Influence Mechanism Of Sodium Sulfate Content And Curing Regime On Compressive Strength And Flexural Strength Of Neutral Sodium-salt Alkali-acti-Vated Fly Ash-based Cement

The output of fly ash in China grew rapidly in recent years,Using fly ash in cement production largely is an effective way to utilize it.Sodium sulfate is an economic and effective activator which can stimulate the activity of high-volume fly ash cement,but low strength is one of the biggest disadvantages of neutral sodium-salt alkali-activated fly ash-based cement(NSAAFC).Strength of cement mortar is less than 30 MPa when content of fly ash is 70%and sodium sulfate content is 3%.The purpose of this paper is to find a way to improve the strength of NSAAFC and cut the cost,therefore extend the application of it.Fly ash contents were 90%,80%,70%,60%,50%,respectively.Compressive strength and flexural strength were tested with parameters of sodium sulfate content(0%,2%,4%,6%)and curing regime changed.XRD,ESEM and pore size analysis were used to analyze kinds and morphology of hydration products and pore structure.NMR and hydrochloric acid selective dissolution method were used meanwhile to analyze reaction degree of Portland cement and fly ash of NSAAFC.Combining macro-analysis and micro-analysis,influence mechanism of sodium sulfate content and curing regime on compressive strength and flexural strength of NSAAFC was analyzed.The achievement was showed as follows:1.When sodium sulfate content increases(no more than 6%)with standard curing:main hydration products of NSAAFC with different fly ash contents are hydrated calcium silicate,hydrated calcium aluminate and a small amount of ettringite,amount of ettringite increases with higher sodium sulfate content;reaction degree of fly ash first increases then decreases,reaction degree of Portland cement decreases;average pore diameter first decreases then increases,harmless pore and little harmful pore first increase then decrease;compressive strength first increases then decreases,flexural strength first increases then decreases.2.When sodium sulfate content increases(no more than 6%)with autoclaved curing:main hydration products of NSAAFC with different fly ash contents are hydrated calcium silicate,hydrated calcium aluminate and a small amount of Tobermorite,Xonotlite and AFm,amount of AFm increases with higher sodium sulfate content;reaction degree of fly ash first increases then decreases,reaction degree of Portland cement decreases;average pore diameter first decreases then increases,harmless pore and little harmful pore first increase then decrease;compressive strength first increases then decreases,flexural strength first increases then decreases.3.Compared with standard curing,amount of hydration products with high crystallinity increases with autoclaved curing;reaction degree of fly ash increases significantly with autoclaved curing,autoclaved curing has little effect on reaction degree of Portland cement;average pore diameter decreases and sum of harmless pore and little harmful pore increases.4.When sodium sulfate content is 4%with autoclaved curing and ratio by weight of fly ash,Portland cement,water and sodium sulfate is 60:40:30:4,the compressive strength and flexural strength is 84.0 MPa and 16.3 MPa,respectively.The cost of NSAAFC with above mix proportion is 30%lower than that of 42.5 Portland cement and the compressive strength and flexural strength of NSAAFC with above mix proportion are higher than that of Portland cement 42.5.Therefore,NSAAFC with above mix proportion has a good prospect in steamed precast aspects.