Study On The Preparation And Strength-Expansion Coordinated Regulation Of Retarding And Expansive Road Base Cement

Cement stabilized base course is the most widely used base structure in China due to its high strength,water stability and good integrity.However,its insufficient permitted delay time will cause strength loss during the rolling process,and its large shrinkage characteristics are easy to produce pavement reflective cracks,eventually leading to the destruction of road.Therefore,how to improve the crack resistance of cement stabilized road base and prolong the permitted delay time has become the key to improve the road service performance.According to the material design of phosphogypsum,clinker and high-aluminum supplementary cementitious materials,the retarding regulation and shrinkage compensation of cement were studied.In addition,the coordinated development mechanism of strength and expansion was revealed through the formation of cementitious and expansive phase,and the preparation method of slow-setting and expansive road base cement was finally put forward.The main research results are as follows:(1)The cement hydration magnetism-heat model was established based on the cement hydration heat release and water phase transition process;thus,the retarding mechanism of phosphogypsum and fly ash was revealed.The setting and hardening of cement depend on the hydration of C3S.At the end of C3S induction period,C-S-H gels began to form in large quantities,and the establishment of the paste microstructure led to the formation of initial setting.This process was confirmed by the change of the ~1H NMR weighted mean relaxation time T2.By inhibiting the hydration of C3S and reducing the content of C3S,respectively,phosphogypsum and fly ash would reduce the amount of C-S-H gels,retard the formation of the paste microstructure,and finally achieve the retarding effect.(2)The performance development of different SCMs system road base cement was studied,and a coordinated development model of strength and expansion was established.The strength of fly ash cement develops slowly,but the expansion rate is significantly increased at early age.And the expansion window gradually increases with the addition of phosphogypsum.When the phosphogypsum content is 17%,the expansion window can be extended to 60 days.While strength of the slag cement developed rapidly,its 28d flexural and compressive strength could reach 10MPa and40MPa respectively.However,compared to the fly ash cement,a weaker expansion ratio and a shorter expansion window which ended within 7 days were obtained with the increase of slag content.According to the microstructure evolution,it can be concluded that expansion is caused by the expansion energy destroying the matrix microstructure,which eventually leads to a decrease in strength;and the main reason for the strength increase could attribute to the C-S-H gels continuously repairing microcracks and filling pores.To obtain better expansion performance and ensure sufficient strength at later ages,the growth rate curve of strength and expansion should conform to the trend of typical curve 3 in the model and transfer to stage 2faster.(3)The formation of AFt contributing to expansion and C-S-H gel contributing to strength were quantitatively analyzed,and the crystallization stress of paste based on thermodynamics and poremechanics theories was calculated.Thus,the quantitative relationship between the macroscopic stress and expansion increment was established,which interpreted the influence of different SCMs on expansion.The expansion of hardened paste would be promoted by AFt formation and its supersaturation,while restricted by the formation of C-S-H gel.Gypsum in the paste is the key to affect the AFt formation and its supersaturation.When gypsum is excess,the concentration of S and Ca in pore solution is stable at 45mmol/L and 23mmol/L,respectively,and the supersaturation of AFt reaches 12.5 and as a result,the large expansion energy generates and the local maximum crystallization stress reaches 42MPa.SCMs with different activity possess different consumption rate to gypsum,and thus differently affect the action time of expansion energy,while the various formation rate of C-S-H gel results in huge difference in dynamic modulus.(4)To realize coordinated development between strength and expansion,based on the mechanism of SCMs influencing expansion,fly ash was adopted to regulate the performance of road base cement with high usage of slag.After FA partly replacing the slag,the consumption rate of gypsum is slowed down,which prolongs the expansion energy;in addition,the decrease of C-S-H gel formation at early age leads to a lower dynamic modulus,and thus the expansion ratio in early age is improved greatly.Although early strength is lowered,the later hydration of FA generally repairs the microcracks generating in early age,which ensures the stable improvement of the strength in later age.The best replacement of FA is 40%,of which the 14d expansion ratio is 0.76%,7d compressive strength and flexural strength are 23.7MPa and6.8MPa,90d compressive strength and flexural strength are 41.9MPa and 8.9MPa respectively.The coordinated development between strength and expansion is realized and the shrinkage-compensating effect is remarkable.