Short‐term length change measurements and sorption isotherms for AAC compared to OPC

The interaction of adsorbed water with the microstructure of cementitious materials leads to effects on macroscopic properties. However, precise quantitative correlation of adsorbed water with short‐term changes in macroscopic properties, such as dimensional changes, remains a challenge. We compare recent results from precise dual‐microscope‐based short‐term length change measurements and discuss differences in length‐change isotherms and adsorption isotherms arising from various cement types. Water vapor adsorption happens already at very low humidity in the interlayer space of the C‐S‐H gel. Adsorption and desorption on cement surfaces due to changing relative humidity induces strong forces in the cement gel, leading to detectable changes in dimensional stability for the hardened material. A significant hysteresis in the water desorption isotherm is often accompanied by hysteresis in dimensional change as well. For precise length change measurements, an optical method for tracking surface patterns on thin cement slices was applied. Short‐term length change results are compared to corresponding water vapor sorption isotherms and pore size measurements. Hypotheses and conclusions regarding microstructural changes and shrinkage mechanisms are discussed in order to understand the experimental data.