Effect of Fine-Ground Silica Marl on the Structure and Properties of Composite Gypsum Binder

The article presents the results of a study of fine-ground silica marl (SM) with a specific surface area of 500 m²/kg as an active mineral additive in the composition of a composite gypsum binder (CGB). The phase composition, microstructure, surface morphology and grain size of SM were determined. The heat release kinetics of CGB with mineral additives SM and SM+HMK-45 upon contact with water was studied. The concentration of active adsorption centers on the particle surface of SM and HMK-45, used as a mineral additive, which have a positive effect on the physical and mechanical properties and the composition of CGB hydration products, was established. The required amount of fine-ground SM in the CGB composition was determined experimentally, which makes it possible to reduce the CaO concentration in the samples at 5 and 7 days (according to TU 21 31 62 89). To achieve the required reduction in CaO concentration, a metakaolin additive HMK-45 (0.5% by weight) was additionally introduced into the CGB composition with fine-ground SM, which made it possible to increase its water resistance and durability. The rational ratio of CGB components (parts by mass) was adopted as «cement : SM : HMK-45» = 1 : 1 : 0.2. Comprehensive studies of phase transformations occurring during the hardening of CGB with highly dispersed mineral additives SM and HMK-45 were carried out. In the studied samples at the age of 28 days, traces of portlandite and ettringite were detected, which indicates a high degree of completion of the hydration processes of the cement component of the gypsum-cement binder and the activity of the mineral additives. Based on the tests, the feasibility and expediency of using fine-ground SM with a specific surface area of 500 m²/kg (if necessary, together with highly active HMK-45) as a mineral component in CGB was established. This is due to its pozzolanic activity – the ability to bind Ca(OH)₂ with the subsequent formation of low-basic calcium hydrosilicates and other sparingly soluble new formations, providing increased density, strength and durability of the modified gypsum-cement stone, eliminating subsequent self-destruction of its structure.