Assessment of the efficacy of ethyl silicate and dibasic ammonium phosphate consolidants in improving the durability of two building sandstones from Andalusia (Spain)

We performed a comparative study of the effectiveness of ethyl silicate (TEOS) and dibasic ammonium phosphate (DAP) on two varieties of natural stone used in the architectural heritage of Andalusia (Spain): Arenisca Ronda (calcarenite) and Molinaza Roja (arkose). The consolidants were applied on healthy samples with a paintbrush, a method frequently used in real building works, and the study was divided into three phases: (a) petrophysical analysis and analysis of the porous system prior to application of the consolidants; (b) evaluation of the changes that have taken place after each application; (c) evaluation of the durability of the two varieties of treated rock when subjected to the salt crystallization ageing test. The results obtained show that there is a compositional affinity between the consolidant and the rock and that this affinity has an influence on the efficacy of the product. This explains why DAP performed better in Arenisca Ronda, which is rich in carbonates, while TEOS was more effective for Molinaza Roja, which is rich in silicates. The change in the porous system was not important, although it showed positive aspects such as an improvement in the drainage of the water during the drying process. We also measured the level of penetration of both products, which reached a depth of 3–5 mm depending on the product applied. We also noticed a negative result, namely the change in the colour of both rocks after treatment with the consolidants, especially in the case of DAP. Finally, the stones were subjected to a salt crystallization test, the results of which show that the consolidants increased the durability of the materials.