The behaviour of as-applied and artificially weathered silica\u2013epoxy consolidants on a typical Mediterranean porous limestone: a comparison with TEOS

Sophie M Briffa,Daniel A Vella

doi:10.1186/s40494-019-0270-1

Abstract

A water based epoxy–silica consolidant and variants of the same consolidant containing silica nanoparticles (NP) were applied to Globigerina Limestone (GL) blocks by brushing and full immersion techniques. The consolidants were applied for their improved strength, toughness and adhesion to the stone surface. As-consolidated dried stone blocks were characterised (colour, total porosity, pore size distribution and microstructure) and their physical and mechanical properties assessed by a water uptake by capillary test, drilling resistance and resistance to salt crystallisation test. In another set of experiments, the consolidated limestone blocks were subject to 28 wet–dry cycles (8-h wet, 16-h dry) in a weathering chamber. The physical and mechanical properties of the weathered consolidated limestone blocks were then re-assessed. Parallel tests were also performed on tetraethyl orthosilicate (TEOS)-consolidated and untreated GL blocks. The water absorption coefficients of limestone blocks consolidated by brush-applied epoxy–silica dropped marginally. No significant changes to the water absorption coefficients were recorded after weathering. The drilling resistance curves showed a clear improvement of the mechanical properties of epoxy–silica treated GL, but the effects of the consolidants were reversed with weathering. The epoxy–silica consolidants reduced the resistance of GL to salt crystallisation. This resistance was partly restored after weathering, corroborating the drilling resistance results, and supporting the consolidation reversal hypothesis. Test results for the epoxy–silica consolidants applied by immersion showed similar trends, albeit complicated by pore blocking. The formation of an undesirable surface crust with epoxy–silica consolidants remains a pertinent issue with such hybrid materials. The behaviour of TEOS-consolidated GL was very different to the epoxy–silica consolidants and this can be traced to the hydrophobic nature of TEOS that persisted months after application.

Highlights

The weathering and deterioration of outdoor exposed stone monuments and buildings is an inevitable natural process
Epoxy–silica consolidants applied by immersion led to considerable darkening, reddening and yellowing of Globigerina Limestone (GL)
In this work an epoxy–silica consolidant, and variants loaded with silica NP were applied to GL

Summary

Introduction

The weathering and deterioration of outdoor exposed stone monuments and buildings is an inevitable natural process. Consolidants based on alkoxysilane precursors [2] are commonly employed in the preservation of stonework. An advantage of these materials over organic polymers is that they are applied as low molecular weight monomers or oligomers that polymerize to form consolidants inside the stone [4]. TEOS is known to be very effective over sandstones but less so on limestone [4] In the former, the consolidant forms strong covalent links with the stone surface silanol groups (≡Si–OH) [4]. These bonds do not occur over limestone surfaces [4]

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Results

Discussion

Conclusion