Abstract. Recent researches have indicated the manufacturing of several useful products by using different industrial wastes all over the world. The materials remain as a waste till their potential to right use is understood. The solid waste in India has emerged as a great threat to the environmental health of the country. The sources for solid waste are industrial wastes, wastes from municipal, agricultural, mining and other processes. The disposal of solid waste is gradually becoming a burden for society, particularly in heavily populated regions. Nowadays, building materials are manufactured from various environmental wastes based on needs and availability. An overview has been given on the manufacturing of Autoclaved Aerated Concrete (AAC), one of the potential building materials. AAC, a building material, is manufactured by utilizing the solid wastes like fly ash, glass waste, bottom coal ash, rice husk, slags, perlite wastes, natural zeolite for its production. Efforts to use the solid waste for the production of AAC materials have been addressed in this work. The replacement of major raw material (sand) with the solid/industrial wastes in manufacturing of AAC leads towards a sustainable process. In this paper, present status on manufacturing of AAC as well as possible utilization of industrial wastes for its production is presented. The observations of a complete industrial manufacturing process of AAC block along with advantages, applications, cost benefit analysis, challenging issues and future scopes have been highlighted.
Introduction The conservation of natural resources, eco-friendly manufacturing, energy saving and comfort living have forced to search a sustainable manufacturing process for a sustainable building material. The construction industry in India is responsible for the largest share of CO2 emission into the atmosphere . The solid waste in India has emerged as a great threat to the environmental health of the country. The urban India alone generates 1.5 lakh metric ton of waste per day that is increasing every year by 5 2 %. This increase in the solid waste generation is due to the rising population and consumption patterns . On the other hand, clay bricks are manufactured using top fertile soil causing soil infertility, threat to soil erosion and also a concern of food production.
Huge amount of energy is required for the production of clay bricks. A large amount of fertile top soil gets consumed and locked into structures in the form of red clay bricks. The CO 2 emissions during manufacturing of red clay brick has significantly contributed to the global warming . The air pollution and global warming have become now a global environmental problem. Nowadays, the construction industry in India has vastly highlighted the concerns of damage on earth due to manufacturing of red clay bricks. For greener environment, it is recommended to replace the red clay brick with the equivalent sustainable material. The autoclaved aerated concrete (AAC) block or cellular lightweight concrete brick has emerged as the best alternative of clay brick as a sustainable building material.
Further, the kilns meant for heat treatment source in clay brick create air pollution and is replaced by steam based heat treatment, so called autoclave, in the AAC production industries. This work aims to study researches happened on use of solid/industrial wastes as replacement of major raw material (sand) in manufacturing of AAC blocks. The use of solid/industrial wastes leads to sustainable manufacturing process for building bricks in place of burnt clay brick. The observations of a complete industrial manufacturing process of AAC blocks and its cost benefit analysis have been studied. The detailed manufacturing stages of AAC, its industrial observation and the cost benefit analysis for its application have been rarely presented in any of the past literature.
History The AAC block was invented in the mid-1920s by the Swedish Architect Dr. John Axel Eriksson and was patented in 1924 [4, 5]. The first AAC blocks plant in India was setup in 1970s by Siporex at Pune. However, in the North-Eastern part of the country, it was produced in the year 2014 and opened for commercialization in 2015 . The AAC block is a complete new product in North-Eastern part of the Indian subcontinent which falls in seismic zone V. The AAC is produced by adding a proper amount of aluminum powder and other additives into the slurry of fly ash or silica sand, cement, lime and water . The Fly ash used as aggregates in combination with silica, is the waste product from power plants . Aluminum powder, used as a foaming agent in the manufacturing of AAC, is responsible to make it lightweight [6-9]. AAC blocks provide opportunities to increase the building quality at reduced cost. The recent developments in AAC production have highlighted the need of industrial wastes utilization in AAC manufacturing. In the past, several researchers have investigated the possibilities for the use of industrial wastes in the production of AAC, such as use of air-cooled slag in place of sand and limes as revealed by Mostafa . The use of coal bottom ash , 3 efflorescence sand  copper trailing  and carbide slag  have been also studied for the substitution of quartz sand in the production of AAC. Authors  studied the use of different types of glassy wastes instead of sand for designing AAC. The mechanical and physical properties of the final AAC product were also investigated by them. Therefore, the use of industrial wastes for the production of AAC materials leads to the sustainable development, which is environmentally friendly, economical, and also energy efficient. Rozycka and Pichor  utilized the perlite wastes as replacement of sand for the production of AAC materials and investigated the main properties of respective AAC products.
