Characterization of isolates used in bacterial-based strategies for accelerated carbonation of lime mortars

Abstract

Portland cement largely replaced hydraulic lime over the past century because of its rapid hardening. Achieving earlier hardening in lime through faster carbonation is thus essential to help overcome one of lime’s limiting qualities. Here, we isolated two alkaliphilic bacteria, Shouchella clausii and Shouchella patagoniensis, from a lime mortar wall. S. clausii was then further grown in high pH (>11) by adaptive laboratory evolution to acclimate a third isolate. Bacterial suspensions of all three isolates were followed for 14 days in serum bottles at pH 11, and gas composition of the headspace, intact/damaged cell populations, and pH were measured. In parallel, lime mortar samples were incubated in a closed environment with bacterial suspension of the isolates and analyzed with thermogravimetric analysis after 7 and 14 days to quantify carbonation. S. patagoniensis produced more CO2, close to the estimated maximum CO2 uptake rate of lime, and carbonated the lime mortars to a larger extent than the other isolates. Finally, the bacterial suspensions were directly mixed with lime. A linear and homogeneous carbonation of the paste samples was measured compared to water-based pastes, and the development of Liesegang patterns was observed upon phenolphthalein spreading. All this indicated that the organic addition altered the carbonation dynamics of the material, although bacteria did not accelerate carbonation relative to media alone and inhibited it relative to water-based paste. Still, a relationship between bacterial activity, CO2 emission, and carbonation rate was established, but practical aspects of bacterial delivery into lime must be addressed.