Limestone resistance to sodium sulfate degradation after consolidation by hydroxyapatite and TEOS

The use of aqueous phosphate solutions to produce hydroxyapatite (HAP) inside weathered carbonate stones has recently been proposed as a new consolidating technique. In this paper, the resistance of HAP-treated stone to soluble salt crystallization was investigated. Globigerina limestone, a porous stone typically used in historic architecture in Malta and often severely affected by salt crystallization, was used. After preliminary artificial weathering by heating at 400 °C for 1 hour, aimed at producing micro-cracks opening in the stone, cylindrical samples (5 cm height, 2 cm diameter) were treated by brushing application of a 3 M aqueous solution of diammonium hydrogen phosphate, followed by application of a limewater-impregnated poultice. For comparison's sake, a similar set of samples was treated with a commercial TEOS-based product, while a third set was left untreated. After proper curing, HAP- and TEOS-treated samples, together with untreated ones, were subjected to sodium sulfate crystallization cycles, by partial immersion in a 14 wt% sodium sulfate decahydrate solution for 7 hours and then drying at 50 °C for 15 hours. After each cycle, damage evolution was monitored by visual assessment and weight measurement. Five cycles were carried out in total. From the results of the study, the performance of the novel phosphate treatment can be regarded as promising, as HAP-treated samples exhibited less micro-cracking and lower mechanical damage than untreated and also TEOS-treated samples.