Effects Of CO₂ Surface Treatment On Permeability And Mictrostructure Of Cement-Based Materials

Concrete is the most widely used building materials and its durability problems have drawn more and more attention.Carbon dioxide can react with C2S,C3S and the hydration products such as calcium silicate hydrate(C-S-H)gel and calcium hydroxide when surface treatment was applied to fresh concrete with carbon dioxide.This reaction forms dense calcium carbonate and silica gel which fill the pore structure of concrete and improve the durability of reinforced concrete.However,the existence of fly ash and slag make the influences of CO2 surface treatment different,and the pore structure after CO2 surface treatment also varies with different mineral admixture.The mix proportion was designed by a seven-batch factorial design method,several parameters such as compressive strength,surface permeability,water absorption,carbonation depth and chloride penetration depth were utilized to analyze the influence of CO2 surface treatment on the permeability of a ternary system of OPC,fly ash and slag,also thermogravimetric analysis(TGA),fourier transform infrared spectroscopy(FT-IR),Vickers microhardness and scanning electron microscope(SEM)was carried out to study the influence of CO2 surface treatment on the micro structure of concrete containing OPC,fly ash and slag.Results of compressive strength showed that CO2 surface treatment could make the surface layer more hard so that increase the compressive strength of concrete,however,the increase was limited because surface treatment only affect surface layer so that the compressive strength of concrete block didn't increase significantly.As the subsequent curing time increases,this improvement on compressive strength became decreasing,at the age of 90 days CO2 surface treatment even reduced the compressive strength.This phenomenon showed that CO2 surface treatment didn't accelerate subsequent hydration of concrete,instead,it inhibited hydration of concrete.Results of air permeability and water permeability showed that when the dosage of mineral admixture such as fly ash and slag was low,CO2 surface treatment could reduce air permeability and water permeability efficiently,but when the dosage of mineral admixture was high,the effect of CO2 surface treatment would decrease significantly.Results of Water absorption test indicated CO2 surface treatment raised the initial water absorption coefficient and degraded pore structure when the dosage of mineral admixture was high.Results of accelerated carbonation test and rapid chloride migration test showed that CO2 surface treatment could reduce both carbonation depth and chloride penetration depth,especially when fly ash was mixed into concrete.Results of Vickers microhardness showed CO2 surface treatment could significantly enhance the microhardness of pasete within 1.5mm.Fly ash could raise the effect of CO2 surface treatment,slag could reduce the depth of surface treatment to 1 mm.Results of FT-IR and TGA indicated CO2 surface treatment could consume calcium hydroxide and generate calcium carbonate and silica gel,however,the reaction product would cover cement clinker and inhibit the hydration of cement.Results of electron back scattering diffraction(EBSD)showed that the porosity and content of calcium hydroxide decreased within depth of CO2 surface treatment.The content of calcium hydroxide constantly decreased because of pozzolanic effect of fly ash.Results of energy dispersive spectrometer(EDS)indicated CO2 could react with C-S-H gel and reduce its calcium-silicate ratio.After CO2 surface treatment there was lot of calcium carbonate on the surface of paste.Finally,the calcium carbonate would be covered by C-S-H gel and become more compact.