SWBSS 2014 3rd International Conference on Salt Weathering of Buildings and Stone Sculptures pages:117-131
Third International Conference on Salt Weathering of Buildings and Stone Sculptures edition:3 location:Brussels date:14-16 October 2014
Gypsum efflorescence (GE) is a recent problem affecting modern clay brick masonry and permanently alters an aesthetic aspect of a building. This type of efflorescence affects exclusively masonry built in the last 30 years and usually appears a couple of years after construction. The GE characteristics indicate that the components are derived from the masonry itself and not from the atmosphere. The GE genesis and mechanism, and specifically the gypsum crystallisation behaviour in porous materials, are poorly understood.
We have designed and executed a series of crystallisation experiments, to compare gypsum behaviour with common efflorescing salts: NaCl and Na2SO4. Our experimental wick test setup consists of a brick sample mounted in a cell. The salt solution is supplied to the bottom sample side while the drying process exclusively occurs at the top. The experiments are carried out under three drying regimes: ‘accelerated’ (35 °C, 19% RH), ‘laboratory’ (24 °C, 58% RH) and ‘cold’ (12 °C, 80% RH). The first simulates accelerated efflorescence formation, the second is comparable to common wick tests while the third and last imitates early spring conditions. The experiments demonstrated that gypsum accumulates below the surface, unlike natural GE occurrences. Since the basic wick test does not simulate frequent masonry wetting, we have implemented wetting cycles which resulted in a considerable GE formation. We conclude that gypsum intrinsically has a subflorescing tendency, but frequent surface wetting progressively leads to GE formation.