The aluminosilicate microspheres are a light fraction of furnace waste generated during the combustion of conventional coals. The planned research concerns the determination of the hydration heat of cement mortars, without and with 1.5%, 3.5% and 5% of aluminosilicate microspheres (cement substitute) with the use of an isothermal calorimeter for two selected grain fractions of the analyzed research material. The study of the heat of hydration will be carried out for summer conditions, temperature: 23oC with the use of the 3-channel TAM Air isothermal calorimeter (TA Instruments). The implementation of the tests concerns the performance of the compressive strength of the prepared cement mortars stored in the water environment at a temperature identical to the temperature of the hydration heat determination. The obtained research results will allow determining the influence of the selected fractions of aluminosilicate microspheres on the rate of dynamics of mechanical strength development. The obtained results of experimental tests will allow determining whether there is a correlation between the amount of aluminosilicate microspheres and the heat of hydration of cement mortars, and how their presence changes its value. The results of experimental tests and numerical calculations will allow the assessment of the influence of the fraction of aluminosilicate microspheres on the hydration process of the prepared samples. The implementation of the project will allow for the development of new research methods in the field of concrete technology and will positively influence the growth of cooperation between the Gdansk University of Technology and the industry.