Studies On The Measures Of Anti-carbonization Performance Of Alkali-activated Slag Cement

As a new low-carbon environmentally friendly cement,alkali-activated slag cement?ASC?has the advantages of fast setting and hardening,high strength in early and late stages,and good performance under chemical corrosion resistance,showing broad application prospects.However,with lots of disadvantages,such as shrinkage cracking,easy frosting,poor anti-carbonation performance,the engineering application and commercialization of ASC have been greatly limited.To improve the anti-carbonization performance of ASC,calcined layered double hydroxides?CLDH?,ultra-fine limestone powder,Portland cement clinker,MgO expansion agent and CaSO₄ expansion agent were selected as additives.To study the influence of additive types and dosages on the carbonation performance of ASC,the flexural strength and compressive strength of alkali-activated slag mortar?ASCM?before and after carbonation and the carbonation depth with different carbonization ages were used as indicators.The improvement mechanism was analyzed by X-ray diffraction?XRD?and scanning electron microscope?SEM?.The results wer as follows:CLDH of the type of Mg-Al-CO₃ achieved structural reconstruction by adsorbing CO₂ in carbonation environment,which significantly improves the carbonation resistance of ordinary Portland cement mortar?OPCM?.When the CLDH content is 2%?wt%of slag?,compared with the benchmark,the carbonation depth of OPCM with the same carbonation age is significantly reduced,the reduction of the flexural strength after carbonation was reduced,and The increase in compressive strength increased;Under the same dosage,the effect of CLDH on the anti-carbonization performance of ASCM was not as good as that of OPCM.The morphological effect and micro-aggregate effect of ultra-fine limestone powder increased the compactness of ASC hardened,but at the same time,more micro-cracks were produced.After carbonation,the anti-carbonization performance of ASCM was reduced.Compared with the benchmark,the ASC hardened of the same carbonation age was slightly higher in density,but the surface of the micro-cracks were carbonized severely,resulting in more micropores.The reduction of the flexural strength after carbonation was reduced,and the increase in compressive strength increased.Portland cement clinker participated the hydration reaction of ASC,increased the content of carbonizable substances and the compactness of ASC hardened,and improved the carbonation resistance of ASC.When the Portland cement clinker content was 2%?wt%of slag?,compared with the benchmark,the carbonation depth of the ASCM with the same carbonation age were significantly reduced,the reduction of the flexural strength after carbonization was slightly reduced,and the increase of compressive strength is slightly increased.However,when the amount of Portland cement clinker was more than 2%?wt%of slag?,the strength of ASCM before and after carbonation was very unfavorable.MgO expansion agent participated in hydration reaction,which increases the density of ASC hardened.In the carbonation process,some of the periclase react with CO₂ to form hydrotalcite,which significantly improved the carbonation resistance of ASC.When the amount of MgO expansion agent is 6%?wt%of slag?,the carbonation depth of ASCM with same carbonation age was significantly lower than that of the standard,the reduction of the flexural strength after carbonization was reduced,and the increase of compressive strength was improved.CaSO₄ expansion agent participated the hydration reaction of ASC,generated dihydrate gypsum and accumulates in the pores,increases the bending degree of the pores,improves the compactness of the ASC hardened,and significantly improved the carbonation resistance of ASC.When the CaSO₄ expansion agent content was 2%?wt%of slag?,the carbonation depth of ASCM with same carbonation age is significantly reduced compared with the standard,the reduction of the flexural strength after carbonization is reduced,and the increase of the compressive strength is increased.However,when the CaSO₄ expander content was 4%?wt%of slag?,the strength of the ASCM is significantly reduced,although the carbonization depth of the ASCM can be reduced.