Evaluating durability parameters of concrete containing limestone powder and slag under bacterial remediation

Microbiological methods, in which bacteria serve as catalysts and contribute to urea hydrolysis by producing the urease enzyme, can be used to enhance concrete durability and repair the entrained pores and micro-cracks. The hydrolysis of urea in a calcium-rich environment leads to the formation of solid calcium carbonate that fills concrete pores and cracks. At least 7% of the total carbon dioxide available on a global scale originates from cement production. This has encouraged efforts to decrease cement consumption in concrete structures and to replace it with such cement-like materials as fly ash, blast furnace slag, micro silica, or limestone powder. Slag and fine limestone have nowadays gained wide applications as cement replacements for their technical, economic, and environmental advantages. The present study was designed and implemented to investigate concrete durability, water absorption, carbonation depth, and water penetration depth in specimens containing slag and fine limestone powder in their concrete mixes, where exposed to the bacterial treatment. For this study, 78 concrete cubes of 100 mm in size, 78 cubes of 70 mm, and 78 prisms of 120 x 200 x 200 mm(3) were made using six concrete mix designs containing two limestone powder ratios of 0 and 15% and three slag ratios of 0, 20, and 40% as replacements for cement. The specimens were subsequently treated for 28 days in one of the curing media of tap water, seawater, and a solution of calcium lactate and urea. Finally, the surfaces of the specimens were treated for two weeks in one of the above media. Results showed improved durability in both concrete specimens with bacteria in their mix designs and those subjected to surface treatment with bacteria. Moreover, all the specimens with bacteria recorded lower values of water absorption, carbonation, and water penetration depths than those lacking bacteria. As regards the cement replacement materials, it was found that limestone powder served as a source of calcium for bacteria and that slag enhanced the compressive strength of concrete and reduced its permeability. Finally, the solution of calcium lactate and urea from among the curing media investigated was found to have better effects on the specimens either containing bacteria in their mix designs or subjected to surface remediation with bacteria.