Experimental Study On The Effect Of Microwave Heating On The Mechanical Properties Of Alkali-excited High-calcium Fly Ash

Geopolymer is a new type of amorphous to semi-crystalline aluminosilicate polymer material with excellent high temperature resistance,corrosion resistance and mechanical properties.The geopolymer is mainly derived from a variety of solid wastes rich in active silicon-aluminum components,such as fly ash,slag,steel slag and the like.In this paper,high-calcium fly ash and slag are used as raw materials,and alkaline activator is prepared by using water glass and sodium hydroxide.The alkali-excited geopolymer is prepared under normal temperature and normal pressure conditions.The effects of water glass modulus,NaOH concentration,slag content and borax content on the setting time,fluidity and compressive strength of alkali-activated geopolymers were studied.The microwave heating method was used to cure the alkali-excited geopolymer,and the influence of microwave power and irradiation time on the strength was studied.The mechanism of action of borax and microwave was discussed by studying the hydration products and microscopic morphology of fly ash.The main findings are as follows:The alkali-excited high-calcium fly ash has a short setting time,and initial setting occurs in the range of 20 to 25 minutes.The setting time of the geopolymer is shortened with the increase of the modulus of the alkaline activator and the amount of slag.When borax of different mass ratios are separately added,the setting time is prolonged with the addition of borax.The study found that when using liquid water glass with a modulus of 2.3,the compressive strength is the highest.With the increase of NaOH concentration,the compressive strength increases,but the setting time is shortened,and the fluidity,workability and plasticity are deteriorated.When 3%borax is added,the fluidity is suitable and the compressive strength is the highest.The power used is 100 W,300 W,500 W microwave irradiation alkali-activated fly ash paste,the water glass modulus is 2.3,12 M NaOH solution,the water-cement ratio is 0.25,the borax addition amount is 3%,and the power is determined.300 W,curing time 3h,is the best maintenance program.The strength after curing can reach 60%of the standard 28 d strength.Scanning electron microscopy showed that the alkali-activated fly ash pulverization process of borax produced some crystals attached to the surface of fly ash,which hindered the speed of hydration reaction,and the crystal decreased with age.The vitreous body on the surface of the fly ash dissolves and the gel looks denser.Observing the alkali-activated fly ash paste after 300 W power microwave curing,it was found that the vitreous body on the surface of the fly ash basically disappeared,and the hydration product was well-densified.