Study of engineering properties of low-pH self-compacting concrete for concrete plug

Engineered Barrier Systems (EBS) is the primary concept in the design of the Geological Disposal Facility (GDF). In the design of EBS, bentonite is usually used as a buffer/backfill material. Due to its unique design, the concrete plug, used to close the junction between the transport tunnel and the disposal tunnel, is made of self-compacting concrete. Because the high pH solution in normal concrete reduces the performance of bentonite, low-pH self-compacting concrete with pH value less than 11 must be used to manufacture concrete plug. The silica fume (SF) and fly ash (FA) were used as mineral admixtures to discuss the effect of the mineral admixture addition level, Increased use of cementitious materials, and w/p on the engineering properties of Low-pH Self-Compacting Concrete (LPSCC). The test result shows that adding SF and FA could effectively prolong the setting time. Using only SF, the initial setting time of the mix increased up to 4.2 h with the addition of 40 wt% SF, while the combination of 40 wt% SF + 10 wt% FA could also increase the initial setting time to 3.92 h, which was much longer than 2 h working time requirement. Furthermore, at 90 days of curing, the pH value of each mixture was lower than 11, while the compressive strength enhanced to 118–132% compared to the control group. This addition could increase compactness, which leads to relatively strong sulfate resistance. The compressive strength could be enhanced by increasing the cementitious materials, which in turn, increases the pH and dry shrinkage. The LPSCC would lose strength when the w/p was reduced. The SF reacted with Ca(OH)₂ in the LPSCC to generate a C-S-H gel. It was the primary factor in the drop of pH. The existence of SF and FA improved the pore structure in the matrix, compacting the structure of LPSCC. All that has been stated above are the dominant mechanism for improving the engineering properties. The pH value of mixture B300W29 was only 10.6 at 90 d, and the compressive strength could reach 61.66 MPa. Moreover, its drying shrinkage was about 0.047% at 90 d, and 0.055% at 150 d. Therefore, it should be considered as a potential material for concrete plug. The swelling property of bentonite was tested to validate the LPSCC material. No evident influence on the swelling property of bentonite was found. Replacing the conventional concrete with the designed material in disposal facilities can effectively enhance safety.