Optimization of Di-ammonium phosphate application protocols for the consolidation of carbonate substrates

The consolidation of carbonate rocks used in cultural heritage is a burning issue in the field of conservation. Several materials were employed to preserve the carbonate substrates widely used in different countries and cultures; however, identifying a treatment that ensures both efficacy and respect for the artefacts' original characteristics remains a challenge. Some materials revealed important drawbacks after many years of their application and climate change has highlighted the need for a convincing solution. Di-ammonium phosphate (DAP), studied since 2011, is one of the inorganic consolidants used for carbonate substrates. It proved to be a promising material due to its high compatibility with the substrates, lack of toxicity, easy application and good consolidation performance. The consolidation process relies on the formation of low-solubility calcium phosphate phases, resulting from the reaction between the phosphate ions of the reagent and the calcium ions of the substrate. Nevertheless, DAP shows some limitations including colour change, superficial cracking phenomena, microbiological attacks and difficulty in achieving a homogeneous and depth consolidation. This research addresses the challenges of consolidating carbonate substrates developing a new application protocol: the 2-step methodology. The 2-step methodology involves the application of two consecutive solutions with increasing concentration and it was compared with the conventional and spread 1-step methodology. The hypothesis underlying the 2-step methodology is that the first low-concentration solution reacts mildly with the calcareous substrate, forming a thin coating on the pore walls without causing occlusion. This allows the subsequent, more concentrated solution to penetrate deeper into the substrate, reaching fresh carbonate and promoting more homogeneous and deeper consolidation. The application method can influence the expected results, and the 2-step methodology aims at enhancing the known benefits of DAP while minimising its adverse effects. To reach these aims the use of very low concentration was also considered. The protocols were tested using different and comparable DAP concentrations. Specifically, the 1-step methodology was applied at 4.5 wt.% DAP and 9 wt.% DAP, while the 2-step methodology was applied at 0.5-4 wt.% DAP, and 1-8 wt.% DAP. The research was divided into two main experimental lines: the study of synthetic samples and the study of natural stone specimens. The synthetic samples were introduced to test the consolidation protocols and to detect the calcium phosphate produced, whereas the natural stone specimens were used to evaluate the effects of the treatments on different lithotypes. The lithotypes selected for this study included Lecce Stone, Finale Stone and Vicenza Stone with different grain sizes. The synthetic samples consist of tablets made of pressed rock powders. The pressed rock tablets retained the original mineralogical composition of the stones without conserving their texture or porosity. Additionally, the tablets mimic an effectively weathered substrate, providing a controlled surface for studying DAP penetration and calcium phosphate formation. Using a multi-analytical approach, the formation of calcium phosphates, specifically octacalcium phosphate (OCP) and hydroxyapatite (HAP), was confirmed even with the 2-step methodology and at the lowest DAP concentrations. The analysis demonstrated that the application method did not affect the type of calcium phosphate formed but could influence the P distribution on the samples. All the samples treated with the 2-step methodology show a reduced phosphorous concentration on the surface compared to the samples treated with the 1-step methodology. This means that the use of the 2-step methodology prevents the formation of a thick consolidated layer at the substrate surfaces avoiding problematic discontinuity within the substrates and preventing cracks and microbiological colonisations. Regarding penetration efficacy, no substantial difference was observed between the methodologies. In synthetic samples, the penetration depth of DAP was approximately 2.5-3 millimetres. In natural stone specimens, DAP penetrated through fractures and pores. Additionally, the capillary water absorption was reduced following the treatment, and the complete evaporation of water indicated that the gas permeability was not compromised by the consolidant. Colorimetric analysis revealed that the chromatic changes induced by the consolidating treatment posed a medium-low risk. The results of this study suggest that the 2-step methodology, combined with lower DAP concentrations can achieve effective consolidation while minimizing undesirable surface deposits and aesthetic alterations, thus improving the applicability of DAP treatments for the preservation of carbonate stone heritage materials.