Freeze-thaw cycles can cause considerable damage to porous materials and thus have an adverse effect on the durability of mortars and porous stone. To assess the behavior and frost resistance of two types of porous limestone, three commercially available repair mortars and four mixtures of laboratory-prepared repair mortars were subjected to freeze-thaw cycles according to EN 12371. During the test, samples of stone and mortar were bonded together and the weight loss was continuously monitored. The adhesion bond between the stone and the mortar was also observed during the cycles. Petrographic analysis and thin sections were also made before and after the freeze-thaw test. The pore size distribution (mercury intrusion porosimetry, MIP) of mortars and stones was also measured. The test showed that most of the repair mortars were damage more than porous limestone due to frost action. Two exceptions are two commercial available repair mortars. These mortars were able to keep the adhesion with the stone, and the frost did not modify significantly the cubic shape of the test specimens, only rounding of the edges was observed. All other samples were broken during the frost tests; stone/mortar interface was dismounted. Other typical damage features such as delamination, blistering, powdering, and granular disintegration were also observed leading to the gradual surface loss of the material. Our tests proved that low pozzolanic cement content in mortars decreases the material durability. According to the pore size distribution (MIP), the small pores (around 0.1 μm) control the weathering behavior of tested porous materials.
Repair mortar and mixture of repair mortar with porous limestone sand aggregate were tested under laboratory conditions. Water absorption properties and micro-fabric analyses with a combination of strength tests were applied to assess the durability and compatibility of repair mortar with porous limestone. Uniaxial compressive strength and flexural strength were measured after 3, 7, 14, 28 and 90 days of casting. Durability was tested by comparing strength test results of samples kept air dry, water saturated, dried in drying chamber, freeze-thaw and nonstandardized freeze-thaw cycles. The results indicate that with time various trends in strength were observed. In general, limestone aggregate content decreases more the compressive strength more than the flexural strength of the mortar. Standardized freeze-thaw tests of saturated samples caused a rapid material loss after 25 cycles, while freeze-thaw tests of undersaturated samples demonstrated that even after 100 cycles the test specimens still have a significant strength. Water-saturated samples that contain 50% of limestone aggregate have a 50% loss of strength in comparison with saturated repair mortar, while air-dry and water-saturated repair mortar has a minor strength difference after 90 days. The use of smaller amounts of porous limestone aggregate in repair mortar allow the preparation of repairs that are compatible with the monuments of Central Europe that were constructed from porous limestone.
Porosity and water absorption of different binder/aggregate ratios of repair mortar and porous limestone were studied that were used in many Hungarian monuments. Different types of mortars were analyzed by using mercury intrusion porosimetry (MIP) and the water saturation method (WSM). Test results showed that there was a strong correlation between the absorption mechanism and the porosimetric characteristics. Mechanical properties of the tested mortars were observed earlier. Pore size distribution confirms that the total porosity increases with increasing aggregate content. Natural stones mainly have medium and large pore radii (1-100 μm) while repair mortars, even with increased aggregate ratio, have smaller pore radii (0.01-0.1 μm). The comparison of different data allows us to state that pore characteristics such as pore volume, pore geometry, pore size distribution and network connectivity are the key control factors of stone and mortar deterioration.
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