Mechanical, hygric and thermal properties of gypsum produced from different raw materials

Calcined gypsum is a historical binder that was used already several thousands years ago. Nowadays, gypsum is produced from various raw materials. The natural gypsum as a source for calcined gypsum production is not prevailing any more and new sources appear mostly as secondary raw materials. Flue gas desulphurization gypsum as waste material from thermal power plants and chemo-gypsum as waste material from production of phosphoric acid, hydrofluoric acid, citric acid and boric acid are the most typical representatives of these sources. In this paper, -form calcined gypsum produced using three different types of raw materials is studied. The first material is flue gas desulfurization gypsum with purity higher than 98 % of calcium sulfate components. It is produced at the electric power plant Počerady. The second is "white" gypsum produced at Gypstrend Inc. – Kobeřice using chemo-gypsum. The other is "grey" gypsum from the same company produced mainly using natural gypsum. The water/gypsum ratios corresponding to the normal consistence according to ČSN 72 2301 are chosen. Normal consistence is determined using the slump test. The standard slump corresponding to the normal consistence is 180±5 mm. At first, basic parameters of the studied gypsum materials, namely hydration heat and initial and final setting times are measured. The process of hydration heat generation at room temperature of about 25 °C is monitored using the isothermal heat flow calorimeter KC 01. The reaction heat flow data are analyzed and integrated to obtain the hydration heat. The initial and final setting times are measured by Vicat device according to ČSN 72 2301. As for mechanical properties, compressive strength and bending strength are determined. Among the hygric properties, apparent moisture diffusivity is calculated utilizing the results of water sorptivity measurements, water vapor diffusion permeability is measured by the cup method, sorption isotherms are determined by the common desiccator method. Thermal conductivity and specific heat capacity as representatives of thermal parameters are measured by an impulse technique.