AIC’s 39th Annual Meeting- Architecture/Research and Technical Studies Joint Session, June 3, Protecting Marble from Corrosion by Sonia Naidu and George W. Scherer

This paper shares a testing program that incorporated the use of phosphate solutions to create a mineral coating (hydroxyapatite) on stone to impart strength and durability. The project goal was to consolidate the surface of weathered stones (primarily calcareous stones were tested) to prevent loss from dissolution. Naidu shared that the idea of this testing program came from natural patinas (phosphate and oxylate-based) that can be observed on stone surfaces. Calcium phosphate and hydroxyapatite (main inorganic component of teeth and bones) were analyzed.
Testing was conducted to react a phosphate source with calcite (carrara marble used), and diammonium hydrogen phosphate (DAP) was selected for further analysis through SEM/EDS and XRD. Variables of DAP concentrations (1m and 2m) and exposure times (12 hours and 24 hours) were analyzed. SEM images were used to evaluate film formation, and it was found that after 24 hours of 1M DAP concentration exposures that a dense film was created on the stone’s surface. Raising the concentration to 2M created a denser film, though spalling was evident. XRD confirmed the presence of apatite in the film.
Studies also found that during the process of converting minerals the porosity of the stone increased and further testing should consider adding calcium back into the stone. SEM analysis was used to evaluate the addition of calcium ions back into the samples, and the most coverage was seen with calcium chloride at a 1M concentration. Naidu also mentioned a study by Snethlage that reported success of converting minerals using ADP. This testing will continue and explore external ion additions and sequence transformation, control films, and comparison of effectiveness with calcium oxylate.
Naidu discussed the process of consolidation using silicate-based systems, pointing out that sometimes coupling agents such as tartaric acid are used to assist bonding. A comparison study was designed to evaluate Conservare OH100 and 1M DAP on artificially weathered limestone (heat was used to induce damage to samples). The consolidants were applied and the tensile strength (all samples) was tested at 2 days (DAP 25% increase) and 4 days (DAP 28% increase). The results indicated a greater improvement with DAP treated samples. This testing will continue and explore the effect of calcium ion additions, organic additives and extending samples to marble. Tracking the progress of this continued testing will be important, since there are relatively few stone consolidants on the market that meet current environmental and safety standards.