Towards a More Realistic and Effective Use of Sodium Sulfate in Accelerated Ageing of Natural Stone

Abstract

Soluble salts are considered a major cause of damage to historic constructions. The selection of materials for the repair of buildings affected by salts is usually carried out by estimating their resistance to salt crystallization based on laboratory tests. This study aimed to improve the ageing procedure of the European standard EN 12370 (Natural stone test methods: Determination of resistance to salt crystallization) towards a more realistic way of reproducing ageing on-site. This was done through the investigation of various environmental and procedural factors that influence the action of sodium sulfate. Four variations of the EN 12370 procedure considered the effect of different testing conditions (salt solution concentration, contamination procedure, drying temperature). Experiments were carried out on three types of natural stone to investigate the effect of different ageing variables on the deterioration of materials with different properties, namely regarding the porous structure and mechanical strength. The ageing process was monitored using non-destructive techniques: visual observation, mass variation, mass loss, and ultrasonic pulse velocity. After the completion of the ageing tests, the specimens were also examined using destructive methods: salt distribution and microstructural analysis using scanning electron microscopy. The results showed that the salt solution concentration was the most important variable determining damage. The contamination method also had a significant effect, whereas changes in the drying temperature had the least quantifiable impact. The effects the different testing conditions had on the deterioration of the stones are discussed and recommendations for future developments are given.