Materials
Two different lithotypes were used for the tests: Globigerina Limestone and Giallo Terra di Siena, from now on labelled as GL and GS, respectively, differing for porosity and mineralogical composition. In particular, GL is composed for about 90 wt% of calcite and exhibits small amounts of quartz and a total open porosity around 40% [23,24], while GS is mainly composed of calcite (around 80 wt%), quartz and feldspars, and exhibits a total open porosity around 20% [19].
Quarry slabs of both stones were cut into 5 cm cubes and core-drilled perpendicularly to bedding planes to obtain 2 cm diameter and 5 cm height cores. As samples naturally weathered in the field usually undergo alterations in porosity, liquid water transport properties and mechanical strength, all the samples used in this study were artificially weathered by heating prior to consolidants application. According to a previously developed methodology [14,23], samples were heated at 400°C for 1 hour (in the case of GS, after preliminary saturation with water and heating at 200°C for 1 hour), so as to induce new micro-crack formation at grain boundaries.
The HAP-based treatment was performed by using a 1 M aqueous solution of DAP (Sigma-Aldrich).
Two different TEOS mixtures were used for the two lithotypes (a commercial mixture for GL, a mixture prepared in the laboratory for GS), maintaining the same catalyst (1 wt% DBTL) and the same ratio between the active principle and the solvent, so that the only parameter to change was the solvent type. GL samples were treated with a commercial product composed of 75 wt% ethyl silicate and 25 wt% white spirit D40 (Estel 1000 by CTS s.r.l., Italy). GS samples were treated with a mixture of 75 wt% ethyl silicate and 25 wt% isopropyl alcohol.
Methods
All the samples were treated by brushing, as this is the most common application method usually adopted in the field [25]. Cylinders of both stones were treated on the whole external surface, while cubes were treated on one face perpendicular to the bedding planes. As usually recommended in the technical data sheets of commercial TEOS-based products, HAP and TEOS treatments were applied until apparent refusal (defined as the condition when stone surface remains wet for 1 minute after consolidant application [7]), which required about 10 brushing applications for GS and GL cylindrical samples, about 20 applications for GS cubes and 30 applications for GL cubes.
HAP-treated samples were left to cure for 48 hours wrapped in a plastic film to prevent evaporation, while TEOS-treated samples were left to cure for 4 weeks (as recommended by commercial TEOS products technical data sheets) in laboratory room conditions (T = 20 ± 2°C, RH = 50 ± 5%).
The performance of the two consolidants on the selected lithotypes was then evaluated in terms of mechanical strengthening and alteration in microstructure and transport properties. All tests were performed on treated and untreated samples, for comparison’s sake.
Mechanical properties were determined on stone cores in terms of dynamic elastic modulus (by ultrasonic test, Matest instrument with 55 KHz transducers) and tensile strength (by Brazilian splitting tension test), as these properties provide an estimation of the consolidant ability to restore stone cohesion and seal micro-cracks, together with providing an indication of its even distribution inside the sample. Tensile strength determination is particularly relevant when TEOS treated samples are under examination, as the formed silica gel can either create chemical bonds with the samples, hence exerting a proper binding action, or else just deposit inside stone pores, hence not giving significant benefit in terms of cohesion [1].
Stone porosity and pore size distribution were investigated by mercury intrusion porosimetry (MIP, Fisons Macropore Unit 120 and Porosimeter 2000 Carlo Erba) on fragments taken by chisel from the surface of stone cores, in order to investigate the alterations occurred in the consolidated layer of the sample. Microstructural alterations were expressed in terms of pore size distribution, total open porosity and average pore radius, defined as the radius corresponding to 50% of mercury intruded volume. Pore percentage below and above 0.1 μm and 1 μm were also determined, as an increase in the fractions of smaller pores is known to raise stone susceptibility to decay due to salt crystallization [26].
Stones transport properties were determined by sorptivity test, performed according to the EN 15801 [27], water being let penetrate the samples through the treated face, as in the scheme in Figure 1. The test was stopped after 24 hours, when saturation had already been reached for both stones, and the corresponding water absorption value was determined.