Development On A New Aerated Concrete Of Lightweight, High-strength, Insulate Energy-saving For Outer Walls

Autoclaved aerated concrete (ACC) is a new building materials with the properties of waste utilization, lightweight, high-strength, sound absorption, insulation, heat insulating, energy conservation and processability, which makes the construction process much more efficient, energy saving and environment protecting. However, due to AAC's highly porous structure and water absorbing properties, some using maradies such as poor matching of strength and density, walls cracking, leakage, plaster hollowing, flaking, etc. The problems mainly result from AAC's low load-bearing capacity, high shrinkage and bad durability (e.g. weathering resistance and corrosion resistance), All of these seriously affects building beauty and function, which restricts AAC's application and dissemination. The goal of this paper is making a new AAC with B07-A7.5 grade and low thermal conductivity (≤0.16 W/(m-K)) in laboratory, and study completely and systematically in the following five aspects.Through comparison, lime and cement were chosen as the calcarious raw materials of AAC, fly ash as siliceous raw material. For improving pore structure, chemical aerating material was aluminum paste, physical air-entraining material was agent WY; Adjust material was desulfurization gypsum, surface active agent was polycarboxylate superplasticizer and foam stabilizer.Mix proportion of AAC's slurry was researched. The plants could not produce high-strength AAC because they always emphasized air-entraining technology and ignored the hardening strength of concrete. Firstly, the relationship between porosity and concrete strength was deduced and the result is that the concrete strength of AAC of B07,-A7.5 should be 53.0Mpa after autoclaving, so the strength of steamed aerated concrete should be 40.8MPa. Secondly, in order to attend demand strength as well as density, the mix proportion of AAC's slurry was searched. Applying SPSS software to analysis the major influence factors of strength:variance, age, cement and fly ash are significant to AAC's strength is founding, while lime, gypsum, second-order interaction effects and higher-order interaction effects are not significant effect on the strength. Intuitive analysis and variance analysis conclude that the water-binder ratio (w/b), curing temperature and age are the most significant factors; Cementitious material (cement: fly ash: lime: gypsum = 1:3:0.35:0.15) for 10 min is blended for 10 min, and w/b is 0.35. SAC cure under 40℃for 4h after casting, and under 80℃for 8h after forms are removed, then curing specimen has dry density of 1710 kg/m~3 and strength of 38.6MPa.The performance of low w/b AAC's slurry and its air-entraining technology are studied. Using first grade fly ash, the w/b 0.25 and the content of water reducing agent D is 0.12%, SAC with dry density of 1668kg/m~3 and strength of 39.1MPa is gained. As the w/b is getting smaller, the yield value of AAC's slurry is decreased while its plastic viscosity and thixotropic value are increased. When the w/b is 0.25, AAC's slurry appears shear thickened phenomenon. The lower the w/b is, the bigger the viscosity and thickener rate are. So when the w/b is 0.25, the initial fluidity should be around 190mm, so that paste gas-forming and consistency can match each other. When producing≤1100kg/m~3 lightweight specimens, the chemical air-entraining is suggested. When producing 1100～1700 kg/m~3 specimens, the physical air-entraining WY is suggested, dosage (relative to cement) is 0 ~ 0.2%. There is also size effect in SAC. The smaller the mould is, the more nonuniform the SAC is. Under the condition that using the mix proportion mentioned above, the w/b is 0.25 and autoclaving under 190℃for 8h, SAC with bulk weight of 787kg/m~3 and compressive strength of 7.8MPa is produced.The determination of AAC's pore structure–MATLAB Digital Image Processing is studied. The higher the image resolution and the more the pore number are, the more precise the Pore-Test-Software (PTS) is, and PTS is developed on MATLAB. Without dyeing the samples, the test error of PTS is less than±2%. The compressive strength of AAC is decided by the parameters of the pore structure. Most important parameter is porosity; the strength will decrease as the porosity getting bigger. Next important parameter is pore size distribution, especially the percentage of large pore (pore diameter≥2.0mm). PTS can measure the pore parameters of AAC (such as average pore size, area porosity, pore size distribution and so on), so it can be used for AAC plant's product quality control and help to realize AAC's nondestructive testing.Structure and properties of AAC are studied. AAC's volume stability changes as w/b and curing system change. The shrinkage value of AAC equals to 1/6 of the shrinkage value of SAC. With longer curing time, the volume stability is improved. The w/b affects the shrinkage value of AAC, which is lesser than SAC. The total shrinkage value of SAC at w/b= 0.25 is 68% as w/b=0.35 or 0.45. Thermal coefficient increases with the density. Using chemical air-entraining technology, as long as the AAC volume density is less than or equal to 750 kg/m~3, thermal coefficient will be less than 0.16 W/ (m-K). The result of XRD analysis shows that the hydration products of 80℃SAC are mainly consist of calcium hydroxide (CH), C-S-H gel, a little of AFt phase, poorly crystallized tobermorite (0.9nm) and hydrogarnet, however, the hydration products of AAC are well crystallized tobermorite (1.4nm) , a small amount of hydrogamet and C-S-H gel. With the increase of autoclave temperature, tobermorite's crystallinity has a little increase and with the extension of autoclave time (from 6h to 8h, and to 10h), the hydration products'crystallinity first increased and then decreased. When w/b changes from 0.45 to 0.25, the crystal form of tobermorite turns 1.1nm into 1.4nm. The result of SEM analysis shows that there are needle or fibroid tobermorite and hydrogamet inside the stomatal wall of AAC, while crapy or wrinkle tobermorite and hydrogamet in the cross-section of AAC. Though autoclaved system and w/b are different, the type of hydration products does not change, and just their crystal form changes. When w/b is 0.25, tobermorite size is 0.5 um wide, while 0.45, it is 2~3 um and its structure is slightly loose. The longer autoclave time is, the smaller tobermorite size gets.In short, this paper starts from the theoretical calculation. When w/b is 0.25 and the initial fluidity is 190mm, curing system that first steamed curing then autoclaved, B07-A7.5 AAC, which has low thermal conductivity, is successful produced. Meanwhile, PTS for AAC, which is based on MATLAB is successful developed. PTS offers a new kind of test means to study pore structure, so it is practically meaningful to the scientific research and application.