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Variable amounts of dehydrated or partly dehydrated calcium sulfate phases in the dry mortar and different crystal habits of the phases are the reasons for a variable reactivity of the mixture. For example, the low degree of hydration of the sample fired for 5 hours at 600 °C is possibly due to the radial fibrous structure of anhydrite crystals. The degree of hydration after 3 days and 1 year was determined gravimetrically and by X-Ray diffraction (Rietveld method), respectively, for samples fired at different temperatures and for different times. The results are summarized in Table 1.

As stated above the fired material is a variable mixture of components with different hydration reaction rates (Glasenapp 1910). Unlike α- and β-hemihydrate, anhydrite III and low fired anhydrite II which hydrate very quickly, high fired anhydrite II reacts very slowly with water. This retarded reaction of anhydrite II can produce cracks in the plaster if the mortar fabric is very dense and cannot compensate the volume increase of the hydrated calcium sulfate phase. Therefore the hydration of high fired anhydrite II has to be activated by a suitable chemical such as citric acid or potassium sulfate. Experiments and analyses proved superfine calcium hydoxide Ca(OH)₂ to be a suitable activator to prevent a retarded hydration of high fired anhydrite II. The linear expansion coefficient of the high fired gypsum injection mortar measured after 280 days was reduced to less than 2 mm/m. The compressive strength of the set mortar measured after 28 days was 6.1 MPa and decreased to 2.9 MPa after storage of the samples under water. No cracking was observed in test walls which were built in 2016 and injected with this mortar. Obviously its properties are favourable for the repair and reinforcement of the historic masonry and it is now successfully applied since two years.

Table 1: Degree of rehydration of fired gypsum as a function of firing temperature and time.

Run No.	Firing temperature	Firing time	Reaction time	Degree of hydration
1	200 °C	–	3 days	85 %
2	200 °C	–	365 days	~100 %
3	600 °C	5 hours	3 days	4 %
4	600 °C	24 hours	3 days	15 %
5	600 °C	–	365 days	94 %
6	800 °C	5 hours	3 days	13 %
7	≥ 800 °C	> 5 hours	3 days	1 %
8	1,100 °C	5 hours	365 days	72 %