction calorimeter that can simultaneously monitor the heat production rate in 14 samples of cement paste or mortar. As it is not thermostated, its performance is governed by the ambient temperature stability and the heat capacity balance between each sample and its reference. The paper shows that the instrument can be used to detect cement-admixture incompatibility and to quantify retardation time.