1 Introduction
1.1 Waste Substitution in AAC
Waste | Characterization and uses | Observations | Refs. |
---|---|---|---|
Rice husk ash | Silica content is about 92.80%, therefore it can easily replace quartz sand during the formation of AAC | 50% optimize replacement of sand has been done. Due to the higher reactivity, it decreased the autoclaving time by converting C-S-H into tobermorite efficiently | Kunchariyakun et. al. (2015) |
Clinoptilolite (natural zeolite) | Rich in silica content (> 75%) due to which it can work as an aggregate by replacing silica sand in AAC. Its calcined form can be used for generating bubbles in AAC due to the high surface energy after calcination | At 50% of optimum replacement of sand, compressive strength was increased by 27.9% whereas thermal conductance and unit weight (930 kg/m3) was decreased by 11.6% and 14%, respectively. With calcined zeolite, compressive strength was observed as 4.6 MPa without using Al powder | Karakurt et. al. (2010) |
Sugar sediments | It has almost similar chemical composition to that of sand (69.20% SiO2). Therefore, it can be used as an alternative source of sand. Due to the presence of 15.57% CaO, it can be added as alternative to lime | 7.5% and 30% replacement of sand and lime by weight has been done, respectively. As a results, higher compressive strength (6.1 MPa) and lower thermal conductivity (comparatively 26.3% less) was observed | Thongtha et. al. (2014) |
Iron ore tailings (IOTs) and coal gangue (CG) | Around 68% and 34% silica content were presented in IOTs and CG, respectively | Combination of IOTs and CG replaced sand completely in AAC samples of 600 kg/m3 density for which compressive strength was observed 3.68 MPa | Wang et. al. (2016) |
Stone processing Mud (SPM) | Besides, 70% of SiO2 and 15% of Al2O3, 3.5 and 4.5% K2O and Na2O presented, respectively | On 100% replacement of river sand, compressive strength was increased by 3.4% and porosity decreased by 1.3% (average pore size of 0.82 mm) | Wan et. al. (2018) |
Granite dust | 65% SiO2 (average grain size of 15 µm) was presented with 15% of Fe2O3. Therefore, it can be used as an alternative source of silica rich fine aggregate for the preparation of AAC | At 20% optimal replacement, compressive strength was increased by 42%. With 5% solution of sulphuric acid (H2SO4) and hydrochloric acid (HCl), 32% and 54% higher acid resistance has been observed, respectively, when compared to control mix | Zafar et. al. (2020) |
Solid municipal waste (incinerated bottom ash) | Contains 32% SiO2, 29% CaO and 8.6% Al2O3. Works as a replacement of silica rich fine aggregate and aluminum powder | Improved strength with uniformity in pore structure. Decreased drying shrinkage | Song et. al. (2015) |
1.2 Chemical Compositions of Waste Materials
Waste materials | SiO2 | Al2O3 | CaO | Fe2O3 | FeO | Fe2O | P2O5 | SO2 | MgO | Na2O | K2O | CaSO4 | SO3 | LOI | Refs. |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Rice husk | 92.80 | 0.15 | 0.70 | 0.17 | – | – | 1.87 | – | 0.77 | 0.08 | 3.35 | – | – | – | Kunchariyakun et. al. (2015) |
Natural zeolite | 77.07 | 13.56 | 2.36 | 1.59 | – | – | – | – | 1.45 | 0.11 | 3.86 | – | – | – | Karakurt et. al. (2010) |
Sugar sediment | 69.20 | 7.08 | 15.57 | 0.92 | – | – | – | – | 0.12 | 0.90 | 1.20 | – | – | – | Thongtha et. al. (2014) |
Iron ore tailing | 68.96 | 7.68 | 4.35 | 2.32 | 4.47 | – | – | 0.02 | 3.64 | 1.41 | 1.85 | – | – | 2.49 | Wang et. al. (2016) |
Coal gangue | 34.05 | 26.00 | 0.67 | 0.49 | 1.70 | – | – | 0.28 | 0.61 | - | 0.16 | – | – | 32.76 | |
Autoclaved coal | 51.05 | 36.71 | 1.05 | 3.12 | 0.42 | – | – | – | 0.92 | - | 0.23 | – | – | 0.72 | |
Stone sawing mud | 72.66 | 15.09 | 1.56 | 1.15 | – | – | – | – | 0.38 | 4.54 | 3.58 | – | – | 0.42 | Wan et. al. (2018) |
Granite dust | 65.53 | 5.39 | 9.50 | 15.09 | – | – | – | – | – | – | 2.60 | – | – | 1.89 | Zafar et. al. (2020) |
IBA | 32.75 | 8.57 | 29.06 | 10.02 | – | – | 4.77 | – | 1.75 | 2.87 | 1.24 | – | 3.01 | 6.60 | Song et. al. (2015) |
EPS | 70 | 14 | 3 | 2.4 | – | – | – | – | 0.2 | 7.6 | – | – | 1.7 | Bonakdar et. al. (2013) | |
Lime sulphate ash | 0.42 | 0.26 | 43.9 | – | – | – | – | – | – | 2.34 | – | 31.0 | – | – | Hauser et. al. (1999) |
Al-bearing ash | 14.51 | 9.30 | 24.4 | – | – | – | – | – | – | 0.44 | - | 10.7 | – | – | |
Iron tailings | 42.90 | 10.75 | 12.97 | 7.51 | – | – | – | – | 7.10 | 2.06 | 1.96 | – | 9.04 | 4.48 | Ma et. al. (2016) |
ZSM-5 | 94.01 | 0.55 | 0.16 | 0.23 | – | – | – | – | 1.48 | - | - | – | – | 3.57 | Jiang et. al. (2021) |
Black rice husk | 93.70 | 0.40 | 0.92 | – | – | 0.28 | – | – | – | 0.03 | 2.55 | – | – | 4.40 | Kunchariyakun et. al. (2018) |
Bagasse ash | 68.60 | 3.97 | 7.85 | – | – | 3.16 | 1.71 | – | 1.69 | 1.07 | 3.92 | – | – | 5.22 | |
Carbide slag | 2.57 | 1.88 | 65.03 | 0.09 | – | – | – | – | 0.17 | 0.09 | – | – | – | 28.31 | |
