Optimizing a Polycarboxylate Superplasticizer Molecular Structure to Reduce Apparent Viscosity of Cement Paste at Higher Environmental Temperatures

A high environmental temperature can increase the apparent viscosity of high-performance concrete mixtures, causing difficulties in mixing, transporting, pumping, and molding the mixtures. Although polycarboxylate ether–based superplasticizers (PCEs) have been proven effective in reducing the apparent viscosity of cement pastes, little work has focused on how PCEs with various molecular structures affect the apparent viscosity of high-performance concrete mixtures when environmental temperature increases. In this study, a PCE (T54C3.5), which was synthesized by reacting acrylic acid (AA) and isoprenyloxypoly (ethylene glycol) ether macromonomer (TPEG) ($m_w=\mathrm{2,400}\mathrm{g}/\mathrm{mol}$) with a ratio equal to $3.5:1$, was used as the control group. Variations on the molecular structure of T54C3.5, including different carboxylate-to-macromonomer ratios, backbone lengths, backbone compositions, side-chain lengths, and side-chain compositions, were made to investigate the effectiveness of the various PCEs in changing the apparent viscosity of an ordinary portland cement paste with the water-to-cement ratio of 0.23 at different environmental temperatures. A rotational viscometer was used to measure the apparent viscosity of cement pastes containing different PCEs, and the working mechanisms for the effective PCEs were discussed. Results showed that reducing the backbone length was more effective than the other variations in improving the viscosity-reducing ability of PCEs at all tested temperatures. The PCEs containing ester bonds were able to retain or slightly decrease the viscosity of their cement pastes when the environment temperature increased from 20°C to 40°C. Reducing the backbone length and introducing ester groups are two effective ways of improving the viscosity-reducing ability of PCEs at a high environmental temperature. Using PCEs with these structures can significantly simplify the manufacturing processes of the cement mixtures with a low water-to-cement ratio when the environmental temperature increases from 20°C to 40°C, without any special mixing or casting method.

PCEs are the most popular concrete admixtures due to their excellent performance and devisable molecular structures. Using a suitable PCE is beneficial to the quality of concrete structures, especially for high-performance concrete (HPC) structures. During the manufacture of concrete, excess water is used to increase workability but can cause strength loss. The use of PCEs can significantly decrease the water demand of concrete, thus promoting the development of HPC, which has to have low water content. A high environmental temperature greater than 28°C can accelerate cement hydration, thus increasing the viscosity and adding more difficulties to mixing, transporting, pumping, and molding processes. Because the molecular structure of PCEs can be designed to meet different needs, it is of great significance to explore which type of the molecular structure is more suitable for cement mixtures with a low water-to-cement ratio at a high environmental temperature. To achieve this objective, the molecular structure of PCEs was varied to determine the PCEs’ effects on the apparent viscosity of the cement paste having a low water content at different temperatures. The PCEs with short backbones or a high content of ester bonds were found to satisfy the objective.