The Influence Of Aggregate-Matrix Interface On The Carbonization Property Of Cement-Based Materials

Carbonation of concrete refers to that CO2 in the atmosphere dissolves in the pore solution through the pores or cracks into the concrete, then reacts neutrally with calcium hydroxide, CSH gel, ettringite and other phases of the concrete. The neutral reaction of concrete will cause shrinkage cracks in the hardened cement paste, leading to the deterioration and shortening the service life of the concrete. Carbonation can cause changes in the internal microstructure of concrete, while the various microstructure of the concrete will make great effect on the carbonization process. The microstructure of concrete consists mainly of phase composition, porosity, pore size distribution and so on including interfacial transition zone (ITZ) and the cement matrix. ITZ is the weak link in the concrete, and the microstructure of ITZ has an important effect on the transmission performance and durability of concrete. ITZ and its microstructure is bound to be an important role which impacts concrete carbonation process, and it can provide the basis for the establishment of the concrete carbonation model considering the ITZ. The effect of aggregate-matrix interface of concrete on carbonation process is researched according to the carbonation rate of interface, the microstructure characterization of ITZ based on nanomechanical properties before and after characterization, the evolution of ITZ microstructure by Backscattered electron (BSE) image analysis technology, as well as the effect of change of interface conditions on carbonation resistance of concrete.Because of differences in the microstructure of ITZ and cement matrix, their carbonization process must result in different appearance. The characteristics of ITZ make its faster carbonation rate than the matrix such as high porosity, loose structure, higher content than the matrix of calcium hydroxide and directional distribution, less unhydrated cement and so on. In this paper, the experiment including regular aggregate-matrix interface was designed, then the paste specimens of 0.60,0.53 and 0.35 water-cement ratio containing limestone and granite stone (as aggregate) are moulded. The ends of the specimens served as carbonation surface, then the specimens were carbonized for 28d with measuring the carbonation depth of interface and matrix by phenolphthalein. The experimental result shows that:the carbonation depth of interface is 2 to 3 times as large as the cement matrix. The carbonation curve in the vicinity of the interface possesses the feature that the closer distance to interface, the deeper carbonation depth under the interfacial influence of carbonation. Thus, CO2 transmission speed in the interface is several times as quick as the cement matrix, the transmission in the interface is one of the causes of the formation of semi-carbonation zone. This paper presents a kind of physical model considering the impact of ITZ on the cement-based material carbonized.Nanoindentation can test the nanomechanical properties and microstructure characterization of ITZ, which reveals the microstructure evolution of ITZ before and after carbonization. The simulated interfaces of concrete for different water-cement ratio are presented in this paper, and nano-indentation technique was applied to test microstructural features of ITZ before and after carbonization. The result shows that:ITZ still exists after carbonization, and the elastic modulus and hardness values of the matrix and ITZ are both increased. The size of ITZ reduced from approximately 50～60μm before carbonization to about 20～30μm after carbonization. Therefore, the ITZ is still the fast-track of CO2 diffusion after carbonation.The porosity and other microstructural evolution of ITZ before and after carbonization can reveal the mechanism how the performance of concrete change during the carbonation process. In this paper, the interface specimens for different water-cement ratio were tested by BSE, the morphology of ITZ was observed, then the porosity and phase composition of it was quantitatively analyzed before and after carbonization according to the BSE image. Experimental result shows that the porosity (the pore size is not smaller than 200nm) of ITZ was higher than the matrix, and the porosity was increased after carbonation, at the same time there was a decline in varying degrees of unhydrated cement content. The porosity of ITZ is still higher than the cement matrix after carbonization, but the amount of unhydrated cement reduces more than the cement matrix.Finally, in order to explore the impact of change of interfacial microstructure on concrete overall anti-carbonation performance, the carbonization experiment that interfacial conditions was changed was designed by aggregate wrapped slurry of concrete specimens. The result shows that: the aggregate with high water-cement ratio wrapped slurry can improve effectively the overall water-cement ratio of concrete to increase the carbonation depth, weakening the carbonation overall resistance of concrete. SEM image analysis shows that the structure of wrapped slurry aggregate interface was looser and more polyporous than that of non-wrapped slurry interface for the concrete of the same water-cement ratio.