Industrial scale waste utilisation in unfired bricks

Abstract

Lime stabilised fly ash bricks (a type of unfired bricks) are gaining popularity against fired clay bricks due to their green benefits. Several studies have shown the laboratory scale feasibility of waste incorporated unfired bricks. However, the waste utilisation at industrial scale is still limited. The detailed literature review and discussions with industries have been carried out to understand an up-to-date research status and reasons are identified behind the limited industrial-scale waste utilisation in unfired bricks. The key reasons have been identified as insufficient characterization data of locally available wastes, the poor economic viability of wastes available at distant sources and lack of clarity about optimized production parameters. In India, red colour is considered an auspicious colour, making the grey coloured fly ash bricks, difficult to compete against red coloured fired clay bricks in the consumer market. A large amount of coloured stone processing waste is available to use as a filler in unfired bricks, which has not been explored in detail till now. Ensuring the waste blended brick mix's homogeneity is vital for optimized production and needs an appropriate measurement tool for industrial scale implementation. Looking at the various perspectives for improving the sustainability of unfired brick industries, the present thesis primarily focuses on exploring the potential of locally available industrial ash, establishing the suitable characterization protocol, investigating optimized production parameters such as mixing sequence and moulding moisture content, exploring stone waste for improved aesthetics of fly ash bricks and development of a simple non-destructive tool for measuring the homogeneity of the brick mix. In the present thesis, an experimental program comprising micro-level characterization techniques (XRD patterns, FT-IR spectra, Raman spectra, and SEM images) and macro-level tests (loss on ignition, and lime reactivity values) has been used for comprehensive characterisation of industrial ashes and to evaluate the potential of utilizing industrial ashes in unfired bricks. A total of six different ashes have been characterised, including two biomass ash and three coal-based ash from local industries, and one coal-based ash from a thermal power plant. To understand optimum mixing methodology and moisture content, the combined effect of two distinct mixing sequences (two-stage and three stage) at five different moisture contents (10% - 15%) has been experimentally examined on the mechanical properties, and microstructure of hydrated lime stabilised fly ash bricks. The coloured stone processing wastes have been explored to upgrade fly ash bricks' aesthetics for value addition and improved social acceptance. A total of four different stone processing wastes have been characterised using XRD patterns, FT-IR spectra, and Raman spectra. The stone processing wastes are blended with GGBS to produce coloured geopolymer mortars, and their potential is evaluated for producing coloured masonry bricks. To study the homogeneity of a dry binary blend, two stone processing wastes have been characterised adopting a three-stage procedure, and Raman spectromicroscopy has been established as a non destructive tool to study the chemical homogeneity at the microscopic scale. To make the coloured bricks economically viable, the bi-layered bricks are conceptualised, and their manufacturability is demonstrated at the laboratory scale via a four-step process in an existing industrial set-up (used for conventional fly ash bricks). It has been revealed in the thesis that a comprehensive characterization methodology comprising the macro-level tests (LOI and lime reactivity test) and micro-level characterization techniques (XRD, FT-IR, Raman spectroscopy and lime reactivity test) has been found suitable for characterization of industrial ashes. By and large, local coal-based industrial ashes can be suggested for potential utilization in lime-based fly ash bricks based on their comprehensive characterisation. Raman spectroscopy has been found to be instrumental in analysing the trio of amorphous, crystalline and carbon bands present in the industrial ashes. Bricks with 15% moisture content produced by a two-stage mixing sequence are found to have improved microstructure and enhanced physical characteristics. The incorporation of stone waste is found to decrease the compressive strength of geopolymer mortars. However, the stone waste blended coloured geopolymer mortars has still attained compressive strength greater than 10 MPa and corroborate the high potential for producing coloured masonry bricks equivalent to class designation 10 fly ash bricks as per IS 12894. Chemical imaging using Raman spectroscopy has been found a simple and effective tool to understand the homogeneity of the dry binary blend at the microscopic level, which was not possible by other simple techniques. The coloured bi-layered bricks have tremendous potential for cost-saving of up to 35% of the overall cost (accounting for the cost of a finished brick surface). These new bricks combine the desired features of aesthetics and durability, and therefore, show a great promise as a cost-effective alternative to conventional fly ash bricks.