Research On Water Saturation Coefficient Of Metakaolin-Based Geopolymer

Geopolymer concrete is a relatively new engineering material.Compared with Portland cement concrete,it has higher early strength,lower natural resource consumption,and lower cost.With the increasing application of geopolymer materials in recent years,the performance requirements of materials in engineering have also gradually increased.For Portland cement concrete,the frost resistance performance and deterioration mechanism in durability have been studied comprehensively,and the concept of water saturation coefficient has been often used as a new characterization in freeze-thaw resistance since it was proposed by Prof.G.Fagerlund in 1977.For the freeze-thaw damage of geopolymer concrete in water and salt solution,there was few basic mechanism researches,and there were few studies on the water saturation coefficient of geopolymer concrete.Therefore,the frost resistance of geopolymer concrete could be analyzed from the perspective of water saturation coefficient,and the relationship of water saturation coefficient and frost resistance could be established to provide a theoretical basis for the engineering application of geopolymer concrete.In this paper,metakaolin-based geopolymer concrete was prepared with metakaolin and alkali solution.By adjusting the Na₂O content,modulus and water/cement ratio,6 groups of geopolymer concrete with different mixing ratios were prepared.Study on the change of water saturation coefficient and the determination of critical water saturation coefficient were carried out.The effects of heat curing and nano-modification on the freeze-thaw resistance of geopolymer concrete and the change of water saturation coefficient were studied.The effects of modification methods on the water saturation coefficient and freeze-thaw resistance in salt environment were systematical researched.The results showed that:（1）The porosity of metakaolin-based geopolymer concrete was higher than that of Portland cement concrete with the same strength grade.The porosity of G1-G6geopolymer concretes were 1.68-2.17 times of that in same strength grade Portland cement concrete;Compared with cement concrete of the same strength grade,geopolymer concrete has a higher water saturation coefficient under natural conditions,about 1.52 times to 1.61 times of same strength grade Portland cement concrete;（2）The difference between the critical water saturation coefficient and the actual water saturation coefficient of the geopolymer was 75%lower than that of cement concrete with the same strength grade.The lower value results in geopolymer concrete with lower frost resistance than cement concrete with the same strength grade.The theoretical results were consistent with the experimental results;（3）The greater the water saturation coefficient of the geopolymer concrete in the salt solution,the greater the amount of scailing amount during the salt freezing process,and the worse the salt freezing resistance.The water saturation coefficient of geopolymer concrete in three salt solutions were significantly higher than the water saturation coefficient in water;its water saturation coefficient in three different salt solutions were ranked as follows:S_{Na Cl}>S_{Ca Cl2}>S_Na2SO4;（4）The heat-curing-modified water saturation and nano-modified water saturation coefficients of the G1-G3 group of geopolymer concrete have been reduced to varying degrees.The water saturation coefficient of heat curing has been reduced by 2.72%,5.97%,and 3.33%respectively compared with standard curing;Compared with standard curing,the water saturation coefficient were reduced by 6.46%,8.57%,and4.77%.Both methods have improved the frost resistance of geopolymer concrete in water;however,the improving effect of heat curing and nano-modification of metakaolin-based geopolymer concrete on salt freezing resistance were not significant;（6）Taking the mass loss of geopolymer concrete under water freezing and the attenuation of density converted from water saturation coefficient as indicators,the damage evolution equation of concrete was fitted,and the expression of the single-stage damage mode equation was Y=A*X²+B*X+C,the two-stage damage mode equations were Y₁=A*X+B（X≤X₀）and Y₂=A*X²+B*X+C+f（A,B,C）（X≥X₀）.Through equation fitting,it was found that the mass loss of the three sets of geopolymer concrete G1-G3 conformed to the two-stage damage mode,and the compaction attenuation conformed to the single-stage damage mode.The fitted damage evolution equation R value（correlation degree）were all above 0.97.The life expectancy of concrete could also be predicted according to the fitted equation.The research results of the thesis related the water saturation coefficient to the freeze-thaw damage of geopolymer concrete,which laid a theoretical foundation for further improving its freeze resistance.