Isotopic analysis for degradation diagnosis of calcite matrix in mortar

Abstract

Mortar that was used in building as well as in conservation and restoration works of wall paintings have been analysed isotopically (delta(13)C and delta(18)O) in order to evaluate the setting environments and secondary processes, to distinguish the structural components used and to determine the exact causes that incurred the degradation phenomena. The material undergoes weathering and decay on a large proportion of its surface and in depth, due to the infiltration of water through the structural blocks. Mineralogical analysis indicated signs of sulphation and dissolution/recrystallisation processes taking place on the material, whereas stable isotopes provided information relative to the origin of the CO(2) and water during calcite formation and degradation processes. Isotopic change of the initial delta(13)C and delta(18)O in carbonate matrix was caused by alteration of the primary source of CO(2) and H(2)O in mortar over time, particularly by recrystallisation of calcite with porewater, evaporated or re-condensed water, and CO(2) from various sources of atmospheric and biogenic origin. Human influence (surface treatment) and biological growth (e.g. fungus) are major exogenic processes which may alter delta(18)O and delta(13)C in lime mortar.

Tracing formation and durability of calcite in a Punic–Roman cistern mortar (Pantelleria Island, Italy)

“…However, the δ 18 O water value of − 12 ‰ for the remaining water in the cistern is not reasonable to indicate evaporation of cistern water. This is because the evaporation of cistern water of δ 18 O water ≈ − 5 ‰ (VSMOW) results in the opposite trend for the remaining water (δ 18 O water > − 5 ‰), as 18 O is enriched versus 16 O in the remaining H 2 O [66]. Thus, the range and the positive trend of δ 18 O calcite and δ 13 C calcite values of the investigated cistern mortar suggest that calcite was formed during the setting reactions of the mortar of the distinct horizons, but not because of the subsequent recrystallization and/or neoformation of calcite from cistern and pore water.…”

“…This calculation results in δ 18 O water of − 12 ‰ (VSMOW). Such 18 O versus 16 O depleted isotopic composition of the meteoric water is highly unlikely for precipitation at Pantelleria Island (see discussion above). In addition, some water could evaporate from the cistern.…”

“…The distinct shifts in isotopic composition from the left to the right for each horizon, corresponding to the exterior to the interior mortar layers, can be explained by a Rayleigh isotope fractionation process (Figures 4 and 5). As calcite formation continued during setting of the lime mortar, the remaining gaseous CO 2 in the pore space was subsequently enriched in 13 C and 18 O versus 12 C and 16 O. This caused subsequently formed calcite to be isotopically heavier along the setting path from the left to the right for each horizon (Figure 4).…”

“…In the present contribution, the changes in mineralogical, chemical, and isotopic composition of hydraulic lime mortar along a vertical profile of the cistern from Pantelleria Island (Italy) are studied. The 13 C/ 12 C and 18 O/ 16 O ratios of the mortar were investigated to trace the formation conditions, individual processing periods, and alteration of the calcium carbonate bearing binder.…”

“…The information about the chemical composition and distribution of stable carbon and oxygen isotopes of the calcium carbonate binder is a powerful tool to elucidate the environmental conditions of its formation and subsequent transformation [3,[10][11][12][13][14][15][16]. However, the combined isotopic and micro-chemical investigations of an entire profile of hydraulic lime mortar are sparse.…”