Stone consolidation is essential for the preservation of masonry materials from biodeterioration, salts, pollution, and natural weathering phenomena. A stone consolidant not only protects the stone from degradation, but enhances the mechanical integrity of the stone material through penetration into the pores and binding of the material. This current study involves formulation of alkoxysilane-based coating systems to exceed the properties of the current commercially available stone consolidants, focusing on improvements in “breathability” and weathering resistance.
This concept of “breathability” involves the stones’ ability to allow permeation of moisture trapped within the stone to migrate through the coating and out into the environment. However, this coating should selectively prevent moisture from migrating into the pores of the substrate. Taking into consideration this target property, formulations were made using a compound called POSS (polyhedral oligomeric silsesquioxane). This cage-like structure offers this “breathability” aspect in addition to increased mechanical properties such as modulus and hardness, both desirable properties for stone consolidants.
In this study, three of the top-performing experimental stone consolidant coatings (based on previous studies) were sprayed on Royal Danby Marble and Indiana Limestone samples using Master Airbrush model G22 dual action gravity feed (see below). As a means for distinction, the coatings are labeled as 0%, 10% and 20% ethanol, although there are other slight variations within the formulations and therefore may only be compared to the controls and the blank rather than with each other. They were then weathered for 400 hours using a xenon-arc Atlas MTS Ci4000 Weatherometer (see notes at bottom on test methods, 1) and tested for water absorptivity using Rilem tubes as well as color changes (ΔE) using an X-Rite portable spectrophotometer (see bottom test methods, 2,3). The experimental coatings were compared to the two commercial formulations, Silres BS OH 100 and Conservare OH 100, as well as untreated (Blank) stone.
ΔE Color Change Results
ΔE is a useful way to quantify changes in color using L*a*b* color values. A higher ΔE indicates a more notable color difference is detected. The color change results were dependent on the stone type. Limestone substrates showed higher ΔE’s for the experimental coatings upon weathering as compared with the commercial coatings and untreated stone. For marble substrates, however, a decrease in ΔE’s was observed for the experimental coatings compared to the commercial formulations. It is hypothesized that color changes may be reduced by eliminating chromophores within the formulation that could potentially absorb radiation and change the color of the coatings.
Water Absorptivity Results
Water absorptivity was measured after every 100 hours (up to 400 hours) of weathering using the Rilem tube testing method. Absorption measurements were taken at 5, 10, 15, 20, 30 and 60 minutes. Experimental consolidant formulations on Indiana Limestone displayed superior performance to commercial formulations and blank. Commercial formulations exhibited high water absorptivity after only 100 hours of weathering, indicating relatively rapid degradation of the commercial coating.
For the Royal Danby Marble coatings, however, the data is relatively inconclusive. Given the minimal porosity of marble, it is suggested that longer weathering times and water absorptivity testing could potentially give more indicative data as to coating performance.
Overall, we observe promising performance from the experimental stone consolidant formulations as they outperform the commercial consolidants in water absorptivity. Further formulation modifications are underway for improvements in color change results.
Notes on Test Methods:
1. ASTM D6695-16 Cycle 1 includes 102 minutes light at 50% ± 5% RH followed by 18 minutes light and water spray repeated continuously. Uninsulated black panel temperature was 63 ± 2°C with irradiance at 0.35 ± 0.02 W/(m2*nm) at 340 nm. Test continued for 400 hours, with checks every 100 hours for data collection.
2. X-Rite spectrophotomer measure L* a* b* color values, which were recorded. To determine ΔE values, the following formula was used:
3. Rilem Tube test method No. 11.4 from M-Testco, measured water absorptivity using Rilem tubes after each 100 hour weathering check. Water absorption measurements were taken after 5, 10, 15, 20, 30 and 60 minutes.