Influence of calcium formate dosing on early mechanical properties and microstructure of SS-GBFS-DG cementitious materials

Abstract

Steel Slag-Granulated Blast Furnace Slag-Desulfurized Gypsum (SS-GBFS-DG)-based cementitious materials frequently exhibit low early compressive strength. This study examines the impact of calcium formate (CF) on compressive strength, pore distribution, microstructure, and hydration kinetics of these materials. The alkalinity, hydration activity, pore characteristics, and hydration process of (SS-GBFS-DG) in the presence of CF were both qualitatively and quantitatively characterized and evaluated using X-ray fluorescence (XRF), X-ray diffraction (XRD), thermogravimetric-differential thermal analysis (TG-DTA), infrared spectroscopy (IR), mercury intrusion porosimetry (MIP), and heat of hydration analyses, respectively. Results demonstrate that CF significantly improves early compressive strength. Specifically, the 1-day compressive strength of the CC3 group increased by 156 %, reaching 18.81 MPa, whereas the 28-day compressive strength rose by 19.55 %, reaching 75.79 MPa. Microanalytical techniques revealed that CF promotes early hydration reactions in the CC3 and CC5 groups. The addition of CF shortened the induction period by nearly 7 hours, increased the exothermic rate of hydration, improved the formation rate of hydration products such as C-S-H gel and ettringite, accelerated the dissolution of Al³ ⁺, facilitated the breakdown of GBFS, reduced the equilibrium pH, and enhanced the overall hydration process. In the CC3 group, the proportion of pores smaller than 20 nm increased by 9.1 %, whereas the proportion of 20–50 nm less-hazardous pores decreased by 4.9 %, 50–200 nm harmful pores decreased by 3.7 %, and pores larger than 200 nm decreased by 0.5 %. Furthermore, CF improved the Si/Al ratio of C-A-S-H gels. The Si/Al ratios in C-S-H gels at 1 day ranged from 1.5 to 2.0, 1.8–2.3, and 2.0–2.9 for the CC0, CC3, and CC5 groups, respectively. The aluminum released from the breakdown of GBFS was primarily incorporated into ettringite crystals. This study aims to enhance the theoretical framework and control index system for calcium formate in (SS-GBFS-DG) cementitious materials.