Modification and Solidification of Phosphogypsum-Based Materials for Subgrade Filling

Abstract Phosphogypsum (PG)-based subgrade filling materials can reduce cement usage and environmental risks, but they require improved strength and impurity removal for wider use. Traditional pretreatments such as acid washing or calcination may weaken the material. This study introduces a novel in-situ composite curing modifier composed of polyaluminum chloride (PAC), calcium chloride, and dimethyl silicone oil. To verify generality, we test two raw feedstocks—PG-A (lower initial leaching) and PG-B (higher). At just 0.2% dosage by mass of phosphogypsum, the modifier increased compressive strength by 79% (PG-B: 82.6%) compared to the control group without modifier and significantly reduced environmental risks by immobilizing free phosphorus and fluoride, lowering their leaching from 1.000 to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mo form="prefix">&lt;</mml:mo> <mml:mn>0.035</mml:mn> <mml:mtext> </mml:mtext> <mml:mi>mg</mml:mi> <mml:mo stretchy="false">/</mml:mo> <mml:mi mathvariant="normal">L</mml:mi> </mml:mrow> </mml:math> (PG-B: 39.510 to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mn>0.049</mml:mn> <mml:mtext> </mml:mtext> <mml:mi>mg</mml:mi> <mml:mo stretchy="false">/</mml:mo> <mml:mi mathvariant="normal">L</mml:mi> </mml:mrow> </mml:math> ), and from 16.850 to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mn>3.100</mml:mn> <mml:mtext> </mml:mtext> <mml:mi>mg</mml:mi> <mml:mo stretchy="false">/</mml:mo> <mml:mi mathvariant="normal">L</mml:mi> </mml:mrow> </mml:math> (PG-B: 139.200 to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mn>33.530</mml:mn> <mml:mtext> </mml:mtext> <mml:mi>mg</mml:mi> <mml:mo stretchy="false">/</mml:mo> <mml:mi mathvariant="normal">L</mml:mi> </mml:mrow> </mml:math> ), respectively, after 14 days. The modifier also optimized hydration kinetics and pore structure. SEM-EDS and XRD (X-ray diffraction) analyses showed that strength improvement was due to flocculation from the modifier, rather than new hydration products. This approach offers an efficient and eco-friendly way to enhance the performance and resource use of phosphogypsum, paving the way for broader future in subgrade filling.