Study On Frost Resistance Of Metakaolin Based Geopolymer Concrete

Geopolymer is a new type aluminosilicate cementitious material,low carbon environmental protection during production and it has the characteristics of high strength,acid/salt erosion and high temperature resistance.There is a broad application prospect for repair,solidify heavy metal ion,aerospace and other fields.Freeze-thaw damage of concrete is the main form in cold regions.Concrete freeze-thaw damage is serious in the three northern regions of China.At present,the research of geopolymer mainly focuses on the fields of high temperature resistance,acid/salt erosion resistance,reaction mechanism and so on.The long-term durability is insufficient,specially the research on frost-resistance is rare.With the deep understanding of geopolymer,it has been applied in some projects,for which it is urgently needed to systematically study the frost resistance.Therefore,to understand the frost resistance of geopolymer concrete,explain the freeze-thaw damage mechanism,and propose measures to improve the frost resistance,which will provide a theoretical basis for understanding geopolymer concrete and accelerate its engineering application.In this paper,we employ Na₂O equivalent content,alkaline modulus and water-solid ratio to design and prepare geopolymer.The basic properties such as the setting time and micro-morphology of geopolymer were investigated.The frost resistance of geopolymer concrete under the quick freezing method and slow freezing method was studied referring to the test standard of cement concrete,and the freeze-thaw damage rule was obtained.It is known how parameters such as Na₂O equivalence content and alkaline modulus affect the frost resistance of geopolymer concrete.The effect of slag content and different types of air-entraining agents on the frost resistance of geopolymer concrete was explored,and measures to improve the frost resistance of geopolymer concrete were proposed.Finally,the pore structure and SEM images of geopolymer concrete before and after freeze-thaw and measuring the porosity,capillary saturation,and the ion concentration of geopolymer pore solution were analyzed to determine the freeze-thaw failure mechanism.Results showed that metakaolin based geopolymer concrete has poor frost resistance,and the limit freezing-thawing times less than 75 times.Na₂O equivalence content and activator modulus have great effect on the freeze-thaw performance and mechanical properties of geopolymer concrete.The metakaolin based geopolymer concrete with a Na₂O equivalent weight of 16%and modulus of 1.5 is the best which limit freezing-thawing times could reach 75 times.The frost resistance of geopolymer concrete has a positive correlation with Na₂O equivalents content that correlation coefficient is 0.845,and the correlation with activator modulus and other parameters is not significant.Slag can shorten the setting time of geopolymer,enhance the mechanical properties of the geopolymer concrete,specially strengthen the tensile strength by more than 20%.The frost resistance of geopolymer concrete advance with the increasing of slag content.The limit freeze-thaw times of geopolymer concrete with 30%slag increase from 43 to135.The effect for improving the frost resistance of geopolymer concrete is sodium lauryl sulfate>sodium dodecyl sulfate>sodium arosinate,which best mixing amount are 0.06%,0.08%and 0.02%respectively.The limit freezing-thawing times of geopolymer concrete could enhance from 36 times to 60-75 times by entraining air.The three air-entraining agents introduced bubble spaces and pore sizes exceed 400μm and 300μm.Common air-entraining agents have poor compatibility in high-alkaline,high-viscosity fresh geopolymer concretes.The slag can better improve the frost resistance of the geopolymer concrete than the air-entraining agent.And there is better impact for improving frost resistance using both slag and air-entraining agent.The degree of saturation is the most important factor influencing the frost resistance of geopolymer concrete,and the degree of saturation is greater than the rate of temperature decrease.The porosity of the geopolymer concrete exceeds 15%.The balance capillary water saturation is 0.84 to 0.92 within 12 hours.Here is filled with sodium silicate solution in geopolymer pores,the concentration of Na⁺ions is very high about 0.6mol/L.Pore structure of geopolymer concrete is terrible,the pore exceeding 50nm is more than 75%and the proportion of harmful pores in freeze-thaw process is further increased.They cause the freezing pressure and osmotic pressure higher in the freeze-thaw process,and even salt precipitation produces crystallization pressure,ultimately resulting in poor frost resistance.The freezing-thawing failure mechanism of geopolymer concrete is mainly based on hydrostatic and osmotic pressure theory.Results clearly define the freeze-thaw damage laws and how parameters effect the frost resistance of geopolymer concrete.And propose measures to improve frost resistance,and explain the freeze-thaw damage mechanism.It provides experimental data and theoretical basis for the application of geopolymer concrete in cold regions.