To obtain a restoring and protective calcite layer on degraded limestone, five different strains of the Bacillus sphaericus group and one strain of Bacillus lentus were tested for their ureolytic driven calcium carbonate precipitation. Although all the Bacillus strains were capable of depositing calcium carbonate, differences occurred in the amount of precipitated calcium carbonate on agar plate colonies. Seven parameters involved in the process were examined: calcite deposition on limestone cubes, pH increase, urea degrading capacity, extracellular polymeric substances (EPS)-production, biofilm formation, zeta-potential and deposition of dense crystal layers. The strain selection for optimal deposition of a dense CaCO(3) layer on limestone, was based on decrease in water absorption rate by treated limestone. Not all of the bacterial strains were effective in the restoration of deteriorated Euville limestone. The best calcite precipitating strains were characterised by high ureolytic efficiency, homogeneous calcite deposition on limestone cubes and a very negative zeta-potential.

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Concrete specimens weathered for over a decade in the moderate Belgian climate, showing a black organic outer layer that mainly consisted of lichens, were cleaned with a new biological technique. A mixture of sulphur oxidising bacteria of the genus Thiobacillus supplemented with an appropriate nutrient was applied to a fouled concrete surface, either by sprinkling or by submersion. The aim was to remove the fouled layer in such a way that the surface is uniformly cleaned. The general effect of the technique was evaluated by colorimetry and microscopy. Two sets of weathered concrete specimens, containing blast furnace slag cement or ordinary portland cement, were investigated. The effectiveness of the technique depended on the cement type of the concrete specimens. The effect on the ordinary portland cement concrete specimens was in some cases up to a factor 2 stronger than the result on the blast furnace slag cement specimens. The sprinkling treatment was about 50% as effective as the submersion treatment but was very promising in the case of in situ acidification. A side effect was the fort-nation of a gypsum layer on some of the specimens, resulting in a whiter colour

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