The purpose of this article is to study the effect of ground limestone additives on the basic physical and mechanical properties of the composite gypsum binder. It includes portland cement and pozzolanic additive of thermally activated clay as hydraulic components to reduce the consumption of more expensive calcining components of the binder. Using standard methods of research building materials we have obtained dependencies that characterize the effect of adding ground limestone on the water demand of the composite gypsum binder, its density, strength, water absorption, and water resistance of artificial stone based on it. It was established that the introduction of limestone into the composite gypsum binder composition with specific surfaces 500-800 m2/kg in an amount of up to 5-10 % by weight causes an increase in compressive strength by 15 % compared to control samples. When introducing up to 10-15 % by weight of limestone with a dispersion of 300 and 500-800 m2/kg, respectively, the strength remains at the level of additive-free samples with the preservation of the softening coefficient corresponding to water-resistant binders.