| Cemented soil has been widely used in engineering constructions due to its high utilization ratio of original soil,low cost,convenient construction,versatile reinforcement form,and environmental benefits.With social and economic developments,the engineering construction environment has been becoming increasingly complicated,which requires the engineering properties such as strength,anti-permeability,and durability of cemented soil to be greatly improved.In addition,massive productions of Portland cement have led to significant consumption of energy,rapid depletion of natural materials and serious environmental pollutions.Consequently,it has become a hot topic to find suitable admixtures to partially replace cement and improve engineering properties of cemented soil.Coal-bearing kaolin,a kaolin resource associated with coal,is often discarded as gangue.Shanxi is a large coal-producing province in China.Great quantity of gangue deposited in this province not only occupies farmland,but also seriously threatens the surrounding environment.In this study,the coal-bearing metakaolin formed by calcining coal-bearing kaolin at 800℃ for 40 mins was used to partially replace cement in cemented soil.The engineering properties such as strength,permeability and sulfate corrosion resistance of composite cement sandy soil and silty soil were tested.The chemical compositions,pore structure and microstructure of cement hydration productions were thus examined in detail for analyzing the mechanism of coal-bearing metakaolin on engineering properties of composite cemented soils.The following conclusions can be drawn:(1)The unconfined uniaxial compressive strength of cemented soils with five dosages of coal-bearing metakaolin was tested at curing ages of 7 days,28 days and 90 days,respectively.The results show that the unconfined compressive strength of cemented sandy soil and cemented silty soil can be effectively improved by addition of coal-bearing metakaolin.The optimum mass ratio of coal-bearing metakaolin to cement is located to 1:6-1:4.In the range of optimum mixing ratio,the strength of cemented sandy soil can be increased by 84.55% at 7 days,112.90% at 28 days,64.35% at 90 days,the 7d strength of cemented silty soil was not increased significantly,however,28 d strength can be increased by 77.15%,and 90 d strength can be increased by 66.95%.(2)The constant waterhead permeability test of cemented soils with five dosages of coal-bearing metakaolin was tested after curing for 7 days,28 days and 90 days,respectively.The results show that the anti-permeability of cemented sandy soil and cemented silty soil can be effectively improved by addition of coalbearing metakaolin.The optimum mass ratio of coal-bearing metakaolin to cement is in the range of 1:6-1:4.In this optimum range,the permeability coefficient of cemented sandy soil can be decreased by 48.86% at 7 days,76.48% at 28 days and 84.63% at 90 days.The permeability coefficient of cemented silty soil can be decreased by 40.13% at 7 days,69.51% at 28 days and 85.27% at 90 days,respectively.(3)The unconfined compressive strength of cemented soil with five dosages of coal-bearing metakaolin cured in sodium sulfate solutions with five different concentrations was tested after immersion for 28 days and 90 days,respectively.The results show that the strength of cemented soil with coal-bearing metakaolin after immersions in sodium sulfate solutions was significantly improved.The strength of cemented soil was increased initially and then decreased with increasing content of coal-bearing metakaolin.When the content of coal-bearing metakaolin was 3%,the strength of cemented soil was the maximum.The optimum concentration threshold of sodium sulfate was 18 g/L for the coal-bearing metakaolin composite cemented soil.The strength of cemented soil was increased when the concentration is lower than that of the optimum concentration threshold,and the strength of cemented soil was decreased when the concentration is higher than that of the optimum concentration threshold.(4)The electrical resistivity of cemented soil with five dosages of coalbearing metakaolin cured in sodium sulfate solutions with five different concentrations was tested at 28 days and 90 days.The results show that coalbearing metakaolin can increase the resistivity of cemented soil.The resistivity of cemented soil cured in sodium sulfate solutions increased first and then decreased with increasing content of coal-bearing metakaolin.When coal-bearing metakaolin content was 3%,the resistivity of cemented soil reaches its peak value.With the increase of sodium sulfate concentration,the resistivity of coal-bearing metakaolin cemented soil increased first and then decreased.When the sodium sulfate concentration was 18g/L,the resistivity of cemented soil reached its peak value.It is proved that the corrosion strength of coal-bearing metakaolin composite cemented soil is strongly correlated with resistivity,and the nondestructive testing of coal-bearing metakaolin composite cement-soil can be carried out by resistivity.(5)The chemical compositions and microstructure of cemented soil with 0%,3% and 5% coal-bearing metakaolin replacement were analyzed.It is proved that the pozzolanic reaction of coal-bearing metakaolin occurred in the composite cemented soils.The content of calcium hydroxide(CH)in the cemented soil was found to be decreased,while the amount of calcium-silicate-hydrate was increased.The morphology of C-S-H transformed from acicular and flocculent structure with higher n(Ca)/n(Si)ratio to gelatinous structure with lower n(Ca)/n(Si)ratio.(6)The chemical compositions and microstructure of composite cemented soil with 0%,3% and 5% coal-bearing metakaolin cured in sodium sulfate solutions were tested.It was found that the contents of CH and expansive products responsible for reactions with sodium sulfate were reduced,and the resistance of cemented soil to sulfate sodium corrosion was increased.(7)Through theoretical analysis and micro-test,it is proved that the mechanism of improving the engineering properties of cemented soil by coalbearing metakaolin is the formation of the maximum compactness of binary cementitious materials(cement and coal-bearing metakaolin),the pozzolanic effect of coal-bearing metakaolin and the microstructure compactness caused by its physical filling effect on the reinforced soil.The interface area,pore structure and mechanical properties of the composite structure were effectively improved. |
