Abstract

Physical and mechanical properties of building stones can vary due to different degradation mechanisms caused by temperature and chemical agents. The problem of chemical and thermal weathering on marble rocks is an important issue to consider for designing building façades since it may cause sugaring, bowing, cracking and spalling. Moreover, an accurate comprehension of induced damages is required for restoration and conservation of heritage monument purposes. While thermal weathering has been widely studied in the last years, the combined effect of thermal and chemical weathering (that here is intended as the combined action of rainwater and atmospheric pollutants) is still poorly understood. In this study, no-destructive (ultrasonic pulse velocities) and destructive tests (bending tests) were performed on Carrara marble slabs in natural and after thermal (with target temperatures respectively of 50 and 90°C) and thermo-chemical treatment. Thermo-chemical treatments were performed by soaking the specimens in a 5x10−6 mol/l solution of sulphuric acid at pH=5 to simulate the acid rain behavior, at constant target temperatures, for one week. In general, for each weathering mechanism, progressive degradation of the physical and mechanical properties of marble specimens was observed. In particular, a marked drop in flexural strength, mirrored by a wide variation in P- and S-wave velocity, was found in specimens chemically treated at target temperature equal to 90 °C.

Introduction

The physico-mechanical degradation of rock properties due to exposition to natural weathering factors (as rain, snow, ice and wind), temperature and chemical agents (air pollution) is fundamental for designing many mining and civil constructions and for planning restoration in cultural heritage. Usually, two or more weathering factors contemporary occur [1], increasing the effect of deterioration on rocks and the strength reduction.

Since ancient time, marbles were used as building material as a facing stone for temples, mosques, churches, palaces and other monuments. Marble is a metamorphic rock mainly made of calcite (CaCO3), with a massive crystalline structure and very low porosity (0.1-0.5%). It is highly sensitive to acid attack that compromises its durability and its mechanical properties. Acid rain causes dissolution of the marble, increasing porosity and allowing a higher circulation of salt within microstructures [2] (and references therein). Moreover, if the effects of acid rain are coupled with thermal gradients, a more severe degradation of the physico-mechanical properties is expected. In fact, due to the anisotropic thermal expansion of grains, thermal stresses arise within the marble structure, leading deflection, bowing and spalling phenomena [3].

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