Research Article Effect of Bacillus cohnii on Some Physicomechanical and Microstructural Properties of Ordinary Portland Cement Ngari Reginah Wangui , 1 Joseph Karanja Thiong’o, 1 and Jackson Muthengia Wachira 2 1 Department of Chemistry, Kenyatta University, Nairobi, Kenya 2 Department of Physical Sciences, University of Embu, Embu, Kenya Correspondence should be addressed to Ngari Reginah Wangui; ngarireginah@gmail.com Received 8 August 2020; Revised 7 September 2020; Accepted 14 September 2020; Published 27 September 2020 Academic Editor: Nenad Ignjatovi Copyright©2020NgariReginahWanguietal.isisanopenaccessarticledistributedundertheCreativeCommonsAttribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Cement-made materials face durability and sustainability challenges. is is majorly caused by the presence of cracks. Cracking affects the mechanical strength of cement-based materials. Microbiologically induced calcite precipitation (MICP) has been found to enhance compressive strength, thus enhancing on the mechanical and durability properties of these materials. is paper presents the findings of a study conducted to investigate the effect of Bacilluscohnii on compressive strength development of OPC mortar prisms and the effect of Bacillus cohnii on cement setting time and soundness. Microbial concentration of 1.0 × 10 7 cells·ml −1 was used. Compressive strength tests analyses were carried out for each category of mortar prisms. Compressive strength tests were carried out on the 2 nd ,7 th ,14 th ,28 th ,56 th , and 90 th day of curing in distilled water and microbial solutions. All microbial mortars exhibited a greater compressive strength compared to the control with the highest observed at 90 days. Highest percentage gain in compressive strength was observed at 90 days which is 28.3%. Microstructural analysis was carried out using a scanning electron microscope (SEM) after 28 days of curing. e results indicated the presence of calcium carbonate and more calcium silicate hydrate (CSH) deposits on the bacterial mortars. e bacteria did not have an effect on cement soundness. Setting time was significantly accelerated. 1. Introduction ere is an increasing need to enhance the durability and sustainability of cement-made materials. ese materials are expected to have a long service life of more than fifty years [1]. However, due to degradation, this is sometimes im- possible [2]. Due to this, there is a need for continuous monitoring and repair of cement-made materials which is expensive in the long run [3]. To a great extent, durability aspects of mortar/concrete are possibly connected with its pore structure characteristics [4]. Deleterious processes, such as rebar corrosion, caused by ingress of chloride or carbonation, are aided by connectivity within the pore system of the cement matrix. e vulner- ability of these cement-based materials to the deleterious processes is more if the pore structure is more open and interconnected [5, 6]. Densification of the cement matrix microstructure is one method of sealing the pores. is increases the resistance of the material for ingress of the deleterious materials such as CO 2 , chlorides, and sulphates [7]. Ordinary Portland cement (OPC) exhibits shorter set- ting times and achieves high compressive strength earlier (at 28 days of curing) compared to blended cements [8]. Due to these properties, OPC is preferred in the construction of dams and bridges in Kenya [8, 9]. OPC is, however, prone to attack by deleterious substances such as chlorides and sul- phates because of high amount of Ca(OH) 2 released as a hydration product [8]. e pore structure of OPC is more open compared to that of blended cement structures [9, 10]. Hence, there is a need for densification and refinement of its microstructure. Cement-made structures using OPC have a higher permeability due to their higher tendency to form cracks. ese cracks tend to form a continuous pathway for Hindawi Journal of Chemistry Volume 2020, Article ID 7816079, 8 pages https://doi.org/10.1155/2020/7816079