Influence of superplasticizer dosage on the viscosity of cement paste with low water-binder ratio

•Underlying mechanism of SP dosage on viscosity of cement pastes is proposed.•A high concentration of un-adsorbed SP is found in interstitial solution of the paste.•Hydroclustering and SP entanglements could be induced in paste with low w/b. In this paper, the influence and underlying mechanism of superplasticizers (SP) dosage on the viscosity of cement pastes with four water-binder ratio (w/b) were investigated. The results showed that apparent viscosity of the pastes with w/b of 0.24 and 0.32 decreased with SP dosage. Whereas, it is reverse for cement paste with w/b of 0.20 and 0.16. The addition of SP increased the packing density and the water film thickness of pastes with a w/b of 0.32 and 0.24. However, the increase of SP dosage had little effect on the packing density and the water film thickness of pastes with a very low w/b (0.16). For the cement pastes with a very low w/b (e.g., 0.16), the small spaces between the binder particles and the high concentration of the un-adsorbed SP in the interstitial solution may be the primary factors responsible for the increase in viscosity of the pastes.