Control the early-stage hydration of expansive additive from calcium sulfoaluminate clinker by polymer encapsulation

Abstract

Ye′elimite (C₄A₃$\overline{S}$), a main compound in calcium sulfoaluminate (CSA) clinker, is an important ingredient as expansive additive in shrinkage compensating cement. This study proposes to modify the expansive additive by encapsulating it with polyethylene glycol (PEG). The polymer provides a matrix structure, in which the ye′elimite particles are embedded. When the modified expansive additive come into contact with water, the polymer matrix acts as a water barrier, but can dissolve away. This slowly exposed C₄A₃$\overline{S}$ to hydration, resulting in gradual early-stage ettringite formation; hence control early expansion in expansive cement. The study compared the ettringite formation between the unmodified and the modified expansive additive using thermogravimetric analysis (TGA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) from 1 hour to 3 days. The results show that the unmodified expansive additive generated more ettringite than the modified ones at the same hydration time. The study subsequently investigated the mortar properties with the unmodified and modified expansive additives admixtures. The results showed that the modified expansive cement showed superior flowability and drying shrinkage behaviours, while the compressive strength of the finished products underperformed that of the untreated expansive additives.