Drying Shrinkage And Mechanical Prop Erties Of Fly Ash Based Geopolymer In A Mbient Temperature

Geopolymer is a potential substance alternative to cement-based materials.Even through several properties of fly ash based geopolymer has been studied,there are some drawbacks waiting for solution.The shrinkage in fly ash based geopolymer is one of the major problems to affect its practical application.And using different raw material to synthesis alkali-activated material has different characteristics and the mechanism of hydrogen reaction.These revealed that the nature of shrinkage in fly ash based geopolymer is related to many variables.This paper mainly studies the drying shrinkage and mechanical properties of fly ash based geopolymer at room temperature.The factors such as water-to-binder ratio,alkali solution concentration and Na-water glass modulus are used to test the drying shrinkage and mechanical properties of fly ash based geopolymer mortar.The purpose is to prepare a fly ash based geopolymer mortar with suitable mechanical properties and small drying shrinkage,and provide a reasonable explanation for the mechanism of each factor through scanning electron microscopy and Fourier transform infrared spectroscopy.The main test conclusions of this paper are summarized as follows:(1)In the equal concentration of sodium hydroxide solution at ambient environment(25℃,65% relative humidity),the lower the water-binder mass ratio,the better the mechanical properties of the fly ash based geopolymer specimen.Under the same waterbinder mass ratio,the greater the concentration of sodium hydroxide solution,the better its mechanical properties.Thus,the best mechanical properties of the N4 and N8 groups are the0.4N8 samples.(2)The Na-water glass modulus has a significant effect on the drying shrinkage of the fly ash based geopolymer mortars,and its influence exists an inflection point value,and the dry shrinkage reaches the lowest value at the inflection point.In the two groups of NS4 and NS8,the inflection points were 1.4 and 1.2,and the 28-day drying shrinkage was 2000 μm / m and1670 μm / m.(3)The concentration of NaOH solution also has such an inflection point value.In other words,the concentration of sodium hydroxide solution and the modulus of sodium water glass have a synergistic effect on the geopolymer reaction,which may affect each other,further illustrating the complexity of the fly ash based geopolymer reaction.(4)Observing 0.4N4 and 0.4N8 samples via scanning electron microscope,it was found that increasing the concentration of sodium hydroxide solution can make the microstructure of the samples more denser.By scanning electron microscopy observations of the 0NS8 sample,1.2NS8 sample and 2.0NS8 sample,it was found that the microstructure of the samples in the NS8 group became denser with the increase of the sodium water glass modulus.(5)The results of Fourier transform infrared spectroscopy show that increasing the concentration of sodium hydroxide solution and the modulus of sodium water glass can promote the reaction of the fly ash based geopolymer,and the concentration of sodium hydroxide solution is greater influence than that of sodium water glass.The greater the concentration of sodium hydroxide solution,the more favorable the fly ash based geopolymer reaction is,and the greater the sodium water glass modulus,the more disadvantageous is the reaction nature of Al-containing groups in fly ash.(6)Finally,the optimal mixing ratio selected in this experiment is a sodium silicate solution prepared with a concentration of 8 mol / L sodium hydroxide solution with a sodium silicate modulus of 1.2 as the alkali activator,the water-binder mass ratio is controlled to 0.4,and the sand-fly ash mass ratio is 2.75.