Study On Some Key Issues Of Foam Concrete Composite Board Preparation

For adapting the requirements of energy saving, fire prevention and environmental protection for modern building structures, inorganic heat insulating material is becoming the important development direction of energy-saving building materials. Foam concrete presents many advantages, e.g. light weight, heat insulation, sound insulation, non-combustible and earthquake resistant, thus can meet the requirements of energy-saving and security for buildings, and be widely investigated in the building energy-saving area. However, foam concrete also has some disadvantages, such as low compressive strength, high shrinkage, water absorption and easy efflorescence, seriously limiting its application. In order to solve these problem, in this study, the foam concrete composite board(with high heat insulation and fire resistance) based on foam concrete and calcium silicate board were prepared. The investigations in this study were focused on the efflorescence of foam concrete during the preparation process of the composite board the bonding strength between the foam concrete and calcium silicate board, and the cracking resulted from the inconsistent drying shrinkage between the foam concrete and calcium silicate board. The main contents are summarized as follows:(1) The efflorescence degree of foam concrete were evaluated by the appearance comparison and pencil hardness method. Based on the investigations on the compositions of hydration products in foam concrete with different densities, the efflorescence mechanism of foam concrete was revealed, and the strategies to restrain the efflorescence were proposed. The results indicate that the water loss of foam concrete at early-age is the main reason for the efflorescence: the rapid evaporation delayed the cement hydration, and reduced the content of hydration products, leading to a porous microstructure and easy falling of powder particles from foam concrete. The foam concrete with a lower density presented a higher water loss at the early-age(the water loss ratio of the foam concrete with a density of 300kg/m3 was higher than 40% at 1d curing age), leading to a more pronounced efflorescence. The efflorescence degree can be reduced by using water-retaining agent, improving the curing condition at early-age or increasing the water cement ratio of foam concrete.(2) The influence of density and water cement ratio of foam concrete, dosage of emulsion powders, and different processing methods of calcium silicate board on the bonding strength of the composite board were investigated. The results show that emulsion powders can improve the bonding strength between foam concrete and calcium silicate board. For foam concrete with a density of 500kg/m3, when the dosage of emulsion powder increased from 0% to 1%, the bonding strength increased from 178 k Pa to 280 k Pa. The calcium silicate board absorbed the water from fresh foam concrete, which increased the bonding strength, but lead to the collapse of foam concrete. When the calcium silicate board with a relative moisture content of 50% was applied to prepare the composite board, the high bonding strength of the composite board can be guaranteed, and the collapse degree can be reduced. Thickening agents can improve the stability of fresh foam concrete in composite board, without adverse effect on the bonding strength.(3) The drying shrinkage of composite board was analyzed by finite element analysis software ANSYS, and the internal stress distribution of composite board was obtained. The influence of the emulsion powders, fine river sands and polypropylene fibers on the mechanical property, drying shrinkage and cracking behavior of foam concrete was investigated. Polypropylene fibers significantly increased the toughness of foam concrete, and reduced the degree of drying shrinkage. For foam concrete with a density of 500kg/m3, the drying shrinkage was reduced to 3750×10-6 by using river sands and polypropylene fibers, mitigating the cracking degree of foam concrete. The results indicate that fiber is the key factor to control the cracking of foam concrete. By using polypropylene fiber in 500kg/m3 foam concrete, composite board without crack could be produced.