Optimum proportion of supplementary cementitious materials to produce blended cement under tannery wastewater

Abstract

Portland cement concrete is a major construction material worldwide. In Bangladesh, mainly two types of composite cement termed as CEM IIA and CEM IIB are commercially produced. However, different adverse conditions may require different portions of blended components and hence, utilization of single type of blended cement in all instances often fails to provide required durability. In recent years, a rapid growth in leather industry is resulting in construction of a large number of tannery factory buildings and related treatment plants. The tannery wastewater contains several detrimental constituents that severely affect concrete strength and durability. It is therefore, of immense importance that a blended cement be developed that perform satisfactorily under aggressive condition like tannery waste water. This research is aimed at studying the behavior of cement mortars prepared with various types of cement mix combination of fly ash and slag when in contact with aggressive aqueous environment of tannery wastewater. Tannery wastewater samples were simulated in the laboratory following recipe developed from raw wastewater. Several variations of cement mix combinations with fly ash and slag were taken which enabled to study the performance and durability of particular mix in tannery wastewater. Cement mortar specimens were casted and immersed in the laboratory simulated tannery wastewater. The study involved evaluation of compressive strength, expansion, weight loss and monitoring surface deterioration through images. Properties of blended cement mixes were studied through their consistency, setting time and flowability tests. In addition, Scanning Electron Microscope (SEM) images and Energy-dispersive X-ray Spectroscopy (EDS) techniques were used to study the change in microstructure of the cement mortars due to being exposed to the harsh aqueous conditions. The compressive strength results over a period of 180 days revealed that blended cement mixes achieved higher strength, resulted in lower expansion and weight loss than those of Ordinary Portland cement. The loss of compressive strength after 180 days submergence period in simulated tannery wastewater, in only slag mixed cement mortars was about 55% which was significantly less than that of OPC cement mortars 63%. The expansion test results revealed that OPC mortars underwent greater expansion of 0.6% compared to only fly ash blended cement mortars (almost no expansion) over a period of 26 weeks. It was inferred from EDS data that there was greater loss of calcium bearing phases in OPC mortars compared to blended cement mortars, which was coherent with the strength and weight loss results. Considering expansion limit for cement mortar, only fly ash blended cement has been found to exhibit desired performance. Hence, the optimum replacement level for fly ash in cement is recommended to be 30% for producing composite cement to be used in tannery wastewater.