The role of salt fog on alteration of dimension stone

doi:10.1016/j.conbuildmat.2009.06.044

Construction and Building Materials

Volume 23, Issue 11, November 2009, Pages 3321-3327

https://doi.org/10.1016/j.conbuildmat.2009.06.044 Get rights and content

Abstract

Limestones and silicate rocks used as dimension stone were submitted to cycles of salt fog, totalizing 140 days in a salt spray chamber. Rock samples comprised three types of limestone, three different marbles, six types of granites, two syenites, one gabbro and one basalt. Mineral changes and variation on surface luster, weight loss and salt crystallization on rock surfaces were some of the features observed along the exposure. While losses of weight and surface luster were the most striking features observed on carbonate rocks, mineral degradation was the main modification on silicate rocks, in which biotite showed to represent a key mineral in the alteration process due to its cleavage and possibly to its composition.

Introduction

Salt fog is a powerful rock weathering agent which has been shown to effectively act on rocks of any composition. The first signs of alteration appear on the rock surfaces, even on silicate rocks thought in the past to be a very resistant, long lasting material. Changes on the rock structure follow as the salt action goes on.

Historic buildings and monuments showing signs of degradation have called the attention of researchers and technicians involved in cleaning, conservation and restoration of such heritage. In this sense, due to their significance to society, they should not be lost. The methodology in which any relevant work can be based on for recovery of those buildings is a complete knowledge of the material (rock), its chemistry and mineral composition and thereafter, the knowledge of the causes of alteration, their extent and how to remediate the problem. This implies, in many instances, an operation that starts with the removal of the cause, if possible, followed by the repair of the damaged object (building). Among the several types of rock alteration, salt activity inside the rock is one common factor that plays an important role on rock decay. It is necessary then, to get to know the relationship between the original rock and new materials added to it, a process developed in many circumstances. The fact that salt activity is not an isolated feature restricted to the surface leads to the need of understanding the way it reaches the rock, how it acts on the rock surface and the type of exchange which takes place within the rock, between components. Buildings exposed to salt atmosphere near the ocean usually give some hints to researchers what lead to diagnose of the occurrence and the proposal of solutions for remediation.

Salt fog activity comprises mechanisms such as salt crystallization and deposition of salt on the rock surface, salt solution penetration in rock pores and further salt expansion causing the increase of voids in the rock framework, followed by removal of soluble salts and secondary minerals. Several authors have pointed out the various ways in which salt weathering affects different types of rocks [1], [2] and mechanisms acting on rocks [3].

The wide variety of silicate rocks used as dimension stone constitutes an important field of research. It opens the opportunity of observing rocks of different origins and mineral compositions answering in different ways to the action of salt fog. Following this idea, different rock types used in buildings, either having structural function or having finishing purposes on façades, were chosen and submitted to salt spray cycles. These rocks were observed during several cycles and a comparison between the material just prior to exposition and after each salt fog cycle could be made recording changes on the rock surfaces and on the rock as a whole. The findings do not represent the only alteration of each material, but they certainly show evidence of possible “starters” or “promoters” of a certain type of alteration for those rocks.

Rocks chosen for the experiments are commonly used as dimension stone in Portugal. They come from quarries having different geological origins, either in Portugal or in Brazil, being all of them of common use as building materials and dimension stone.

Section snippets

Samples selection

Limestones and marbles, granites, syenites, gabbros and basalt were tested in the laboratory through the exposition to salt fog during 13 cycles of 10 days each. Intentionally: (1) limestones of different porosities and (2) silicate rocks containing stable minerals like quartz, in granites, unstable minerals such as nepheline, sodalite and cancrinite in syenites and olivine, in gabbro were selected. Most silicate rocks contain biotite as a common accessory mineral, which was identified as a key

Discussion and final remarks

Chloride penetration in silicate and carbonate rocks has shown to be a process similar to that occurring in concrete, in which case the development of microcracks due to the presence of chlorides has been studied. In the case of concrete, two types of chlorides are present, the so-called bound and free chlorides [16], which act in different ways, either through fixing by adsorption or by chemical reactions with the concrete components (the bound type). The free chlorides, as the name indicates,

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The role of salt fog on alteration of dimension stone

Abstract

Introduction

Section snippets

Samples selection

Discussion and final remarks

Construct Build Mater

Sci Total Environ

Eng Geol

Cem Concr Res

Construct Build Mater

The mechanism of masonry decay through crystallization