Preferential adsorption of polycarboxylate superplasticizers on cement and silica fume in ultra-high performance concrete (UHPC)

UHPC is fluidized particularly well when a blend of MPEG- and APEG-type PCEs is applied. Here, the mechanism for this behavior was investigated. Testing individual cement and micro silica pastes revealed that the MPEG-PCE disperses cement better than silica whereas the APEG-PCE fluidizes silica particularly well. This behavior is explained by preferential adsorption of APEG-PCE on silica while MPEG-PCEs exhibit a more balanced affinity to both cement and silica. Adsorption data obtained from individual cement and micro silica pastes were compared with those found for the fully formulated UHPC containing a cement/silica blend. In the UHPC formulation, both PCEs still exhibit preferential and selective adsorption similar as was observed for individual cement and silica pastes. Preferential adsorption of PCEs is explained by their different stereochemistry whereby the carboxylate groups have to match with the steric position of calcium ions/atoms situated at the surfaces of cement hydrates or silica.