Quartz tailing | 93.23 | 1.68 | 0.33 | 0.56 | – | – | – | – | 0.14 | – | 0.64 | – | – | 0.78 | Jin et. al. (2016) |
Phospho-gypsum | 10.64 | 1.22 | 25.39 | 0.54 | – | – | – | – | 0.19 | 0.23 | 0.50 | – | – | 22.91 | |
Quartzite | 97.58 | 0.31 | 0.14 | 1.20 | – | – | – | – | 0.10 | 0.10 | 0.03 | – | – | 0.03 | Albayrak et. al. (2007) |
Glass cullet | 67.1 | 0.90 | 7.4 | 0.20 | – | – | – | – | 4.2 | 19.4 | – | – | – | – | Walczak et. al. (2015a) |
Hematite tailing | 24.4 | 10.95 | 6.2 | 44.52 | – | – | 2.78 | – | 0.99 | 0.28 | – | – | 0.24 | 6.95 | Zhao et. al. (2012) |
Copper tailing | 44.52 | 5.36 | 13.56 | 1.94 | – | – | – | – | 19.92 | 1.00 | 1.20 | – | – | 9.26 | Huang et. al. (2012) |
Blast furnace slag | 32.7 | 15.4 | 38.79 | 0.4 | – | – | – | – | 8.97 | 0.23 | 0.36 | – | – | 0.76 |
2 Constituents of AAC
2.1 Binder
2.2 Micro-particles
2.3 Air-Entraining Agent
2.4 Fiber Addition
2.5 Hydrophobic Agents
2.6 Superplasticizer
3 Preparation Method of AAC
4 Physio-mechanical Properties
4.1 Density
4.2 Compressive Strength
4.3 Flexural Strength
4.4 Drying Shrinkage
4.5 Water Absorption
5 Microstructural Analysis
5.1 Mineralogy
5.2 X-ray Diffraction Analysis
5.3 Pore Structure
6 Functional Property: Thermal Conductivity
7 R&D at CSIR-CBRI
8 Recommendations
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In order to reduce the carbon emission, replacement of cement by different wastes should be acknowledged in the preparation of AAC.
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Impact of various wastes on the acoustic properties of AAC must be studied.
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Utilization of demolished AAC waste should also need attention in construction.
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As micro- and macro-pores are the main characteristic of AAC, therefore how the utilization of different waste affects the porosity of AAC should be studied.
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Stone wastes contain various other components (except quartz) like potassium and soda feldspar. Their involvement in the hydration process is still not much explored.
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Strength of AAC is related to the moisture content. Detailed study of this parameter will be helpful for further research studies.
9 Conclusions
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As most of the raw materials of AAC are silicious in nature. It offers wide opportunity for the various silicious waste throughout the world. For e.g., Rice Husk, Natural zeolite and Sugar sediments, etc., highly rich in silica content (> 65%) can easily replace natural resources like sand (up to 20–40%).
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As it is light in weight, therefore application becomes very easy. As well as it requires less reinforcement in the foundation work of buildings.
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All the ingredients used in the production of AAC are eco-friendly and utilization of fly ash as one of the constituents is leading it towards greener development.
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The pore size in AAC is always in the millimeter range, mostly having diameters in the range between 0.5 and 3.0 mm.
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Presence of air voids in AAC, provide thermal insulation properties to it. Due to its thermal insulation property, about 50% consumption of building energy reduces. Hence maximize energy efficiency in buildings.
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AAC block generated around 67% less carbon emissions than clay brick.
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AAC can be used in constructions of panel in both load and non-load bearing for walls, roofs, floors, etc.
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Fiber-reinforced aerated concrete (FRAC) had nearly half compressive strength than that of AAC samples but flexural strength of FRAC was 100 times greater than that of AAC.
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On increment in the autoclaving pressure (i.e., > 0.8 MPa) tobermorite developed well, whereas when the autoclaving pressure was more than 1.2 MPa other hydrothermal products were also formed which have lower strength.
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Tobermorite crystals are majorly responsible for the compressive strength of AAC. Tobermorite appears at nearly 2 h of autoclaving and profound up to 6–8 h and further, no formation takes place on increasing time.
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Preparation of AAC by using different industrial wastes is a great way to cope with the problems which have been generated by the wastes.