Development of a rapid-hardening cement for low-shrinkage foamed concrete production: Synergistic activation of Na2SO4/Al₂(SO₄)₃ for calcined coal gangue

Using ordinary Portland cement (OPC) in foamed concrete production prolongs the demolding period and increases the risk of cracking, owing to its slow setting, low early strength, and high drying shrinkage. In this paper, a rapid-hardening, low-clinker cement was developed by incorporating 35 wt% calcined coal gangue (CCG) and 60 wt% clinker, synergistically activated with 3.0 wt% Na₂SO₄ and 2.0 wt% Al₂(SO₄)₃. Foamed concretes of different densities (900, 1100 and 1300 kg/m 3 ) were prepared with this cement, and their setting time, compressive strength, pore structure, and drying shrinkage were tested. An 84.2 % reduction in initial setting time, a 61.9 % improvement in 3-day compressive strength and a 74.9 % reduction in drying shrinkage were obtained for the foamed concrete prepared with the rapid-hardening cement when compared with that of the foamed concrete made with OPC at the same density (1300 kg/m 3 ). This enhancement is attributed to the synergistic activation of Na 2 SO 4 and Al 2 (SO 4 ) 3 , which preferentially accelerates the growth of ettringite along the direction normal to the (0001) growth face, forming abundant ettringite with a large aspect ratio within the cement hydrates. Additionally, the shortening of the setting time improves the stability of bubbles in the slurry, thereby reducing the pore size and the number of interconnected pores in the foamed concrete. This study introduces a rapid-hardening cement for foamed concrete production and offers an alternative technique for coal gangue recycling. • A novel rapid-hardening cement with a low clinker content was developed. • The cement significantly lowers the drying shrinkage of its foamed concrete. • The cement optimizes the pore structure of the resulting foam concrete. • The hydration mechanism of the rapid-hardening cement was investigated. • The ettringite formed in hydrates is characterized by a large aspect ratio.