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Crystals growing in confined spaces can generate stress and are a major cause of damage in porous materials. The present paper is an experimental study of the isothermal hydration of MgSO 4 • H 2 O, kieserite, in porous glass filters with different in situ techniques, i.e., X-ray diffraction under controlled conditions of temperature and humidity, scanning electron microscopy after cryofixation of samples, and electronic speckle pattern interferometry. Based on the phase diagram of MgSO 4 + H 2 O derived from a careful evaluation of the available thermodynamic data, all experiments were carried out under appropriate conditions in a controlled environment. Hydration of MgSO 4 • H 2 O and formation of MgSO 4 • 6H 2 O (hexahydrite) is a true solid state reaction below the deliquescence humidity of kieserite. This reaction, however, is kinetically hindered. Above the deliquescence humidity, the reaction proceeds via a two stage reaction pathway involving the dissolution of kieserite and the subsequent crystallization of hexahydrite from a highly supersaturated solution. The hydration of kieserite in confined spaces generates substantial stresses, resulting in considerable deformations of the glass filters that were used as porous substrate. The deformation measurements confirm that the deliquescence-recrystallization pathway is more efficient than the direct solid state reaction in generating stress. Finally, theoretical considerations confirm that the stress generated during the hydration is sufficient to damage nearly every building material.

Damages to natural building stones induced by the action of frost are considered to be of great importance. Commonly, the frost resistance of building stones is checked by standardised freeze-thaw tests before using. Corresponding tests normally involve 30-50 freeze-thaw action cycles. In order to verify the significance of such measurements, we performed long-term tests on four selected rocks over 1,400 freeze-thaw action cycles. Additionally, numerous petrophysical parameters were analysed to compare the behaviour of rocks in the weathering tests according to the current explanatory models of stress formation by growing ice crystals in the pore space. The long-term tests yield more information about the real frost sensibility of the rocks. A clear deterioration cannot be determined in most cases until 50 weathering cycles have been completed. In the freeze-thaw tests, the samples are also stressed by changing temperature and moisture, indicating that different decay mechanisms can interfere with each other. Thus, thermohygric and moisture expansion are important damage processes.

The deterioration of three marbles (Palissandro, Sterzing and Carrara) differing in composition and rock fabric has been studied using measurements of the thermal dilatation in the temperature range from −40°C up to 60°C. A long-term freeze-thaw experiment was performed to characterize the frost weathering via Young’s modulus. The results show that the combined effect of heating and cooling under dry and water-saturated conditions significantly influences the material properties. The thermal dilatation and its anisotropy can be explained by the crystallographic preferred orientation of calcite and dolomite as well as with the thermal expansion behaviour of these minerals. The residual strain, i.e. the permanent length change, after thermal treatment is different for the investigated samples and less pronounced for the dolomitic marble from Palissandro. The hygric expansion is of only minor importance and weak in the phlogopite-bearing Palissandro sample within the direction parallel to the foliation.Fresh and artificially weathered marbles were exposed to 204 freeze-thaw cycles. The Young’s modulus for the Carrara marble decreases from 55 GPa to 28 GPa while the porosity increases from 0.25% to 0.62%. The effect on the Palissandro and the calcitic Sterzing marbles is less pronounced while the artificially weathered ones clearly exhibit a drastic reduction in Young’s modulus.The progressive loss in strength is caused by progressive microfracturing or the loss of cohesion along grain boundaries due to the crystallization pressure of ice growth. The experimental data along with existing theoretical models lead to the conclusion that the physical weathering of marble is influenced by cooling and heating under mid-European climatic conditions.

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