Limiting salt damage : a study on the effectiveness of salt inhibitors on Globigerina limestone

Abstract

The aim of this research study was that of investigating changes that occur in varieties of Globigerina Limestone in the presence of contaminant salts and studying a method to modify these changes to limit damage induced, by the use of crystallization inhibitors. The mechanism of action of the specific class of polycarboxylates proposed is twofold: to reduce pressure generated upon crystallization by targeting crystal habit, and altering the position of the crystallization front within porous substrate, to promote efflorescence formation, other than harmful subflorescence precipitation. The macroscopic characteristics of crystallization and growth of sodium sulfate, sodium chloride and sodium nitrate within the local stone, but also resulting damage were studied and evaluated through salt crystallization/evaporation tests. Simultaneous applications of the proposed crystallization inhibitors were performed through different application techniques, including partial immersion, brush and poultice applied. Both pre-treatment and pre-contamination were tested. Next, a thorough assessment of rates of flux obtained in the crystallization/evaporation tests was carried out to determine efficacy of the various systems used. The treatment application gives strong indications that flux is affected and hydraulic continuity in the systems persists in approximately 50% of the cases; this was slightly more evident in the soil variety (at 56%) of Globigerina limestone investigated, compared to the bajda (at 42%). Assessment of efflorescence and damage formations, supplemented by microscopic (SEM) observations, allowed for a study of salt crystallization habit and distribution. Results showed that differences were limited, considering the low salt concentrations that were adopted; manifestation of efflorescence, its distribution, but also damage induced by these salts, varied primarily according to substrate type and environmental conditions. The study was also accompanied by tests on crystallization in bulk solution, in the presence of the crystallization inhibitors and without, to understand the type and extent of influence that these chemicals have on crystallization modification. Here results showed that the presence of modifiers tested, does affect supersaturation ratios, v induction times and crystal morphology, in the unconfined context and under specific environmental conditions. The data collected were extensively analysed statistically, both through univariate and multivariate techniques. The most statistically significant parameters were identified to be salt type and temperature. Such data provides the basis for a template of what can be termed as an 'efficacy model' and which can be used as a reference for future applications under different environmental conditions, to assist interpretation of results in similar treatment applications. It is now possible to develop a template modelled on this data. Given the extent of positive results, mainly in flux, it can be concluded that the proposed treatment with polycarboxylates on salt contaminated stone can be applied to limit damage, although further investigations are required, due to the system specificity that affects results.