Investigation On Foam Concrete And Its Application In Composite Sandwich Panel

With the improvement of building energy conservation, thermal insulation building materials have rapidly developed. Because the requirements of building fire protection, inorganic thermal insulation materials become more and more popular. Among them, as a special inorganic material, foam concrete presents a much lower bulk density and larger porosity compared to ordinary concrete, leading to excellent insulation properties. Meanwhile, the preparation of foam concrete also has many advantages, e.g. simple preparation procedure, easy process, wide raw material sources, low cost and etc. Therefore, foam concrete attract increased attentions from researchers and industry. However, due to the lower mechanical properties compared to ordinary concrete, its application in retaining material as its own is limited. The foam concrete composite wall panels prepared by high-strength panels and foam concrete can improve its mechanical strength, thus significantly expanding its application in the construction engineering field. However, during the preparation process of foam concrete composite panel, usually there are some problems, e.g. the collapse of foam concrete, pulverization and poor adhesion between the panels and foam concrete. In present, the reported studies were mainly focused on the prevention methods for single problem, but the detail mechanisms related to these problems and influencing factors were not systematically investigated, restricting the preparation and application of foam concrete composite panel with excellent engineering properties. Therefore, in this thesis, the investigations were mainly focused on the mechanisms、influencing factors and prevention methods for the above mentioned problems in the preparation of foam concrete composite panel, in order to provide the theoretical guidance and technical support for the production, promotion and application of foam concrete composite panel. The main research works in this thesis include three aspects:1. The specific research contents related to the bubbles in foam concrete slurry are summarized as follows:(1) A test method for the surface tension of foam concrete slurry and the strength of the bubble wall of foam concrete is presented.The force model of the bubbles in foam concrete slurry both during the Stirring process and stagnant process was established. Based on this model, the forces on the bubbles was analyzed and the influence of the surface tension and strength of the bubble wall on the bubble stability was revealed. Further, the mechanical boundary conditions for the stable existence of bubbles in slurry was analyzed and the lowest diameter for the stable existence of bubbles in slurry was determined.(2) The influence of bulk density, viscosity and setting time of slurry, fly ash addition, foaming agent types(film strength), grain size of sand particles, sand amount, grain size of ceramic balls, mixing methods, mixing speed, mixing time and etc. on the bubble stability in foam concrete slurry was investigated. The results indicate that a higher bubble stability in foam concrete slurry was related to a larger bulk density and viscosity and shorter setting time of slurry. A larger force on bubbles was harmful for the stable existence of bubbles in slurry. Further, the bubbles prepared by protein composites foaming agent presented a best stability in foam concrete slurry.(3) The key issue for collapses in foam concrete slurry is the match between the setting time of slurry and bubbles lifetime. Therefore, by extending the bubbles lifetime(e.g. increasing the bulk density and viscosity of slurry and strength of liquid film(bubbles film)) and shortening the setting time of foam concrete(e.g. adding accelerating agent or using quick-setting cement), a stable skeleton structure can be formed in slurry before bubbles burst, thus solving or mitigating the collapse problem in foam concrete.2. The microstructure parameters of foam concrete were characterized and their influence on the cement hydration process and pulverization process of hardened foam concrete was investigated. The results indicate that the pulverization in foam concrete was related to the rapid water evaporation and insufficient cement hydration, resulted from its porous, thin wall structure with a huge internal surface.(1) Pencil hardness method was proposed to characterize the surface hardness of foam concrete; by using this method, the pulverization degree of foam concrete was determined.(2) A pore structure model of bubbles in foam concrete was established. In this model, the pore structures of bubbles were divided into closely packed and non-close-packed bubbles, and the theoretical equation for the pore structure of bubbles(the number, internal surface area and wall thickness of bubbles) in foam concrete with different bulk densities was deduced. The results indicate that with a some bulk density, a higher thickness of the bubble wall was related to an increased diameter of bubbles and reduced porosity. The bubbles in foam concrete presented the following properties: porous structure and thin-wall, coexistence of materials with small volume and bubbles with large volume, materials with small volume exposed to the gas in bubbles with a huge area.