Surface Consolidation of Maastricht Limestone by Means of <i>Bacillus Sphaericus</i> under Varying Treatment Conditions

Abstract

The exploration of biodeposition treatment for the surface consolidation of natural stones is limited by thus-far used study-specific treatment conditions. This study explores the surface consolidation performance of Bacillus sphaericus in varying treatment conditions and compares the results with the application of a tetraethyl orthosilicate (TEOS)-based chemical consolidant to suggest a more standardized treatment procedure. Initially, the biodeposition medium was optimized for maximal ureolytic activity. Then, consolidation procedures with different times and numbers of applications were conducted on sound (no aging) Maastricht limestone. Consolidation performances were evaluated by comparing the changes in the ultrasonic pulse velocity (P-wave) through the specimens and the drilling resistance of the specimens. Upon two or more subsequent biotreatments, the transmitting velocity of the P-wave increased by up to 25% in the first 20&nbsp;mm of the specimen. The drilling resistance of the specimen increased by 100% up to a depth of 35&nbsp;mm from the surface. A minimum of two subsequent biotreatments are suggested for a decent consolidation of Maastricht limestone, and a minimum of four subsequent treatments are suggested to replace TEOS.