Assessment of long-term water resistance of modified pressed gypsum composites

Abstract

Introduction. The most important indicator of the durability of gypsum materials is their water resistance, which is most often assessed by the degree of strength reduction after short-term water saturation. In our view, the durability of composite building materials based on gypsum and other air binders can be more accurately predicted based on the results of longer laboratory tests, which include prolonged immersion of control samples in water, as well as an assessment of their ability to restore their original strength after re-drying. The aim of this study is to assess the effect of a complex modifier, consisting of a monosubstituted salt of orthophosphoric acid and a fine carbonate filler, on the water resistance properties of pressed gypsum composites after prolonged immersion in water. Methods. The technical characteristics of the studied composites were determined using standard methods with control samples manufactured by pressing. The long-term water resistance of the material was assessed based on the change in the values of the softening and water resistance coefficients, calculated based on the results of testing control samples stored for 1, 7, 28 and 90 days in water. Results. We show that the high-strength fine-crystalline structure of pressed gypsum binder without modifying additives is characterized by extremely low strength, both during short-term and long-term immersion of control samples in water, as well as reduced ability to restore its original strength after drying. This is revealed in the comparison of the water resistance indicators of the material, even with similar characteristics of cast samples made from pure gypsum binder, and especially with pressed samples from composite binders containing the proposed complex modifier. We established that gypsum-modified pressed composites are characterized by fairly high durability during prolonged immersion in water. After 90 days of testing, the softening coefficient of the studied compositions of composite binders ranged from 0.62 to 0.64, and the water resistance coefficient ranged from 0.90 to 0.95. This indicates the possibility of using products based on them in building enclosures, as well as in rooms with humidity above 75 %, provided that the water resistance of the studied pressed composites is maintained for a long time only if complete hydration of the gypsum binder is ensured.