Effect Of Curing Regimes And Sulphate Attack On The Properties And Microstructure Of Ultra High Performance Concrete

Curing regimes and sulphate attack can affect the hydration process of cementitious materials in ultra high performance concrete（UHPC）,thus influencing microstructure formation and evolution mechanism of hydration products such as calcium silicate hydrates and Al-bearing phases.As the basic unit of concrete,composition and microstructure of hydration products decide its mechanical properties and durability.Based on NSFC Straits Foundation“Preparation and Engineering Application Foundation of ultra high performance concrete”and project of National 973 Plan“Microstructure Formation and Regulation of Long-life Concrete Materials under Harsh Environments”.By using advanced test technology such as ²⁹Si and ²⁷Al Magic Angle Spinning Nuclear Magnetic Resonance（MAS NMR）,XRD、SEM、MIP,effects of curing regimes（standard curing,80℃steam curing and 210℃autoclaved curing）and sulphate attack on microstructure of cementitious paste in UHPC was studied,relation between performance and microstructure of UHPC was discussed.Principal results are as follows.1.microstructure formation and evolution mechanism of hydration products such as calcium silicate hydrates and Al-bearing phases were studied.Under standard curing,the main hydration products of UHPC are C-S-H、CH、AFt、AFm and TAH,with the curing age,Al[4]/Si of C-S-H decreases,the main molecular chain length（MCL）increases.After 80℃steam curing for 48h,continuing standard curing to different curing ages,the main hydration products of UHPC are C-S-H、CH、AFt、AFm and TAH,with the curing age,Al[4]/Si of C-S-H decreases,the main molecular chain length（MCL）increases firstly,then decreasing,and increasing finally.3d⁷d,relation of silica fume and CH dominates,promoting the conversion from dimer to polymer,thus enhancing the MCL of C-S-H.7d²8d,hydration of cement dominates,developing the amount of dimer,thus reducing the MCL of C-S-H.After 28d,relation of silica fume and CH continues to dominate,the MCL of C-S-H develops.After 210℃,2MPa autoclaved curing for 8h,the main hydration products of UHPC are tobermorite、CH and TAH.Continuing standard curing,C-S-H gels by relation of cement and silica fume increase,TAH unites with SO₄^2-to form AFt and AFm,the MCL of C-S-H increases firstly,then decreasing,Al[4]/Si decreases.2.Microstructures of UHPC on the same strength scale under different curing regimes was comparative analyzed.Relation between mechanical properties and microstructure of UHPC was discussed.When UHPC under different curing regimes reach the same strength,hydration degree of cementα_c and silica fumeα_SF:standard curing>80℃steam curing≈autoclaved curing.The polymerization and MCL of C-S-H:autoclaved curing>80℃steam curing>standard curing.Al[4]/Si:autoclaved curing>80℃steam curing>standard curing.Porosity:standard curing>80℃steam curing>autoclaved curing.Types of hydration products:the main hydration products of UHPC under standard curing and 80℃steam curing are C-S-H、CH、AFt、AFm and TAH,and tobermorite、CH and TAH under autoclaved curing.Mechanical properties of UHPC depends not only on microstructure of cementitious paste,amount and the pore size distribution of hydrated cement mortar,but also types,amount of Al-bearing phases.3.Effects of sulphate attack on microstructure evolution of hydration products under different curing regimes in UHPC was studied.Obvious dealuminization appears in HSC with water-binder ratio of 0.35 and BFS when being subjected to sulphate attack for 180d.But only weak dealuminization appears in cementitious pastes of UHPC under different curing regimes after sulphate attack,and it enhances the hydration degree of cement,thus the MCL and Al[4]/Si decreases slightly.Effect of sulphate attack on the Al-bearing phases of autoclaved curing is more obvious,which promoting the conversion from TAH to AFt and AFmResearch results of this paper can provide some reference basis for the optimization,regulation of microstructure and performance improvement of concrete in UHPC.