Research On Microstructure And Properties Of The Steam-cured Concrete With Metakaolin

The paper focuses on the key technical problems of steam-cured concrete precast elements and combines the major demand of steam-cured concrete precast elements on industrialization of construction industry in China currently.The effects of metakaolin(MK),as a mineral admixture,on the properties and micro-structure of steam-cured concrete were systematically studied.Compared with the standard curing condition,the effects of water-binder ratio,age,the amount of MK and mixing with other mineral admixtures on workability,mechanical properties and durability of steam-cured concrete were studied.The effects of MK on hydration degree,micro-morphology and micro-structure were also investigated by means of scanning electron microscopy(SEM),pore structure and thermal analysis.The main conclusions are as follows:(1)Incorporating MK has a great effect on workability of concrete.To control the fluidity of mortar between(135 ± 10)mm,the dosage of water reducer increases with increasing the amount of MK.When using MK at a replacement level of 25%,the dosage of water reducer(water aqua)is 6%.The dosage of water reducer also increases with increasing MK surface area.(2)The presence of MK is beneficial in mechanical properties of steam-cured concrete.The demoulding compressive strength of steam-cured concrete improves significantly.With15% MK singly,the demoulding strength improves 19.8%.The later strength of steam-cured concrete with metakaolin increases continuously.The improvement effect of MK on steam-cured concrete is equivalent to silica fume,and better than fly ash and slag.The higher fineness of MK has better effect on mortar strength.(3)The addition of MK helps in improving the capillary water absorption and chloride ion permeability of the steam-cured concrete.With the increase of curing age,the increase of MK content or the decrease of water-binder ratio,capillary water adsorption coefficient and diffusion coefficient of chloride ions are both decreased and gradually stabilized,the rate declines faster in the first 28 d.When the water-binder ratio is lower than 0.35,the amount of MK reaches to 15%,the capillary water absorption and chloride ion permeability are similar to that of standard-cured concrete at the age of 28 and 90 d,or even better than that of standard-cured concrete.(4)MK replacement of cement is effective in improving resistance of steam-curedconcrete to sulfate crystallization attack.Under the condition of partial soaking,the corrosion resistance coefficient and quality of the blank group increase firstly and then decrease.After adding MK,the corrosion resistance coefficient increases and the mass change range is lower than that of the blank group.(5)SEM,thermal analysis and pore structure data indicate that MK can significantly accelerate the hydration process as well as promot the formation of hydrated gel and other products.Added MK into steam-cured concrete can reduce the content of Ca(OH)2,decrease the porosity and increase the content of C-S-H gel.At the age of steam-cured demoulding,the calcium hydroxide in steam-cured concrete mixing with 20% MK is basically consumed by the pozzolanic reaction.Steam-cured concrete with 15% MK singly,the content of Ca(OH)2is only half than that in the blank group at 90 d.