In the present study, early-age heat evolution of over 120 mortar mixes was monitored for 24 hours after mixing using a simple isothermal calorimetry device. This paper describes the processes of calorimetric characterization and performance prediction of these mortar materials. The features of the heat evaluation curves obtained from the calorimetry tests and the effects of the mortar materials, mix proportions, and curing conditions on the heat evaluation curves were studied. A derivative method was employed for exemplifying the physical changes in the tested materials from the heat evolution curves, and the results were compared with set times from ASTM C403 tests. The study indicates that the simple calorimetry technique can be used to flag changes in cementitious materials, pre-screen concrete materials and/or mix design, identify incompatibility of cementitious materials, and forecast setting time and early-age strength.