Effect of Polycarboxylate Superplasticizers on Mechanical Properties and Modification Mechanism of Ultra-Fine Portland Cement

This study systematically explored the influence of Polycarboxylate Ether (PCE) content on the fluidity, setting time, and compressive and flexural strength of Ultra-fine Portland Cement (SPC) cement-based grout through the external admixture method. The microstructure and evolution of hydration products were analyzed using XRD and SEM to reveal the modification mechanism. The results showed that the optimal PCE content was 0.25% (calculated based on the mass of SPC), at which the fluidity of the grout reached 273 mm, and the initial and final setting times were extended from 130 min and 235 min to 268 min and 310 min, respectively, reflecting significant plasticizing and retarding effects. The mechanical properties were particularly improved, with the compressive strength of the hardened paste at 7 d and 28 d increasing by 28.78% and 37.09%, respectively, and the flexural strength increasing by 11.20% and 14.52%, respectively. Microscopic analysis indicated that PCE optimized particle packing through adsorption–dispersion effects and moderately delayed the early hydration process, promoting the more thorough and uniform growth of hydration products (such as C-S-H gel), thereby generating a denser microstructure. This is the fundamental reason for the improvement in macroscopic properties. This study provides important theoretical and experimental basis for the performance optimization and engineering application of SPC-based grouting materials under low water–cement ratios.