Research On Autoclaved Aerated Concrete Fabricated With Flushed Sand In Yellow River

The application of lightweight wall materials has become wider and wider with the development of architectural structures.Autoclaved aerated concrete is the main type of lightweight building enclosure materials.Because fly ash as raw material is valuable rather than wasted,it has been not more advantageous for fabrication of aerated concrete.Artificial discharge is performed in the middle and lower reaches of the Yellow River every year.When the river retreats,a layer of " flushed sand" is left on the surface of the river bed.It is a good silicon material with little impurity and high content of silicon oxide.However,the silicon oxide of flushed sand in Yellow River is crystalline.It is not reactive at normal temperature.The purpose of this paper is to study the optimal proportion and reasonable autoclave curing system for autoclaved aerated concrete fabricated with flushed sand in Yellow River.By studying the influence of four factors,such as calcium-to-silicon ratio,water-to-material ratio,cement content and aluminum powder content,on the performances of sand aerated concrete,the orthogonal test of four factors and four levels was designed to get the best combination.On this basis,the performances of the aerated concrete were optimized by adding additives.A series of comparative tests were designed to investigate the effect of fineness and autoclave curing system on the reactivity of crystalline silica in sand,and to analyze the hydrothermal reaction products and the pore structure of products.According to the range analysis and variance analysis of the orthogonal test,the calcium-silicon ratio has the greatest impact on the compressive strength of sand-aerated concrete,followed by the ratio of water to material.The ratio of water to material is the decisive factor influencing the dry apparent density of sand aerated concrete.Considering the two performance indexes of compressive strength and dry apparent density,the optimum ratio of sand-aerated concrete is: calcium-silicon ratio 0.25,water-to-material ratio 0.38,cement content 14%,aluminum powder content 0.15%.The calcium-silicon ratio affects the alkalinity of the slurry and the type of hydration product.The water-to-material ratio affects the fluidity of the slurry.The cement content affects the thickening rate of the slurry.The amount ofaluminum powder affects the total amount of gas generated in the slurry.On the basis of the optimal ratio,a certain amount of alkaline activator was added to improve the internal porosity of the product.The performance of the product was optimized.According to the maximum strength method,the Na OH activator is better than the Na2CO3 activator.The optimal amounts of two alkaline activators is: Na OH activator 0.05%,Na2CO3 activator 0.10%.The ground degree of the sand affects the hydration reaction rate and reaction degree of the slurry.When the specific surface area of the ground sand is 325 m2/kg,the sand aerated concrete has the highest strength,at the same time,the slurry has a suitable fluidity.Through the study of the autoclave curing system,the best curing system for autoclaved aerated concrete fabricated by surface area of 325m2/kg ground sand was determined as follows: heating time 135 min,constant temperature 175 ℃ 8 h,cooling time 2.5 h.When the heating time is 135 min,the temperature difference between the inside and outside of the billet and the stress caused by the temperature difference can be well balanced,so that the phenomenon of "insufficient core" and the phenomenon of affecting the production efficiency can be avoided.The autoclaved curing system of sand aerated concrete with the surface area of 325m2/kg has been studied in detail and found that in the process of increasing the constant temperature from 165℃ to 195℃,its compressive strength increases first and then decreases with the increase of constant temperature.It indicates that at any constant temperature,there is a corresponding isostatic time corresponding to the best temperature.The compressive strength reaches the maximum value.At this time,the degree of reaction of crystalline silicon oxide in the corresponding sand reaches the maximum.Sand aerated concrete is a porous structure.Under the premise of certain ratio and autoclave curing system,the internal pores of the product are mainly affected by the pouring temperature and pre-curing conditions.It was determined by tests that the optimum pouring temperature of the slurry was40℃,and the pre-curing of 18℃ was better than that of constant temperature water bath.Under certain conditions of total porosity(or dry apparent density),the compressive strength of the product gradually decreases with the decrease of the closed porosity and the increase of the open porosity.Through proportioning optimization and reasonable autoclave curing system,crystalline silica can be used as a siliceous raw material to completely replace fly ash to fabricate autoclaved aerated concrete.It has technical significance for the fabrication and performance improvement of autoclavedaerated concrete,and it is valuable to improve the utilization rate of sand in Yellow River.