Study On Corrosion Characters Of Cement-based Materials In Aggressive CO₂Environment

With the development and needs of society, more and more buildings and structuresare used in the underground environment, such as subway, tunnel engineering, the drillingengineering, underground drainage engineering and etc., however these structures andbuildings in underground environment will be subjected to a series of more complex andsevere physical chemistry erosion, of which the aggressive CO₂in underground waters isconsidered to be one of the key factors. To cope with climate change, CO₂storagetechnology will be applied at large scale, the aggressive CO₂which is caused by thisprocess will result in more severe damage on the underground concrete structures. In recentyears, geological disposal radioactive waste by using cement-based materials andcement-stabilized municipal solid waste fly ash are widely used. It is important to ensurethe durability of cement-based materials during the application process mentioned above, sothe degradation of cement-based materials in aggressive CO₂solution is of particularinterest to the relative researchers. This thesis which studied the degradation ofcement-based materials in aggressive CO₂solution has some of the prospective andimportant theoretical significance.The experimental device which was designed and made for simulating aggressive CO₂solution environment in this thesis can prepare aggressive CO₂solution with differentconcentration. By regulating the experimental device, the corrosion test in aggressive CO₂solution on different conditions such as immersion, flow and scour can be simulated.The degradation of cement pastes were studied in the continuously renewed aggressivecarbonic acid environment at flow speed of50L/d, of which pH value was4.5-4.6,aggressive CO₂concentration was about340mg/L and temperature was13-15°C.Byanalyzing microstructure, mineral compositions and chemical compositions of differentregions in leached zone from3days to81days leaching, the corrosion mechanism andkinetics of reactive transport were discussed in this thesis. The results showed that thecorrosion of cement paste in aggressive CO₂solution is a combined actiont of acidcorrosion and carbonation, and can be divided into three stages: the early stage ofcarbonation on the surface, calcite and other insoluble carbonates are the main corrosion products in this stage; the mid-term stable corrosion stage, during this stage the leachedzone are composed of three layers, gel layers, carbonation layer and CH dissolution layerfrom the outmost surface to the sound zone; the late accelerated degradation stage, which iscaused by large through cracks on local areas.Based on this, the corrosion of cement pastes mixed with different supplementarycementious materials （SCM） was further studied. The results showed that the depths ofleached zone in different paste samples are ranked in this order: mixed with GGBS and flyash （high）, mixed with GGBS, mixed with fly ash, mixed with none SCM, and mixed withsteel slag; however it is just the reverse order for the cracking degree in different pastessamples. For the depth of leached zone, it has some relationship with the content of CH; andfor the cracking degree, it probably relates to the Ca/Si ratio in the gel layers.The changing regularity of mechanics property of mortar and concrete samples inaggressive CO₂was primarily discussed. Due to the higher density, the corrosion rate ofmortar and concrete samples is smaller than that of paste samples, and shrinkage crackingof gel layers in mortar and concrete samples weakened because of aggregates in thesematerials. To60and90days, degradation on mechanics property of mortar and concretesamples was light but indeed existed.