Study On The Preparation Of Autoclaved Blocks With Low Quality Fly-ash

There are a large number of low quality fly ash in our country.Because of its large particle size,high water demand ratio,large loss on ignition and other problems,its utilization is seriously affected.The key to improve the utilization of low quality fly ash is to improve its activity.In this paper,two kinds of low quality fly ash are selected.The ball milling treatment is used to reduce their fineness and the influence of ball milling treatment time on the grader retained percentage,water requirement ratio and strength activity index of the fly ash.Graphene is one of the materials with the highest strength and good toughness.The influence of graphene content on the strength activity index of fly ash was also explored by adding a small amount of graphene.In addition,alkali,sulfate and chloride are selected to add into fly ash and the effect of activator type and dosage on the activity of fly ash are explored.The optimal amount of chemical activator are determined by experiments.Based on the previous experiments,the modified low quality fly ash was used to prepare small autoclaved block test blocks.The effects of milling time of fly ash and chemical activator amount on dry density and compressive strength of autoclaved block were studied.Studies have shown that the increasing of the ball milling time of fly ash reduces the particle fineness,improves particle gradation,reduces the water requirement ratio,and thus increases the fly ash strength activity index.But the ball milling time should not be too long as considering to the effectivity and cost,the perfect balling milling time is less than 4 hours.When balling milled for 4h,the strength activity index of fly ash A is 80%with an increase of32%;the strength activity index of fly ash B is 94%with an increase of 43%.Graphene adsorbs on the surface of fly ash,changes the secondary hydration reaction process of fly ash,and improves the strength and activity index of fly ash.The experimental results show that the optimal content of graphene is 0.05%,when the strength activity index of fly ash A is 79%and the strength activity index of fly ash B is 68%.When Na₂SO₄is added into the slurry,the p H value of the solution increases and the Si-O bond and Al-O bond on the surface of the glass sphere are destroyed,which result in the hydration reaction was promoted.The strength activity index of two kinds of fly ash increased with the increase of Na₂SO₄content.When the content of Na₂SO₄was 2.5%,the strength activity index of fly ash A was 78%,and that of fly ash B was 92%.Incorporation of Ca?OH?₂increased not only a large number of calcium ions but also the alkalinity of the system,which promoted the hydration reaction to produce more gel products.However,too much addition amount of Ca?OH?₂will make the alkalinity too high,the solubility of Ca²⁺will decrease,the hydration rate will be slowed down and the generation of hydrate will be inhibited.Therefore,the strength activity index of the two kinds of fly ash will firstly increase and then decrease with the increase of Ca?OH?₂content,and the optimum content is 10%;The Ca²⁺and Cl^-in Ca Cl₂penetrate the hydration layer on the surface of fly ash particles and react with active Al₂O₃to form hydrated calcium aluminate,which increases the internal and external osmotic pressure of particles and promotes hydration.The strength activity index of fly ash increases with the increase of Ca Cl₂content.The results show that the effect is remarkable when the fly ash is milled for 4 hours and mixed with activator especially 10%Ca?OH?₂and 3%Ca Cl₂.with113%of activity index of fly ash A,and 97%of activity index of fly ash B..For autoclaved blocks,the ball milling time of fly ash is not the longer the better.After grinding,the water demand ratio decreases and the water material ratio of slurry decreases,which makes it more and more difficult to foam.When ball milling time is 0.5 hours,the ratio of compressive strength and dry density of the two kinds of autoclaved fly ash blocks are the highest.With the increase of Na₂SO₄content,the dry density of the autoclaved block decreases while the strength increases.Moreover,adding Na₂SO₄after ball milling of fly ash can solve the problem of superfine powder foaming.Since Ca Cl₂exotherms when it meets water,it raises the temperature of the system during gassing which make the gassing smoother and the hardening rate of the slurry increases.As a result,better pore structure forms which bring a stable strength.However,when Ca Cl₂is too excessive the reaction rate is too fast,which is bad to genrate a sable pores.Therefore,with the increase of Ca Cl₂content,the strength firstly increases and then decreases.The optimal Ca Cl₂content of fly ash A is 2%,and that of fly ash B is 1%.With the increasing amount of Ca?OH?₂replacing lime,the effective Ca O content decreases,the heat production is insufficient,resulting in that aluminum powder fails to form foaming.The addition of Na₂SO_x`after ball milling can alleviate the difficulty of fine powder gas generation.The addition of graphene has an enhanced effect on the preparation of autoclaved blocks with low quality fly ash.The optimal content of graphene for fly ash A is0.05%,and for fly ash B is 0.1%.