Study On Several Durability Problems And Mechanism Of Cement-based Materials

The durability of concrete is one of the key problems in cement and concrete field that many scholars are concerned with. In recent years, the focus of durability research is transferred from single degradation factor to multi ones. Two of durability indexes of cement-based materials, freeze-thaw and carbonation, are related with each other, for that they both affect and are affected by structure and permeability. Water permeability is related with these two indexes, because it will cause erosion and changes above. So in this paper, damage features of cement-based materials after freeze-thaw, permeability behavior with a micro-crack, especially the interactive effect of freeze-thaw and carbonation are researched. BET SEM methods are also used to study the relevant mechanisms.Freezing and thawing test shows that the figuration and cross-section of cement-based materials (paste, mortar and concrete) are different after they damage because of their different pore structures. Study on the micro-structure of specimens' shows that freeze-thaw damage is a kind of uniform one, which happens from the surface to the center. And the interior becomes looser; the amount of large pores increases, so the total pore volume becomes larger.After it is carbonated, the pores of hardened cement paste are filled with finer calcium carbonation crystals. So the total pore volume decreases, and the pores become finer. The larger ratio of W/C, the more effect carbonation has to the surface area and pore structure of hardened cement paste.Considering the interaction of these two indexes, it can be concluded from the experiment that carbonation has some helpful effect on samples' ability to resist freeze-thaw, but the effect is not notable; on the other hand, the freeze-thaw cycling will accelerate carbonation. And the changing process with time of concrete under the interaction of carbonation and freeze-thaw is analyzed.The result of the leaching test to the sample with a micro-crack shows that the amount, the pH value, electric conductivity and Ca2+ content of leached water decrease with the prolong of time. At the beginning, all indexes have a trend of increasing, then decrease, and cease in the end.