An experimental investigation into the salt-weathering susceptibility of building limestones

Simona Scrivano,Laura Gaggero

doi:10.1007/s00603-020-02208-x

Abstract

Dimension and ornamental stones used for built heritage are frequently affected, under all climates, by intense weathering induced by the transport of saline solutions. Accordingly, researchers addressed the weathering mechanisms induced by salts (primarily Na2SO4), the stresses generated in the host material, and the durability of ornamental and dimension stones.This study investigates the behaviour of four sedimentary lithotypes, differing in texture and structure (Breccia Aurora, Rosso Verona, and fine- and coarse-grained Vicenza Stone) exposed to a selection of saline solutions, aiming at evaluating their susceptibility to weathering and the influence of petrographic features on durability. The chosen Na2SO4, MgSO4, Na2CO3, NaCl, and KNO3 solutions include the anions characterizing the major saline sources affecting built heritage: capillary rise, acid rain, atmospheric particulate deposition, bird or other animal scat, and de-icing salt contamination.The petrophysical properties of unweathered and salt-weathered samples were characterized by relative mass variation estimation, capillary rise, and hygroscopic absorption-based porosimetry. Scanning Electron Microscopy coupled with microprobe allowed studying the morphological changes triggered by weathering and analysing the chemical composition of the efflorescence crust.The whole of data, consistently with literature, allowed inferring pore type and distribution as the main intrinsic parameters controlling salt weathering susceptibility. Moreover, Na2SO4 and Na2CO3 proved to be the most harmful, probably due to their ability to crystallize in different hydration stages at room temperature. The comparative analysis of the induced weathering can help to choose suitable dimension stones for new buildings or restoration replacements.

Highlights

Within natural materials, dimension stones in monumental structures and artefacts are studied correlating mineralogicpetrographic and petrophysical characteristics to durability
The relative mass loss data recorded for the different salts and lithotypes (Table 4) highlights that Na2SO4 and Na2CO3 saline solutions are exceptionally damaging to carbonates (Fig. 2)
The use of different saline solutions enabled determining the impact of each salt on the stone. Na2SO4 and Na2CO3 were found to induce the highest weathering rates in the tested materials due to their chemical interaction with the rocks and the liability to crystallize under different hydration states

Summary

Introduction

Dimension stones in monumental structures and artefacts are studied correlating mineralogicpetrographic and petrophysical characteristics to durability. Salt crystallization has a significant effect on the weathering of exposed ornamental and building stones (Amoroso and Fassina 1983; Siegesmund and Snethlage 2014). Crystallization is the process common to all salt species inducing weathering in the host stone. The most common model considers a saline solution circulating inside the pore network, where salt precipitation begins when water starts to evaporate or temperature raises (Cardell et al 2008; Castellazzi et al 2013; Grementieri et al 2017). This process occurs where the dynamic balance between evaporation rate and solution supply rate slows down or ceases. The evaporation rate depends on rock properties (permeability and pore size) and on environmental conditions

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