Mineralogical analyses of old (78 and 98 years) concrete - 59093

Because Ca-dissolution from cementitious material is considered a source of long-term alteration of the performance of radioactive waste repositories, much research including dissolution tests has been conducted on this topic. These studies have introduced models such as Atkinson's model [1] to calculate the leaching of cementitious material. These models have been used to verify that the results of many studies do represent the alteration of cementitious minerals and the composition of the leachate. They have also been used to make numerous estimates of long-term mineralogical alteration in repositories, such as cement-clay interaction in cementitious barrier systems, and to evaluate the change in repository performance. However, immersion tests using bulky cementitious material have often indicated that the actual alteration of cementitious material might be slower than the rates calculated by these models. This difference may be due to a change of mass-transport characteristics, either in the bulky cementitious material or at the interfaces with other materials. In this study, a mineralogical analysis was conducted on two types of old concrete. Drilled cores from the foundations of rotary kilns at two cement factories were collected beneath the groundwater level. Both concrete structures were made from Japanese ordinary Portland cement (OPC), which is similar to European type 1 cement. One structure had been set into a fresh groundwater environment for 78 years (78-F), and the other had set sunk into a saline environment for 98 years (98-S). We evaluated the composition of hydrated cement minerals and the microstructures of old concrete in order to estimate the actual alteration of cementitious material over a long period. The types of old concrete examined consist of one that had been in a fresh groundwater environment for 78 years (78-F) and one that had been in a saline environment for 98 years (98- S). The carbonated area was several mm for both concrete samples evaluated in this report, and its depth did not extend very far from concrete surface. Measurements of pore size distribution show there were few comparatively large pores in.the microstructure, and many pores were small ones of 0.05 μm or less. Therefore, it can be concluded that both the 78-year sample and the 98-year sample had dense structures. Neither sample had any remaining alite or belite in the cement paste in the concrete interior, and both samples showed that cement hydration has sufficiently progressed. One factor contributing to the dense structure appears to be the high degree of hydration. Moreover, traces of Ca(OH)2 leaching were not confirmed in the non-carbonated area, even though the concrete had been in contact with groundwater for a long period. However, the amount of Ca(OH)2 in cement paste was very low in the non-carbonated area compared with OPC paste that had been cured for 28 days. On the other hand, the samples of old concrete showed higher C-S-H content than the OPC paste. It is possible that Ca(OH)2 was consumed by the concrete through a Ca-Silica reaction, given that the carbonated area was limited to the concrete surface. As a result, we surmise that was C-S-H generated and its content increased. From the results of this study of concrete placed in an underground environment, it has been learned that alteration can be limited to a depth of several mm from the concrete surface if the concrete has a firm structure, even if 70 years or more have passed since the concrete was placed. Moreover, the results suggest that the hydrate composition of old concrete differs from that of young concrete. One likely conclusion is that the mineral composition of concrete changes when there is no more unhydrated cement; however this result depends on the materials used and the environment where the concrete was placed. (authors)