An experimental method to measure crystal growth rate in porous materials and quantity deformation developed during crystal growth

Student thesis: Doctoral Thesis

Abstract

The aim of this thesis work is to develop a new experimental method to quantify dam-ages produced by salt crystallisation in porous building materials, a phenomenon long considered as one of the most important cause of deterioration problems, occurring in monuments exposed to a wide range of environmental conditions [125]. Sodium sulphate in particular is widely recognised to be the most damaging salt because of the difference in solubility between phases and the very temperature-sensitive solubility of the stable phase at room temperature. It is known to have two hydrated phases at ambient conditions: the metastable heptahydrate (Na2SO4 .7H2O) and the stable decahydrate called mirabilite (Na2SO4 . 10H2O). Damage is caused by mirabilite crystallisation from supersaturated solutions, either directly or via heptahydrate dissolution, which develops a high crystallisation pressure and relative high strain level on the porous matrix. Crystallisation of sodium sulphate hydrates is followed by ice formation when temperature drops below the solution eutectic point (c.-3°C). These mechanisms result in micro and macro fractures of the building materials. Damages entail costs from repair/re-placement, hence a deeper quantitative understanding of strain produced from salt and ice crystallization in building materials is extremely relevant.With the new methodo-logy presented here it can be measured how fast sodium sulphate and ice crystals grow through building stones and what strain is developed as a result. We achieved this by developing a new experimental apparatus which measures sample strain and crystal propagation rate using an LVDT and thermocouples respectively. For this thesis work we also used X-ray computed tomography (X-CT) to investigate where salts tend to distribute within stone cores. This technique enables one to image the morphology and internal structure of porous material, at very high resolution (µm), hence salt distribution within the porous matrix and at its outer surface could be detected. Understanding where different salts tend to distribute is fundamental to estimate and predict damage they can cause. Lastly, we also explored how specific stone physical and chemical parameters can affect damage caused by salts to masonry, and how damage is related to environmental parameters such as relative humidity and temperature and their cyclical changes. We did this by carrying out a field investigation at the Shetland Island Town Hall where sodium sulphate was found. For the investigation we used the apparatus developed during this work plus other diagnostic techniques.
Date of Award26 May 2020
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde
SupervisorAndrea Hamilton (Supervisor) & Cristina Gonzalez-Longo (Supervisor)

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