The Relationship Investigation Of Cement-Based Materials Between Microstructure Features And Carbonation Model

Carbonation is a kind of chemical reaction process occuring between alkaline substances of cement-based materials and CO₂ of the air under suitable humidity conditions. It which weakens the protection effects of reinforced concrete and increases the steel corrosion and volume expansion speed then the concrete cover cracks. During design of concrete structures, in order to forecast the service life, numerous domestic and international scholars have investigated and established carbonation models. In recent years, the characteristic of cement-based material microstructure research reveals the relationship between microscopic mechanism and macro phenomenon, which establishes the basis of microstructure carbonation models. However, the influence of the complete carbonization zone on CO₂ transfer rate and partial carbonization zone on reaction rate are ignored.The thermogravimetric （TG）, X-ray tomography （X-CT）, nuclear magnetic resonance （NMR）, nano indentation, physical adsorption （BET）, and mercury intrusion （MIP） methods are used to quantify the phase composition and pore structure parameters within dirrerent carbonation depth,such as CH, C-S-H, CaCO3, saturation, porosity, tortuosity and so on. This provides experimental basis to modify and verify the traditional model. The main research results are follows:Firsly, the gray and composition of solid substances which belongs to different depth of 0.35W/C and 0.53W/C cement paste are tesed by using the method of X-CT, TG, after carbonation. The results showed that the X-CT method of can detecte the distribution of complete, partial and non-carbonated zone in cement paste. X-CT and TG measures result are basically identical.Secondly, Combining TG and NMR methods the carbonation degree of different depth CH, C-S-H which derived from 0.35W/C cement after 28 days carbonated are calculated. The results show that carbonation degree of CH, C-S-H differ from complete carbonation zone to partial. In complete carbonation zone, C-S-H carbonation degree is higher than that of CH, while it is reverse in partial carbonation zone. At the same time, nano indentation method using to analysis the solid phase volume fraction of different depth reflects that the solid phase composition changes gradually by depth. The volume fraction of LD C-S-H and HD C-S-H HLD gradually increased from surface to inside of sample and the C-S-H carbonation rate fester than that of HD C-S-H.Thirdly, the layer porosity and pore size distribution refers to different carbonation depth of 28 days carbonated 0.35W/C cement paste are tested by MIP and BET method. The paper proposes a method to unify calculation of pore size probability distribution which can avoid their defects. The research results show that the pore structure parameters and transmission performance is various from surface to inside, the CO₂ diffusion coefficient order is complete carbonation zone< partial carbonation zone< non-carbonated zone.In addition, combining TG, MIP and NMR test results, the C-S-H average chain length evolution and porosity are calculated, in terms of 0.35W/C and 0.53 W/C cement paste’s different carbonation age. Finally, a new method is put forward which does not depend on the C-S-H formula to calculate porosity variation leading by C-S-H. The results also show that the C-S-H volume change is the main factor to change cement porosity. For low C-S-H carbonation degree, the C-S-H volume expands, which accounts for 70% of the total volume variation; for high C-S-H carbonation degree （88.7%）, the C-S-H volume shinks.Finally, considering the complete carbonation zone pore structure and saturation degree determines the CO₂ transfer rate and partial carbonation zone solid phase determines the reaction rate, Papadakis model is modified. The calculation result show that the predict the carbonation depth by revised model is more close to the experiment values and the error of revised model is 44.4% smaller than original model.