Non-adsorbing small molecules as auxiliary dispersants for polycarboxylate superplasticizers

[Display omitted] •Non-ionic molecules improve cement paste fluidity in combination with MPEG-PCEs.•Their performance as auxiliary dispersant is particularly strong at low w/c ratios.•Molecules with a low molecular weight and high content of non-polar groups are very powerful.•Non-ionic co-dispersants act as spacer molecules keeping the cement particles apart.•Portion of non-adsorbed components becomes crucial for the stabilization at low w/c. The dispersing ability of polycarboxylate superplasticizers (PCEs) can be ascribed to the electrostatic and steric stabilization of cement suspensions. For this purpose, PCEs need to adsorb on the cement particle surface to become effective at all. In this study, it is demonstrated that at low water to cement ratios ≤ 0.30, even non-ionic molecules like diethylene glycol or 2-methyl-2,4-pentanediol which do not adsorb on cement, but remain dissolved in the interstitial pore solution, can greatly enhance cement dispersion when combined with PCEs such as conventional methacrylate ester based comb co-polymers. Their effect as co-dispersant is particularly pronounced for PCEs possessing a low side chain density and long side chain length. Relative to the co-dispersants it was found that especially non-polar small molecules like neopentyl glycol greatly enhance the paste fluidity. These molecules significantly reduce the surface tension of the pore solution and thus increase the wettability of cement. Based on results from spread flow tests, adsorption measurements and pendant drop tensiometry it is concluded that in cementitious systems formulated at low w/c ratios, non-adsorbing molecules with a molecular weight of ≤ 1000 g/mol induce repulsive depletion forces which prevent cement particles from agglomeration. This way, such small molecules act as auxiliary or co-dispersant when combined with PCEs.