Study on Preparation and Properties of Phosphogypsum-Based Lightweight Thermal Insulation Materials

Abstract

Highlights: What are the main findings? The best overall performance was achieved with a 1:4 binder–bead ratio, water–binder ratio of 1.6, 0.1 wt.% HPMC, and 24 wt.% WAE solid content. This formulation yielded low density and thermal conductivity while maintaining adequate strength. Adding 8% sulfate aluminate cement improved microstructure densification via interlaced ettringite and gypsum crystals, increasing compressive and flexural strengths by over 20%. Incorporating 0.9% basalt fibers formed a uniform network that dispersed stress and prevented brittle cracking, raising flexural strength by ~18% and improving ductility. The combination of 2% paraffin emulsion and spraying 5×-diluted sodium methyl silicate created inner and outer hydrophobic layers. Water absorption decreased by ~23%, and the softening coefficient rose to 0.53, meeting water-resistance requirements. SEM and XRD results revealed that cement hydration products (C–S–H gel, ettringite) filled voids among gypsum crystals, while fibers bridged microcracks—jointly enhancing the integrity and stability of the matrix. What are the implications of the main findings? The research provides a sustainable method to recycle industrial phosphogypsum waste into high-performance building insulation materials, reducing solid-waste accumulation and related pollution. The material’s low thermal conductivity and stable mechanical performance make it a viable alternative to traditional organic insulation products, supporting energy-saving building envelopes. The optimized formulation and modification strategy (SAC + fiber + dual waterproof treatment) offer a practical pathway for large-scale production of durable, fire-resistant, and water-resistant inorganic insulation boards. The study elucidates the coupling mechanism of cement hydration, gypsum crystallization, and fiber reinforcement in composite matrices—providing a theoretical basis for future design of eco-friendly insulation composites. At present, phosphogypsum, as an industrial by-product, is a solid waste in phosphoric acid production, and its accumulation has caused serious environmental pollution. Furthermore, due to the insufficient insulation properties of traditional wall materials, the issue of a rising proportion of building energy consumption in total social energy consumption has become increasingly pressing. The study investigated vitrified beads as a light aggregate and phosphogypsum, mineral powder, and quicklime as an inorganic composite cementitious system to prepare the phosphogypsum-based lightweight thermal insulation material. The effect mechanism of the initial material ratio on the mechanical properties and micro-morphology of insulation materials was studied by macroscale mechanical property testing, X-ray diffraction, and scanning electron microscopy. Meanwhile, in order to meet the performance indexes specified in relevant standards, insulation materials were modified by adding sulfate aluminate cement, basalt fibers, and a waterproof agent to improve the strength, toughness, and water resistance. Based on the single-factor experimental design, the optimal dosage of various admixtures was obtained. The results indicated that the optimal properties of the sample were achieved when the binder–bead ratio was 1:4, the water–binder ratio was 1.6, the dosage of hydroxypropyl methylcellulose was 0.1%, and the solid content of waterborne acrylic emulsion was 24%. The optimal dosages of cement and fibers were 8% and 0.9%, respectively. The cement hydration products and gypsum crystals lapped through each other, filling the pores in the matrix and increasing the strength of the sample. In addition, the fibers could form a disordered network structure inside the matrix, disperse external force, weaken the stress concentration at the tip of internal cracks, and significantly improve the toughness of the modified sample. By incorporating 2.0% paraffin emulsion in the mortar and spraying 5 dilutions of sodium methyl silicate on the external surface, dense protective layers were formed both inside and outside the modified sample. The water absorption rate reduced from 30.27% to 23.30%, and the water resistance was increased to satisfy the specified requirement for the insulation material.