Bioconsolidation of Damaged Construction Calcarenites and Evaluation of the Improvement in Their Petrophysical and Mechanical Properties

Abstract

Bioconsolidation treatment using bacterial carbonatogenesis has been proposed as an environmentally friendly strategy for the efficient preservation of damaged stones, particularly suitable for carbonate stones. The study presented here deals with the evaluation of the performance of this treatment, applied to damaged carbonate stones in two historical buildings in Spain. The methodology applied in this research serves as a reference for future similar studies. Results showed significant improvement in the petrophysical and mechanical properties of the damaged stone following the treatment through the production of calcite and vaterite by the abundant carbonatogenic bacteria inhabiting the stone. These bacteria were able to effectively consolidate weathered areas if an adequate nutritional solution was employed, thereby augmenting the stone’s resistance, as evidenced by the Drilling Resistance Measurement System (DRMS). FESEM images showed calcified bacteria and calcified exopolymeric substances (EPS) consolidating stone minerals without blocking their pores. In addition to consolidation, this biotreatment improves the stone’s behavior against water absorption and increases the contact angle of water droplets without significant modifications in the pore size or diminishing vapor permeability. No color changes are observed. Overall, these results show that the application of the nutritional solution (M-3P) for in situ consolidation of different types of porous carbonate building stones is a highly effective conservation method, with no modification of the chemical composition of the treated materials.

Evaluation of Eco-Friendly Consolidating Treatments in Pugliese Tuff (Gravina Calcarenite) Used in Italian Heritage Buildings

“…The FESEM observation (Figure 9b) of the bioconsolidated samples at 5000× magnification shows that the microtopography of the stone is completely covered by a biofilm of calcified bacteria of about 2 microns in average size. This behavior is similar to that observed in other cases in which bioconsolidants have been used on calcarenitic rocks [49,50,76].…”

“…The FESEM observation (Figure 9b) of the bioconsolidated samples at 5000× magnification shows that the microtopography of the stone is completely covered by a biofilm of calcified bacteria of about 2 microns in average size. This behavior is similar to that observed in other cases in which bioconsolidants have been used on calcarenitic rocks [49,50,76]. The bulk density before and after the test is similar for the usually bioconsolidation treatment, while in the double bioconsolidation, the density increased by 2.3% on average, which implies an improvement in the compactness of the treated specimen (data obtained from the increase in weight of the cubic test specimens).…”

“…The average value of the measurements has been assumed 1.24 N of force up to 2 cm depth. These results indicate a low mechanical resistance compared to other studies of similar calcarenites [50].…”

“…Two possible eco-friendly treatments have been selected, including "Lime Water" [48] from the waste from aerial lime slaking and bioconsolidant [49,50], which generates a biomineralisation by bacterial carbonatogenesis that produces calcium carbonate capable of consolidating the treated stone.…”

“…Bioconsolidant-treated specimens were tested with two different application methods as follows: the first with 7 days of application [50] (which is the usual method) and the second with 7 days of application, 7 days of rest and 7 days of application. This second method has been carried out because, with the normal 7-day application, not many improvements were observed.…”