Use of a Supplemental Agent to Improve Flowability of Ultra-High- Performance Concrete

摘要

Ultra-high-performance concrete (UHPC) possesses a very low water-cement ratio (< 0.25). Additionally, a large amount of fines, such as silica fume, are used to achieve optimum packing density. Because of its specific surface chemistry and higher surface area, silica fume is more difficult to disperse than cement. Previously, it was found that methacrylic acid-MPEG methacrylate ester type PCEs disperse cement effectively whereas allylether-maleic anhydride-based PCEs work better with silica fume. Apparently, PCEs with different molecular architectures are required to achieve optimum coverage of the different surfaces of cement and silica fume. Thus, a blend of methacrylate- and allylether-based PCEs used at approx. 0.5% by weight of cement is more effective than when they are utilized individually.To further enhance the performance of the formulation, sodium gluconate was introduced as a "supplemental" agent. The combination of PCE with gluconate allowed a reduction of approximately 50% in the dosage of PCE. The final blend contained 0.28% of allylether-based PCE and 0.10% of gluconate by weight of cement.A mechanistic study established that sodium gluconate adsorbs very strong on cement and to a less extent also on silica fume, whereas the allylether PCE almost exclusively adsorbs on the silica surface. Thus, the surface of cement is covered by gluconate molecules whereas the silica surface shows concomitant adsorption of both PCE and sodium gluconate molecules. The small gluconate molecules fill the space between the huge PCE molecules on the silica fume surface.