Study On Carbonation And Chloride Resistance Of Autoclaved Nano-modified Concrete

According to the current situation of accelerated curings for precast concrete members in China,in which high-temperature and high pressure conditions can easily cause the deterioration of the protection of concrete for prestressing tendons,the paper adopted a combination method of theoretical and experimental studies on the chloride and carbonation resistance of concrete,which were modified by two kinds of different pozzolanic activity nanomaterials—nano-SiO₂ and nano-CaCO₃ and under autoclave and secondary water curing conditions.The corresponding mechanism of the improved carbonation and chloride resistance of concrete from microscopic point by scanning electric microscope and mercury intrusion porosimetry were also conducted.Finally,the prediction of service lives of nano-modified concrete under chloride environments were obtained.Some main findings obtained are shown as follows:?1?Nano-SiO₂ can significantly improve the carbonation and chloride penetration resistance for autoclaved concrete.The improvement of concrete carbonation resistance was almost the same as that under standard curing,while the chloride penetration resistance was slightly worse than that under standard curing.The specific improvements were ranked as 2wt.%>1wt.%>3wt.%.Dosage exceeding the optimum was not beneficial on the anti-corrosion protection of steel bars in autoclaved concrete.Nano-CaCO₃ produced limited effects on the carbonation and chloride penetration resistance on autoclaved concrete at its early age,however,it did contribute beneficial effects at its later age.The specific improvements were listed as 3wt.%>2wt.%>1wt.%.?2?The secondary water curing can significantly reduce the carbonation depth and Coulomb electric flux values of autoclaved concrete.However,the improvement effects of short-term subsequent hydration reaction were still not enough to compensate the thermal damages caused during the autoclaved curing.After the application of secondary water curing,the carbonation and chloride penetration resistance of the nano-SiO₂modified autoclaved concrete were significantly improved,which was mainly attributed to the coupling effects of nano-SiO₂ and secondary water curing.The improvements of carbonation and chloride penetration resistance of various nano-CaCO₃ modified concrete were slightly worse than that of nano-SiO₂.The nano-characteristics of nano-SiO₂ and nano-CaCO₃ have great influences on the anticorrosion performance of steel bars in autoclaved concrete after secondary watering curing.?3?Nano-modification can significantly reduce the porosity of concrete,optimize the distribution of pore structures,and decrease the proportion of more harmful pores obviously.The more harmful pores are gradually transformed into harmful pores,and the proportion of harmless pores and less harmful pores increase at varying degrees.In addition,there are still some differences in the improvement of pore structures between nano-SiO₂ and nano-CaCO₃ particles.The secondary water curing has beneficial effects on the micro-structures of autoclaved concrete,makes concrete dense,and the micro-voids and micro-cracks in concrete tend to be reduced.Nano-SiO₂ usually has good pozzolanic effects,micro-aggregate filling effects and nucleation effects,therefore the hydration products are oriented differently and overlapped each other,which makes concrete relatively flat and compact.Many microvoids and microcracks caused by accelerated autoclave curing are filled after the incorporation of nano-SiO₂.The amount of calcium silicate gel in nano-CaCO₃ modified concrete is relatively low,and many micropores and cracks are not fully filled,and the improvements of connectivity is worse than that of nano-SiO₂.?4?The equation of Cl^-diffusion coefficient D_n and the amount of nano-materials is obtained by regression fitting:D_n=K₁n²+K₂n+K₃.According to Fick's second law and existed scholars'research,combined with the functional relationship between the nano-modified concrete Cl^-diffusion coefficients and the nano-material contents,the anti-Cl^-attack service life prediction models of nano-modified concrete are derived:T=0.56c²??K₁n²+K₂n+K₃???tDn?/?t'??^m.The service life of 2.0wt.%nano-SiO₂ modified concrete under autoclaved curing is the best,which is 20.78 years longer than that of the control with an increase of 119.70%.The service life of 2.0wt.%nano-SiO₂ modified concrete under secondary water curing is extended to 60.98 years,nearly double that of the control.After modification by nano-CaCO₃,the service life of concrete against Cl^-attack is also improved,however,the overall improvement is lower than that of nano-SiO₂.The service life of 3.0wt.%nano-CaCO₃ modified concrete against Cl^-attack under the secondary water curing reachs 41.22 years.