Research On The Strength Development And Rehydration Characteristics Of UHPC Matrix Incorporated With Metakaolin

Metakaolin(MK)is a high pozzolanic material that derived from calcined kaolintic clay by dehydroxylation under low temperature of 500-800℃ and has ultra high specific surface area and excellent activity after moderate milling.MK is a kind of low-carbon and environmentally-friendly pozzolanic material since only vapour is discharged in the production process of MK,which has not adverse effect on environment and MK production consumes low energy.Ultra high performance concrete(UHPC)is a kind of rising creationary cementitious material in recent years possessing ultra high mechanical and durability properties.However,UHPC has some disadvantages such as high cost of production and low utilization rate of cement et al.Based on this point,the UHPC matrix with metakaolin as reactive powder and limestone replacing part of cement was manufactured in this paper.Mechanical properties,hydration and microstructures of UHPC matrix with standard and steam curing conditions were studied.In addition,late age rehydration of UHPC matrix placed in high temperature water for a short period and and long term normal temperature water were also studied.Above-mentioned studies can furnish reliable basis for the application of UHPC component in engineering structure.The UHPC matrix in this study contains binary matrix blended with MK and ternary matrix incorporated with MK and limestone.The main research work and achievements are as followed:(1)The influence law of MK replacement level(0-20%)on compressive and flexural strength of UHPC matrix with standard curing and steam curing at 55 or90 ℃ for 24 or 48 h were studied.In two series,the UHPC matrix containing 15%MK have optimum compressive strength at early or late ages.Steam curing can obviously improve the early age compressive strength of matrix.However,steam curing can increase the brittleness of matrix thus decreasing the flexural strength and inhibit late age strength development.(2)The studies on the hydration and microstructure of MK contained UHPC matrix with standard curing indicate that rapid MK reaction rate occurred in the former 7 days.While the MK reaction rate significantly decrease relative to that in the former 7 days.However,there is continuous MK reaction in UHPC matrix from7 to 90 days.The bounding water amount of gels in UHPC matrix increases with MK scale while the quantity of portlandite decreases with MK dosage.Also,the pore structure of matrix was apparently refined with MK scale.The quantities of bounding water,portlandite and microstructures are similar in UHPC matrix when the MK level is over 10%.The synergistic effect of MK and limestone is measurable in ternary matrix containing MK and limestone when the MK level is not exceeding5%.(3)The studies on the hydration and microstructure of steam cured UHPC matrix containing metakaolin indicate that increasing early-age curing temperature can significantly enhance MK reaction degree at early age.Although the hydration rate of matrix incorporating MK is very slow after the end of steam curing,the MK reaction degree is still obviously larger than that under normal temperature curing from 28 days onwards.The cement hydration degree of steam cured UHPC matrix has a certain extent decrease relative to that of standard cured matrix.Steam curing can further refine pore size of matrix and increase the proportion of small pores(<20 nm)according to the test results of micro-structures.(4)Significant rehydration occurs in standard cured UHPC matrix blended with MK after 28 days when they are immersed in 80 ℃ water for 28 days.While the rehydration degree of corresponding matrix under steam curing conditions significantly decrease after going through high temperature water curing.Moreover,the rehydration of steam cured matrix is primarily composed of MK reaction.Rehydration of UHPC matrix placed in high temperature water for a short period cannot obviously degrade the compressive strength.However,the rehydration of matrix can cause different level of decrease for the flexural strength.It is noted that decreasing early-age curing temperature can efficiently alleviate the degrading effect of flexural strength.Pore structure analysis indicated that rehydration can not only refine the pores lower than 20 nm in matrix but also increase the proportion of pores larger than 200 μm.(5)The rehydration of UHPC matrix immersed in normal temperature water from 90-365 d is composed of cement hydration and MK reaction.The rehydation degree of matrix with standard and steam curing are similar since early-age steam curing can not evidently inhibit the rehydration of UHPC matrix under long term water curing.The rehydration do not harm the compressive strength of UHPC matrix at 365 d relative to that at 90 d.The flexural strength of ternary matrix blended with MK and limestone has measurable enhancement after rehydration while that of binary matrix containing MK has some extent of decrease.After rehydration,the development law of pore structure of long term water cured matrix is similar to that of matrix cured in high temperature water for a short period.