**1. Introduction**

Coal, being the largest source of power worldwide, contributes maximum in respect of global CO2 Emission. More than 33% of global electricity was produced by 2100 GW cumulative capacity coal based thermal power plants in 2020. China, United States and India are ranked as top three nations for coal combustion linked electricity production [1]. This being fossil fuel, and linked to CO2 emission, worldwide movement has been started to tone down its usage. The ash produced as a fallout of coal burning is classified into fly ash and bottom ash. It is considered that for every 4 tonnes of coal burnt, approximately one ton of coal ash is produced [2]. The ash content actually depends on type of coal burnt, which is classified as anthracite, bituminous, sub-bituminous and lignite. But, the legacy ash already stored in various ash ponds of thermal power plants is huge and continuously affecting the environment by contaminating surrounding air and soil. In spite of various efforts undertaken by authorities, 100% ash utilization could not be made possible. On the other hand, sand, the natural soft mineral is continuously depleted due to mindless sand mining mainly for infrastructure related developmental requirements. In fact, rate of formation of sand is getting outpaced by rate of extraction, thus creating a serious environmental imbalances. Considering the rapid growth in urbanization and particularly huge demand in building sector, the energy consumption by building sector

alone is enormous. Overall, buildings accounted for 36 per cent of global energy demand and 37 per cent of energy-related CO2 emissions in 2020. Since the signing of the Paris Agreement in 2015, greenhouse gas emissions from the buildings and construction sector had peaked in 2019 and subsequently fallen to 2007 levels in 2020 mostly due to the COVID-19 pandemic [3]. Despite the expected rebound in emissions in 2021 being moderated by continued power sector de-carbonization, buildings remain off track to achieve carbon neutrality by 2050. To meet this target, all new buildings and 20% of the existing building stock would need to be zero-carbon-ready as soon as 2030 [4]. Buildings consume energy at different levels of the life cycle. The fastest-increasing end uses of energy in buildings are for space cooling, appliances and electric plug-loads, which contribute buildings sector electricity demand growth. Researchers observed that operational energy requirement by buildings occupy lion's share (around 80%) and rest is shared by material embodied energy including transportation, construction etc. related energy consumptions. It was also observed that a normal residential purpose use building and an office purpose use building consume on an average 275 kWh/m2 /year, and 400 kWh/m2 /year respectively [5]. Though these figures depend on many factors like climatic condition of the area, material choice, orientation / layout etc. the overall life-cycle energy figure can be optimized by appropriate planning and design. Among all the building components, envelop influences in deciding the energy ingress or egress to and from the building core. Such flow of energy ultimately determines the operational energy requirement for the particular building, depending on its functionality. Therefore the design of envelop with appropriate material property can contribute significantly from energy efficiency point of view. This research topic presents an approach in passive design of building envelop by selecting waste based abundantly available material, which might evolve into less heat conducting concrete and mortar. Thus, coal ash substitution in building construction industry address the issues of effective thermal power plant waste utilization, arresting rapid depletion of sand and improvement in building energy efficiency together.
