Synthesis, Performance, and Accelerating Mechanism of a Polymerized Aluminum Accelerator with Alkali-Free, Chlorine-Free, and Fluorine-Free toward Cement-Based Materials

Accelerators that can significantly shorten the hardening time of sprayed cementitious composites are indispensable admixtures. However, traditional accelerators inevitably reduce the durability of sprayed cementitious composites and damage the environment because certain detrimental elements are used as primary components of accelerators, including alkali, fluorine, and chlorine. A novel polymeric aluminum-based liquid accelerator (PAA) with alkali-free, chlorine-free, and fluorine-free properties for cement-based materials was synthesized by an organic-inorganic hybrid method. The initial setting time of cement paste containing 5 wt % PAA was only 4 min, and the hardening interval between plasticity and solid phase can be shortened to 30 s. Meanwhile, the compressive strength and flexural strength of cementitious composites at 1, 7, and 28 days were 129.43, 112.03, 104.16, 108.43, 147.02, and 107.45% of the reference group, reaching 11.48, 32.44, 45.24, 3.22, 6.73, and 7.24 MPa, respectively. The transformation of aluminum sulfate with highly symmetric hexacoordinated aluminum into Keggin-type Al₁₃ polymers characterized by tetracoordinated aluminum and aluminum-oxygen bonds is the classic feature of PAA. The PAA, characterized by high positive charge and abundant hydroxyl groups, can work as nucleation sites to enrich Ca²⁺ and SO₄²⁻. This significantly accelerates the nucleation of AFt crystals, thereby facilitating the rapid setting and enhancing the mechanical performance of the cementitious composites.