Sodium sulfate is a known activator to accelerate the hydration of anhydrite [1, 2]. This work aims for a better understanding of the hydration process of natural anhydrite and the effect of sodium sulfate on this reaction. Besides the natural anhydrite from underground mining, we used a synthetic anhydrite for comparison. The use of two different anhydrites is also supposed to show the discrepancy of lab models and reality. The key questions of this study are: what is the mechanism of sodium sulfate accelerating the hydration of anhydrite and why does it vary for different types of anhydrite? Both anhydrites (natural and synthetic) were characterized for their particle size distribution, morphology, and chemical and mineralogical composition. Anhydrite was hydrated in batches mixed with a liquid to solid ratio of 1.5. We used deionized water and sodium sulfate solution (concentration of 33.33 g/L) as liquid phase, to investigate the effect of sodium sulfate on the hydration process. QXRD and heat flow calorimetry results show, that the sodium sulfate solution accelerates the hydration process both of natural and synthetic anhydrite compared to deionized water. In addition, it could be shown that synthetic anhydrite shows higher and faster hydration rates compared to natural anhydrite in both liquid phases considered. To elucidate the accelerating effect of sodium sulfate on anhydrite hydration we used highly diluted systems (liquid to solid ratio of 3704 – approx. ten times below saturation). The concentration of the sodium sulfate solution was set to 0.0135 g/L. Measurements of electrical conductivity show the amount of ions dissolved and therefore enables monitoring of the dissolution of anhydrite over time from an early stage on. The electrical conductivity results showed no acceleration of the dissolution of anhydrite in the presence of sodium sulfate solution compared to deionized water in highly diluted systems. On the contrary, when sodium sulfate is used as the liquid phase, the equilibrium is reached slower compared to deionized water. Consequently, sodium sulfate seems not to accelerate the dissolution process of anhydrite but it is assumed to facilitate the formation of gypsum. This will be verified in oversaturated systems in a next step of the project. Literature: [1] G. Losch, Zur Wirkung von Additiven auf die Hydratationsreaktion von Anhydrit II, in: T.U.B. Freiberg (Ed.), 2018. [2] E.Z. Carlos Lillo-Polo, Pedro Garcés, Eva G. Alcocel, Optimizatio of the activation process for natural anhydrite, Zement Kalk Gips, (2021) 28-37.      «

Sodium sulfate is a known activator to accelerate the hydration of anhydrite [1, 2]. This work aims for a better understanding of the hydration process of natural anhydrite and the effect of sodium sulfate on this reaction. Besides the natural anhydrite from underground mining, we used a synthetic anhydrite for comparison. The use of two different anhydrites is also supposed to show the discrepancy of lab models and reality. The key questions of this study are: what is the mechanism of sodium su...     »