Mechanism of bacteria controlling gradient mineralization at surface layer of cement-based materials

Microbial mineralization refers to the formation of inorganic minerals by action of microorganisms.1,2 The earliest study in this field could be traced back to the 19th century, where early aquatic and soil microbiologists carried out pioneering studies of non-medical microbial processes in soils, sediments and waters, opening the door to the empire of environmental microbiology, microbial ecology, geological microbiology and microbial geochemistry.3,4 In 1838, for the first time Ehrenberg5 explored the close relationship between ochre iron deposits in swamps and Gallionella ferruginea, investigating the important role of these bacteria in the formation of such sediments. In 1890, Muentz6 started to probe into the action process of bacteria in rock weathering and soil formation, which initiated the study of microbial weathering. Afterwards, Beijerinck7 discovered that microbes were involved in the oxidation and deposition of Mn in nature in 1931, who didn’t come singly but in pairs when Woksman8 reported that the microbial metabolites participated in geological processes, such as nitrifying bacteria, lichens, fungi and other acid-producing microorganisms in 1932. In many studies so far,9–11 it is believed that there are two main forms of microbial mineralization: one is BIM (bacteria-induced mineralization), generally nucleate and grow out of cells. The metabolites secreted by organisms are transported to the extracellular and react with ions and compounds in the environment, causing changes in pH-Eh in the solution or environment, leading to mineral deposition. The second is BCM (bacteria-controlled mineralization). Microorganism cells determine the form of mineral formation.12,13 In BCM, minerals are usually deposited inside the organic matrix or vesicles of biological cells, allowing organisms to control the nucleation and growth of minerals significantly, thus influencing the composition and size of mineralization. In cementbased materials, microbial mineralization is mainly used to repair cracks.14 In this study, the mechanism of microbial mineralization in the pores of cement-based materials and its effects on the pore structure were identified by the help of a numerical simulation of gradient mineralization.