Feasibility of making aircrete with autoclave using local materials in Bangladesh

Abstract

This study is aimed at investigating the feasibility of producing lightweight concrete in Bangladesh. Aircrete; a prominent branch of lightweight concrete is produced by reducing unit weight through introduction of void in the concrete matrix. The potential of lightweight concretes have already been identified and are intensively used in developed countries for various infill and structural purposes. This study demonstrates the feasibility and sequential approach for producing lightweight concrete using indigenous ingredients and acquiescent technology of Bangladesh. First, a thorough search for local materials and appropriate production technique is administered. Bangladesh, being a tertiary-geological-featured country, possesses no natural lightweight concrete that could be incorporated in this research. However, relatively suitable indigenous and easily available foreign material options are identified and utilized in the study. Methods used to produce lightweight concrete in recent times all over the world are found sometimes inappropriate in Bangladesh due to lack of resource options and technical knowhow. Thus, an easily practicable method is identified to carry out the study. It is found that a binder (OPC or PCC), local sand (Local sand and Sylhet sand), a foaming agent preferably aluminium as non-mechanized foam generator and lime to enhance performance of aluminium and admixtures if required are applied in this study. Feasibility study integrating a series of materials revealed the optimistic notion towards the production of lightweight concrete (density<1000 kg/m3) in Bangladesh. Range of ingredient contents is identified through this study. Major problem associated with lightweight concrete is low strength due to high water-cement ratio, low cementation index and foamy honey comb structure. Further detail optimization study with moist curing is carried out involving a series of materials options. From the results, it is seen that density decreased with increased cement and water content, whereas compressive strength increased with increased cement but decreased with water content, thereby indicating a conflicting relation. Consequently, a number of interpretable charts are constructed to correlate density and compressive strength with material contents. However, although target density range is achieved, strength criteria are insufficient for practical application. Therefore, an improved curing technique is eminent. Autoclaving is a special curing method subjected to high temperature and pressure. This method is used for early strength gain of concrete. Due to lack of resource and technical knowledge, use of autoclaving with high pressure is not an appropriate option for Bangladesh. As an alternative, autoclaving without pressure is applied in special curing chamber using local resources. Compressive strength range of produced aircrete is improved to a practicable level. Finally, after optimization, lightweight concrete with density, compressive strength and thermal conductivity within range of 700-1000 kg/m3, 0.5-2.0 MPa and 0.2-0.4 W/m-k, respectively is produced. Cellular concrete produced from locally available materials through non-mechanized foaming and non-autoclaving treatment, resulted in lighter (<1000 kg/m3) but lower strength (1.0 MPa) concrete product. Finer sands were more appropriate to contribute as filler than coarse aggregate in cellular concrete. The drag force exerted by foam generation was unable to disperse coarser aggregate uniformly in the matrix rather allowed settlement due to gravity. Lime used in this research acted simultaneously as filler and expansive ingredient. Unslaked lime is one of the major causes of low compressive strength and through generation of heat affects hydration moisture and the action of admixtures. However, lime contributed the most in reducing strength of produced aircrete. Author suggests future study to be programmed without including lime as a potential ingredient of aircrete. Plasticizer and viscosity modifying admixture were unsuccessful in their performance to improve strength. Reason behind might be elevated temperature due to hydration of lime. Further study is suggested. Moisture loss at the early stage of setting was observed emerging necessity of humidity restoration. Both density and compressive strength were reported to improve by application of steam curing environment. Thermal conductivity and absorption for selected samples were found within the range of 0.2-0.4 W/m-k and 25-35 percent respectively with well-established trend with corresponding density. As per RILEM classification, lightweight concrete produced in this research can be grouped under class-III and Insulating type lightweight concrete. Although not covered in this research, cork granules and polystyrene fiber are expected to be suitable as filler to reduce unit weight of concrete. However, cement-cork and cement-fiber compatibility should be studied rigorously. Mechanized mixing should be preferred in preparing aircrete otherwise ingredients may not disperse uniformly. Foaming agent can be used associated with mechanical mixing to achieve aircrete. This study is a new approach for producing lightweight concrete in Bangladesh. It is believed that the outcome and recommendations may provide ahead start for the next researchers interested in this field.