Key Technology For Improving Performance Of Autoclaved Aerated Concrete (AAC)

Autoclaved aerated concrete(AAC)is a kind of porous lightweight cement-based materials and used widely in construction engineering for its excellent performance,thermal insulation,acoustic,consumption of industrial waste and low cost.Documents showed that AAC was the only single material to achieve 65% building energy efficient.Then AAC will exhibit great potential in construction engineering with the improving requirements of building energy standards and fire protection.However,Previous studies showed that the products of most aerated concrete enterprises had some problems such slow strength,unqualified bulk density,large quality fluctuation.So,the paper focus on the match of slurry hardening process and air expansion process,the regulation of hydration products and pore structure to improve performance and quality control.The work will provide technical support for quality stability and performance improvement of AAC.(1)The rheologicial properties and its development of paste and bubbles in paste were studied.The parameters and techniques of paste stability was discussed.The results indicated:(a)the rheologicial property of AAC paste was according with the H-B model distribution.When the ratio of water to mixture was less than 0.4,the paste exhibited pseudo pastic and when the ratio of water to mixture was greater than 0.4,the paste was expanded plastic fluid.(b)By adjusting the initial rheological characteristics of the paste to control the process of gas expansion,it was found that the optimum yield stress of the initial slurry was 1.8-4.2Pa and the air expansion of paste was successful.The temperature rapid expansion stage,mild expansion stage and gas end stage of paste was controlled at 45-55℃,55-65℃ and above 65℃,respectively.The closer the curves of gas production rate of the paste and the thickening rate and the time to reach the maximum value of the two curve were,the better the matching of the two process was and the more uniform the pore structure distribution was achieved.(c)Additionally the results indicated that the compression strength of CaO-SiO2-H2 O autoclaved system increased most obviously mixed with synthesized C-S-H under the 150℃ with c/s ratio 1.0.Calcum silicate hydrate seed promoted dissolution of Si O2 from fly ash and decomposition of hydrogarnet,accelerated the generation of hydration products,mainlyC-S-H(B)and tobermorite.Then the products had higher strength.The pore size could be refined and the distribution of pore size was uniform with the addition of calcium silicate hydrate.The number of harmless pores and less harmful pores below 50 nm increased and the number of hamful pores above 200 nm reduced.(2)By the optimization design of the CaO/SiO2 ratio of AAC,the type of silica material,the fineness of silica material,silica fume content,gypsum content and research on the effect of these factor on the performance of AAC.The results showed:(a)When the CaO/SiO2 ratio was between 0.6 and 0.7,the compressive strength of AAC was highest for the higher tobermorite content and good crystallinity while the water absorption and thermal conductivity washigher and dring shrinkage was larger.(b)The compressive strength of AAC prepared by fly ash was lower while the water absorption and thermal conductivity was smaller.Hydration products in AA prepared by fly ash contained more bad crystalline calcium silicate hydrate and crystal size was uneven.(c)With the decrease of particle size,the dry shrinkage,the water absorption rate increased and thermal conductivity decreased.(d)Silica fume content between 5% and 10% improved the compressive strength and water absorption rate of AAC obviously,decreased the thermal conductivity of AAC.(e)Gypsum content with the amount of 3%-5% accelerated the hydrothermal reaction between CaO and SiO2 and promoted the conversion of C-S-H gel to tobermorite,which was beneficial to the improvementof compressive strength.(3)AAC included macro pores and micro pores located in the inner wall.The macro pores were tested by mercury intrusion porosimetry(MIP)and the macro pores was tested by optical microscopy in order to analyze the influence of pore structure on performance of AAC.The results showed:(a)The shape of macro pores was mainly oval.The lower the bulk density was,the more the number of pores was and the more serious the merging phenomen between pores was.The macro pores size was generally distributed in the three stages of 100-1500μ m,1500-2500μm and 2500-4000μm.With the increase of porosity the pores in 1500-2500μm and 2500-4000μm increased and the pores in 100-1500μm reduced.Average pore size increased while the porosity increased.(b)The number of micro pores of AAC prepared by fly ash was less than that of sand.The number of pores with below 1μm was more and the number of pores with below 20 nm was less with particles size of sand decreasing.(c)The addition of silica fume refined the micro pores.(d)Thermal conductivity was reduced by 10% and compressive strength increased by 10% by using the optimized method of entraining gas(physical foaming and chemical gas)of aerated concrete.(e)A gas-solid phase model was established to predict the thermal conductivity of AAC based on sierpinski carpet model and the thermal conductivity calculated by the model was in good agreement with the measured values under different porosity.Then,a gas-liquid-solid phase model was established to predict thermal conductivity of AAC with different moisture content by introducing different thickness of water film.The relationship between porosity,moisture content and thermal conductivity was established by adjusting porosity and water content.