Designing high early-strength one-part hybrid cement mortar via incorporating highly-reactive artificial slag and pre-treated gypsum

Abstract

The study focused on developing a high-early-strength hybrid cement mortar using a low binder content of only 10 vol% OPC, supplemented by artificial slag (AS) and finely ground limestone powder (LP). The key objective was achieved with incorporating sodium hydroxide-treated gypsum waste (GW), designated as DAN (dry activator, Na-based). The inclusion of DAN (at 2 wt% sodium oxide equivalent: Na₂Oeq) significantly accelerated early hydration of the hardened mortar, yielding a remarkable 1-day compressive strength of 31.9 ± 1.6 MPa, nearly double that of the control without an activator. However, it concurrently negatively impacted one-year compressive strength, which is linked to an increase in pore diameter and the predicted inhibition of straetlingite formation, as indicated by thermodynamic modeling. Micro-computed tomography (micro-CT) and three-dimensional (3D) visualization revealed that this reduction is associated with clusters of low-density hydration products that formed early on and remain in the microstructure at later ages. In contrast, incorporating untreated GyW resulted in a lower early hydration acceleration rate but significantly enhanced the one-year compressive strength to 67.5 ± 0.8 MPa, surpassing that of DAN-containing mixtures (56.6 ± 1.8 MPa). This investigation elucidates a critical performance dichotomy, whereby untreated GyW emerges as a viable material for achieving superior long-term strength, while the treated GyW is demonstrated to enhance 1-day mechanical performance.