Influence of pore structure on the effectiveness of a biogenic carbonate surface treatment for limestone conservation

Abstract

A ureolytic biodeposition treatment was applied to five types of limestone in order to investigate the effect of pore structure on the protective performance of a biogenic carbonate surface treatment. Protective performance was assessed by means of transport and degradation processes, and the penetration depth of the treatment was visualized by microtomography. Pore size governs bacterial adsorption and hence the location and amount of carbonate precipitated. This study indicated that in macroporous stone, biogenic carbonate formation occurred to a larger extent and at greater depths than in microporous stone. As a consequence, the biodeposition treatment exhibited the greatest protective performance on macroporous stone. While precipitation was limited to the outer surface of microporous stone, biogenic carbonate formation occurred at depths of greater than 2 mm for Savonnières and Euville. For Savonnières, the presence of biogenic carbonate resulted in a 20-fold decreased rate of water absorption, which resulted in increased resistance to sodium sulfate attack and to freezing and thawing. While untreated samples were completely degraded after 15 cycles of salt attack, no damage was observed in biodeposition-treated Savonnières. From this study, it is clear that biodeposition is very effective and more feasible for macroporous stones than for microporous stones.

Fig. 1.

Pore size distributions of the different types of limestone used in this study as determined by MIP (A) and microtomography (B).

Fig. 2.

Influences of the cell number (A), the presence of calcium (B), and the stone type (C) on the amount of urea hydrolyzed by B. sphaericus cultures.

Fig. 3.

Scanning electron micrographs (top view) of untreated (pictures on the left) and biodeposition-treated (pictures on the right) limestone. Note the presence of a newly formed layer of carbonate crystals on the surface of biodeposition-treated limestone.

Fig. 4.

Thin sections of untreated (pictures on the left) and biodeposition-treated (pictures on the right) limestone. Note the differences in the amounts and sizes of the newly formed carbonate crystals (indicated by black arrowheads) on different types of biodeposition-treated stone.

Fig. 5.

2D (left) and 3D (middle and right) microtomographs of biodeposition-treated limestone. Limestone cylinders were treated on all sides by immersion. Newly formed carbonate crystals are yellow. A rectangular region (right picture) was extracted from the limestone core (middle picture) in order to get an appreciation of the distribution of the biogenic crystals inside the pores of the limestone. Note the differences in coverage and penetration depth in the different types of biodeposition-treated stone.

Fig. 6.

Influence of the biodeposition treatment on the water absorption (A and B) and drying behavior (C and D) of stones that differ in porosity. Drying behavior is expressed as percent weight loss, i.e., the weight of the water lost due to evaporation compared to the weight of the water initially present inside the water-saturated stone.

Fig. 7.

Influence of the biodeposition treatment on the resistance of stone to sonication. The lower the weight loss after sonication, the greater the consolidating effect of the treatment.

Fig. 8.

Influence of the biodeposition treatment on the resistance of stone to salt attack. A smaller weight loss after freezing and thawing indicates higher resistance to salt attack. Note the difference in scale between the y axes of the two graphs. Weight loss is expressed as a percentage of the initial dry weight of the stone.

Fig. 9.

Visual appearance of Savonnières (A) and Massangis (B) limestone cubes after 14 cycles of freezing and thawing. The four cubes on the left were biodeposition treated, while the four cubes on the right were untreated.