Effect of biomineralization on the early mechanical properties and microstructure of fly-ash cement-based materials

Abstract

The lower early strength of fly ash (FA) as the partial replacement of cement has always been an inherent question, biomineralization furnishes a developing technology for promoting hydration and improving early compressive strength of FA cement-based materials (FACM). In this paper, the effects on the properties of FACM of biomineralization were investigated by two treatment methods, direct mixed bacteria (BDM) and drying-soaking (BDS). The properties, including microscopic performance and the mineral fraction of the paste were explored by using hydration heat, X-ray diffraction (XRD), thermogravimetric differential heat analysis (TG-DTG), pore structure tests, and scanning electron microscopy (SEM). The experimental results revealed that the early strength performance of the biomineralization treated FACM was significantly improved and achieved a maximum when the FA content was 10 %. With curing ages of 3 days, 7 days, and 14 days, the strength improvement in BDM was 33.01 %, 10.54 %, and 16.24 %, and that of BDS was 25.65 %, 8.68 %, and 3.54 %, respectively. XRD and TG-DTG analysis demonstrated that there were no new hydration products in the FACM specimens after biomineralization treatments. However, the hydration of FACM was promoted and more AFt and CSH gels being facilitated by biomineralization. The microstructure results also showed that the increase in the amount of hydration products changed the micropore structure distribution by refining the porosity, resulting in a more compact structure. The BDM approach more effectively promotes the formation of hydration products than the BDS method. The ability of these bacteria to facilitate the generation of hydration products of FACM can be applied to solve issues with the early strength of such materials.