Research On The Evolution Rules Of Long-term Performance And Microstructure Of Cement-based Materials With Low Water–binder Ratios Under Further Hydration Effect

High-performance concrete(HPC)and ultra-high-performance concrete(UHPC),etc,are produced with low water–binder ratios to improve the compactness of internal structures.However,in terms of the cement hydration process,cement cannot hydrate completely when the water–binder ratio is less than or equal to 0.38.And there must be unhydrated cement particles in the hardened cement paste.These unhydrated cement particles can continue to hydrate when they are further supplied with water.This phenomenon is referred to as further hydration.In wet or water environment,the further hydration of unhydrated cement particles may induce the expansion and cracking of concrete materials,and provide channels for the invasion of external harmful substances,accelerating the deterioration of concrete performance and influencing the long-term performance of concrete materials.In this paper,the evolution rules of long-term performance and microstructure of cement-based materials with low water–binder ratios under further hydration effect were deeply studied through experimental study and theoretical analysis.The influences of hydration environments on further hydration of cement-based materials were studied first,and the rapid evaluation mechanism and test parameters for further hydration were given.During the further hydration process,the growth rate of compressive strength of the cement paste increased as the hydration ambient humidity increased,and the growth rate of compressive strength reached 2.75 times that of insulating humidity state when RH?95%.The growth rate of compressive strength and the expansion strain both increased as the hydration temperature of water increased,and the growth rate of compressive strength and the expansion strain under water soaking at60°C reached 1.83 times and 1.37 times that under water soaking at 20°C,respectively.The completion of standard curing for 28 d was suggested as the time starting point of the further hydration test.Water soaking at 60°C was recommended to be used as the test method for accelerating further hydration.The compressive strength and expansion strain could be used as performance evaluation indexes of cement-based materials under further hydration effect.The further hydration model of cement-based materials based on multiple factors was studied.Considering the influence of water migration on water–cement ratio,the modified model of cement particle hydration was proposed based on the Krstulovi(?)–Dabi(?)hydration dynamics of cement and microstructure model of cement hydration.Based on the result of particle size distributions of cement particles,the relationship between hydration degree of cement particle and hydration degree of cement was clarified,and the modified model of cement hydration was established.Considering the influence of water migration,the influence coefficients of water–cement ratio,amount of silica fume and further hydration effect were introduced into the modified model of cement hydration,and the further hydration model based on multiple factors was established.The predicted results were in good agreement with the experimental results.The evolution rule of long-term performance of cement-based materials under further hydration effect was studied,and the mechanism of further hydration effect was given.The results of the mechanical properties experiment showed that the compressive strength of cement pastes had an alternating tendency of increasing and decreasing with increasing further hydration time.Combined with the evolution process of microstructure,the hydration rate of cement was fast during the early stage of further hydration,and the new C-S-H gel filled the initial pores in cement paste,therefore,the pore volume and average pore size were significantly reduced.Thus the further hydration played an enhanced role.During the late stage of further hydration,the hydration rate of cement slowed down,and the internal space in cement paste was gradually not sufficient to accommodate C-S-H gel.The volume expansion of C-S-H gel led to the increase in internal stress and ultimately led to the formation of microcracks,therefore,the pore volume and average pore size increased.And the performance of the hardened cement paste deteriorated.The expansion prediction model of cement-based materials under further hydration effect was studied,and the damage risk assessment and control methods were proposed.The modelling method for the autogenous volume deformation of MgO micro-expansion concrete was used as a reference,and the relationships between temperature functions of a(T)and b(T),and hydration temperature T of water,expressed as exponential functions,were combined,and then an expansion strain model with a hyperbolic form was established.Considering the influences of water migration and water–cement ratio,the water migration coefficient and water–cement ratio influence coefficient were introduced into the expansion strain model with a hyperbolic form,and then an expansion prediction model of cement-based materials was established.Based on the model values of expansion strains after further hydration for 360 d,and combined with the damaging effect of long-term further hydration,recommended limit values of expansion strains(?_FH)_lv were given.Adding silica fume could effectively inhibit the harms induced by further hydration of unhydrated cement,and damage risk control effect for cement-based materials became more obvious with the increase of silica fume amount.The evolution rules of long-term performance and the assessment method and control measure of damage risk for cement-based materials with low water–binder ratios under further hydration effect are proposed through the research in this paper.This can provide a basis for the evaluation and design of long-term performance of cement-based materials with low water–binder ratios.