Rheoloji değiştirme ajanları: Mikroorganizmalar ile geliştirilen anahtar teknoloji

Recent development in concrete technology enabled the design of highly flowable mixes with improved workability. These advanced mixes require incorporation of fine materials or viscosity modifying agents (VMA) to reduce the possible segregation and bleeding due to the use of high range water reducers (such as superplasticizers). The VMAs used in concrete production are generally produced from acrylic polymers and polysaccharide-based biopolymers obtained from cellulose, starch or bacterial fermentation. Diutan gum, produced by fermentation of Sphinogomonas sp, and welan gum, which is a fermentation product of Alcaligenes sp, are the most commonly used polysaccharide VMAs. Similar polysaccharides can be obtained by fermentation of genetically modified bacteria or using plant cell walls. Most polysaccharide based VMAs are able to increase the viscosity of cement paste and exhibit shear thinning behavior such that increased shear rate results with a substantial decrease in apparent viscosity. This behavior is attributed to the long molecular structure of bio-based polysaccharides. Though highly effective bacterial fermentation products can resist the high PH environment of cement-paste, the ecological population of the species is not known. Thus, they are among the most expensive cement admixtures. Advances in construction technology and risen importance of sustainability initiatives reinforce the use of biological admixtures, however, their relatively high cost can be a major drawback in practical applications. Through the literature, nopal mucilage, brown algae, and bacterial cell walls were proposed as alternatives to these bacterial fermentation products. However, these alternatives also require extra processing which required bigger budget even compared to bacterial fermentation products. This project aims to incorporate bacteria cells to the cement-based mix as VMAs without any extra intervention. To achieve this goal, Sporosarcina pasteurii (S.pasteurii), Bacillus megaterium (B. megaterium), Bacillus subtilis (B. subtilis) and Paenibacillus polymyxa (P. polymyxa) were selected as suitable due to their abundant resource in nature. These Gram-positive bacterial cells include peptidoglycans and polysaccharides in their cell wall structure, which resembles the molecular structure of commercially used VMAs. In addition, these cells, particularly B. subtilis, can influence the viscosity of a suspension due to its motility. Throughout the study, these cells were grown in specified nutrient media and then harvested from the inoculum by centrifuging. Then, these cells were suspended in mixing water and their influence on the rheology of cement paste was evaluated. In addition, the influence of water to cement ratio, the dosage of cells added was evaluated along with the impacts of superplasticizers and fly ash on the performance of bacteria cells as VMAs. There are few established industrial and various small-scale companies that produce biological admixtures for cement-based materials. However, nationwide these biological admixtures (for instance chitosan) are only produced for the food industry. The product obtained by the end of this study is a novel and sustainable practice in Turkey, where the construction industry leads the economy.