**2.1 Conventional materials**

The building envelop influences heat conduction through roof, wall, fenestrations and determine the quantum of sensible cooling/heating load requirement to balance comfort condition. As per the report, published by International Energy Agency in 2013, the demand for space air-conditioning is estimated to rise three fold between 2010 and 2050 on account of more numbers of hot days [6]. To restrict more heat entry inside the building, insulating materials are put to use. The conventional materials, which are used for this purpose are mostly by-products of fossil fuel oil industries, and the cost and embodied energy content of those are very high, besides being hazardous at the end of life disposal scenario. Choice of materials for energy efficient envelop construction should cater to the issues of durability, environmental sustainability, local sourcing of materials to reduce transportation related emission etc. Concrete is an integral composite material in modern building construction with considerable carbon footprint, due to one of its constituent with high embodied energy content, which is cement. It has been observed that Ordinary Portland Cement (OPC) contains the highest Global Warming Potential (GWP), Photochemical Ozone

Creation Potential (POCP) and Abiotic Depletion Potential (ADP) [7]. Paints with high reflection parameter on roof and other types of insulating materials are included in energy efficient building design. The manufacturing energy and GWP of normally used such materials are on the higher side [8]. Bergey had presented in a Symposia about the comparative study of various commercially available insulating materials, among which XPS was observed to be with highest embodied global warming potential (GWP) [9]. High albedo coating with cool roof feature contain embodied energy to the tune of 23 MJ/m2 of roof surface [10].

The walls in a building are conventionally made up of bricks, joined with mortar and covered by plaster on both sides, topped with paint and other finishing. The materials used for mortar and plaster are cement and sand of different proportions and grade (MM3/MM5 etc.) [11].

Since, major carbon intensive component in concrete is cement, sustainable concrete mixes had been adopted for this work with the inclusion of Portland Pozzolana Cement (PPC) (30% fly ash blended), stone aggregate, sand, water and fly ash /bottom ash / marble dust / lime dust.

#### **2.2 Coal ash as constituent material in envelop construction**

Coal ash is basically a combination of lighter fly ash (75–80%) and coarser bottom ash particles, produced out of coal combustion in thermal power plants with zero embodied energy content. Depending upon its CaO percentage in the composition, it is classified as Class C (with some cementatious property) or Class F (with pozzolanic property). Globally, 100% utilization of coal ash from all the thermal power plants has not become possible till date. Cumulative accumulation of the un-utilized coal ash each year in ash dykes are creating groundwater contamination, air pollution etc. On the other hand, due to the rapid growth in global infrastructure sector, unprecedented rate and pace of sand mining from river bed is threatening the ecological balance enormously. Various researchers have explored the suitability of this industrial waste, which is coal ash in building construction, as a constituent material of concrete and mortar. To name a few, Higgins had compared one tone of concrete made of ordinary Portland cement as main constituent and the same quantity of concrete with Portland pozzolana cement with 30% flyash blend. It was observed that 17% less CO2 emission to the atmosphere, 14% less primary energy requirement and 4% less mineral extraction resulted with such substitution [12]. The earlier works related to utilization of coal bottom ash in concrete have been studied. OPC, sand, bottom ash and stone aggregate as the concrete mix constituents were used. The results revealed that 10–30% replacement of sand by bottom ash did not adversely impact the desired strength gain in the concrete, barring some losses in workability and flexural strength parameters [13–19]. Another group of Researchers investigated about the suitability of fly ash and bottom ash as replacement material of cement and normal river sand utilized in concrete making. The compressive strength values at 28 days after casting were noted to be without change in comparison with conventional concrete mix ingredients. The workability parameter of the concrete mix was noted to be stiff, but at a longer maturity period, strength increased considerably. Toxicity parameters and durability aspects including leaching procedure, sulfate and acid attack and elevated temperature effects on concrete blended with coal ash as substitute to cement and sand were also studied, and the test results did not reflect any adverse impact, and as such considered to be used as

clean construction material [20, 21]. Other researchers explored about the usage of fly ash as fine aggregate in masonry mortar and found that up to a considerable replacement ratio, the fly ash blended mortar can be used [22]. Soheil Oruji, Nicholas A. Brake and others tried to see if the finely ground bottom ash can act as an alternative material to cement in mortar preparation. The fineness effect on workability as well as, on setting time were studied. Improvement in micro-structure of cement mortar and increase in the strength parameter of such product was observed [23]. Kim had experimented with sieved and ground coal bottom ash in high strength cement mortar. The ground bottom ash was found to increase the workability and compressive strength values compared to the equivalent mortar made of cement and fly ash [24]. Shahidan et al. had studied the physical and chemical properties of coal bottom ash, as a replacement material for sand. The gradation of particles in bottom ash and sand showed some similarity, and overall, bottom ash is recommended favorably as a replacement material to sand [25]. Abbas et al. had also studied the effect on cement and sand by limestone dust and bottom ash partially respectively. For a number of mixes, sand substitution by bottom ash were done in various replacement ratios, and limestone dust replacement ratio with cement was maintained constant at 5% ratio. Water-cement ratio was same for all mixes. Increase in strength was found consistent up to 30% sand substitution and 5% cement substitution [26]. Ghosh et al. had experimented with coal bottom ash and fly ash separately as sand substitute in different concrete mix proportions. It was observed that with increasing percentage of replacement, thermal resistance parameter increased but the strength parameters decreased. Up to 40% replacement, the blended concrete exhibited desired strength with considerable percentage of decreased thermal conductivity value [27]. In another set of experiments, Ghosh et al. had further observed the effect of coal bottom ash and fly ash separately on masonry mortar of different proportions. The sand in the mortar was replaced by bottom ash and fly ash (separately) in steps of 10% up to 100%. The masonry mortar minimum strength criteria was observed to be fulfilled up to 100% replacement ratio, and specific mortar grade requirement was fulfilled up to 60% replacement with an astounding result of lower thermal conductivity [28].