Advantages of Applications of AAC AAC material is a special kind of concrete, which uses fly ash or sand, lime, cement, water and a foaming agent (aluminum powder) for its manufacturing. The AAC material conforms to be an eco-friendly material with almost negligible environmental impacts. The porosity property due to expansion process imparts lots of interesting features/advantages in this type of material to be well fitted for building application. All the interesting features/advantages have been well described in the following subsections.
Environmentally friendly The present AAC materials help in establishing a green environment due to its environmentally friendly nature. In the AAC production process, no toxic by-products or pollutants are evolved and diffused in air. Since all the ingredients used in the production of AAC blocks are inorganic in nature, it does not contain any toxic substance. The environmental pollution from clay brick-making operations is injurious to human health, animals and plant life. At a global level, the air pollution from clay brick-making operations contributes to the phenomena of climate change and global warming. AAC brick manufacturing puts less tax on environment. Moreover, the AAC materials are also helpful for consuming the power plant by- products (fly ash).
Lightweight The gas formed by the reaction of aluminum with a soluble alkali in the lime/cement slurry makes the aerated concrete lightweight . For same shape and size, AAC weighs 30% of the traditional clay bricks . The lightweight cellular structure of AAC makes ease of handling, transportation and wall installation for the construction activities . The lightweight property of AAC material also imparts high strength to weight ratio. AAC is typically a low density, porous and lightweight, by which it reduces the seismic inertial force acting on the structure as compared to the conventional concrete.
Good insulation With high porosity, thereby low density (~500 kg/m 3 ) and thermal conductivity (0.1 W/mK), the AAC material can serve as a sound-proofing and thermal insulation material . The porosity value for AAC material was found to be 82.1-91.5% for a density range of 250-500 kg/m 3 . It has a better sound absorption property than a smooth dense concrete and clay brick. The total porosity decreases with increase in density. The thermal conductivity of any cellular concrete is due to air filled pores. The value of thermal conductivity, k (0.08 W/mK to 0.21 W/mK) of aerated concrete primarily depends on its density . Further, AAC is non-combustible (fire resistant) and hence in case of fire, it can help to prevent the fire from transfer to other rooms. No toxic gases or vapors emit from the AAC at the time of fire .
Less energy consumption The AAC is energy efficient . Frey and Briesemann  studied the factors affecting the primary energy costs to produce autoclaved aerated concrete. They found that for the plants, raw materials production, raw materials transportation, and actual factory production accounted for 60%, 5%, and 35% of the energy costs, respectively. They indicated that the process is currently very energy efficient. Bave  pointed out other factors relating to energy savings in the use of autoclaved aerated concrete. These factors included raw materials, the production process, transportation and erection of the manufactured units. As an example, he stated that the relatively low weight of aerated concrete building units allows cuts in the energy consumption due to handling and assembly on site.
Application of AAC Autoclaved aerated concrete (AAC) can be used in the form of block and panel for masonry wall constructions, floors, roof insulation, trench fills and for other insulating purposes . There is an increasing use of this material for non-structural applications in the past decade such as infill panels and cladding. AAC has a wide range of application for building construction with residential, commercial and industrial buildings. AAC block, being lightweight, is suitable for the realization of masonry bearing wall of low to medium story building in the seismic zones for good seismic resistances .
AAC is as workable as wood and can be easily cut, shaped or directed to accommodate almost any design . AAC block and panel both are manufactured according to specification/dimension of the customer requirements. As AAC is a prefabricated product, it is of high precision and drillable. AAC panels are used directly in internal wall of concrete structures and steel structures, such as schools, in hotels, offices, houses and market places etc. The internal walls such as, separation walls, household walls, partition walls in bathroom and kitchen also finds the 5 application of AAC panel. The AAC blocks have application in both external and internal walls. Several authors reported the application of AAC material in the structure of wall system, and roof. Researchers  studied the utilization of AAC in green roof structure. Authors  investigated the mechanical characterization of the building wall masonry made of autoclaved aerated concrete subjected to in-plane loading.