Polycarboxylate superplasticizer and viscosity modifying agent: Mode of addition and its effect on cement paste rheology using image analysis

Abstract

The necessity for an increased dosage of polycarboxylate-ether-based (PCE) superplasticizer in conventional and self-compacting concrete (SCC) is inevitable in hot environments. However, the increased PCE dosage leads mostly to initial instability and segregation of the SCC mixture, which mandates high precautions. Under such conditions, the use of a viscosity-modifying agent (VMA) is highly recommended. Because of that, the possible addition modes of PCE and VMA were investigated in SCC cement pastes with two water-to-cement (W/C) ratios of 0.3 and 0.4 using rheological measurements and image analysis. The mode of action of PCE and VMA with cement is shown to rely on the sequence of their mutual additions. Namely, either PCE or VMA first, followed by the other, or add both together at the same time. The effects of mode of addition on mini-slump flow and the rheological properties of cement pastes, represented by yield stress and plastic viscosity, were evaluated. It is proven that the critical PCE dosage has the maximum power to expel water from both cement paste and VMA-cement paste complex when added to their fresh mixtures. The influence of the critical dosage could be identified using image analysis as a supporting technique, which marks the segregation layer in red as an indicator for segregation regardless of the nature of casting molds. For each W/C ratio, there is an optimum PCE/VMA ratio at which maximum stability with enhanced fresh properties can be obtained. Two opposing mechanisms were also recognized as being water repellent and water attractive. Finally, it is concluded that the net mode of action represents the average effect of both mechanisms as visualized. To that end, mathematical models could be derived and verified.