(3) The influence of microstructure properties(porous structure and huge internal surface area) on the pulverization of foam concrete was analyzed. The poor water retention of the thin wall structure, together with the huge internal surface area in foam concrete, lead to a rapid water evaporation and insufficient cement hydration; the good structure of hydration products can’t form, resulting in an easy pulverization. Due to a significantly higher water evaporation rate compared with foam concrete with a high bulk density, the pulverization degree was more severe in ultra-light foam concrete.(4) The results indicate that the pulverization was more severe in foam concrete with a lower bulk density. SEM observations showed that in ultra-light foam concrete(with a bulk density of 300 and 400kg/m3), the hydration products didn’t grow well and the connection between them was also loose, leading to a porous structure and easy pulverization in hardened foam concrete; on the other hand, in foam concrete with a bulk density of 700kg/m3, the hydration products grew well and the connection between them was also compact, thus reducing the pulverization in foam concrete. The results also indicate that carbonation can also acceleration the pulverization rate in foam concrete.(5) Based on the above analysis and investigations, the prevention method for pulverization of foam concrete by improving the pore structure(increasing the thickness of bubble walls), using water retention admixture and proper water/binder ratio was proposed and experimentally verified.3. In order to solve the matching problem between foam concrete and panels, the stability of foam concrete slurry between the panels, deformation consistency and interfacial adhesion between foam concrete and panels were investigated.(1) The influence of water absorption property of calcium silicate board, processing method, thickening agent, setting accelerator and etc. on the slurry stability during the preparation process of foam concrete composite panels was investigated. The results indicate that a better water absorption property of panels lead to a more serious collapse of foam concrete slurry; the process method with "dry + glue + concave" presented the minimum effect on the stability of slurry; the application of thickening agent and setting accelerator was beneficial for the stability of slurry in foam concrete composite panels. On the basis of these results, the methods and techniques to improve the stability of slurry in foam concrete composite panels was proposed.(2) The finite element analysis structural models of both panels and foam concrete core materials were established. By using finite element analysis, the influence of internal stress distribution, drying shrinkage, and elastic modulus of foam concrete composite panels on the shrinkage stress was discussed and the matching studies on the drying shrinkage deformation between the panels and foam concrete core materials were carried out. The results indicate that the cracking of foam concrete composite panels started at the contacting interface in the four corners and spalling of core materials between panels started at the center position. When foam concrete with bulk densities of 700kg/m3 and 800kg/m3 was used as the core materials in foam concrete composite panels, the composite panels were safe and without cracking; When foam concrete with bulk densities of 600kg/m3 was used, the cracking risk existed. By using proper amount of emulsion powders, polypropylene fibers and fine sands, the shrinkage of foam concrete can be reduced, thus avoiding the spalling of panels and cracking of core materials. In this way, there is a good match of deformation between the panels and foam concrete, and the global performance of foam concrete composite panels can be improved.(3) The influence of bulk density of foam concrete, processing method of panels, emulsion powders, water/binder ratio on the adhesive property between foam concrete and panels was investigated. By the combination of proper processing method of panels( "dry + glue + concave"), emulsion powders, bulk density of foam concrete and water/binder ratio, the foam concrete composite panels without spalling of panels and cracking of core materials were prepared. The prepared composite panels presented excellent thermal insulation properties, fire resistance, impact resistance, hanging performance, water resistance and etc.The research works in this thesis enrich the fundamental theory of foam concrete, and innovative progress has been made in test method of the surface tension and the bubble wall strength of the foam concrete slurry, as well as in model of bubble pore structure and its influence on the performance of foam concrete.Also innovative progress has been made in mechanism and prevention method about the collapse and the pulverization of the foam concrete.Part of the research findings in this thesis has transferred into industrial trial stage. It can be believed that the research results of the thesis are beneficial for improving the preparation level of foam concrete and foam concrete composite panels.