The project of “Study on the application of phosphogypsum in roadbase” by Hubei Changyao New Material Co., Ltd (Changyao) won the second prize of 2020 CCPA Concrete Science and Technology Award (Technology Innovation).

Phosphogypsum is the industrial solid waste produced from the manufacturing process of phosphoric acid. There will be 5 tons of phosphogypsum in the manufacturing process of 1 ton phosphoric acid. The main chemical composition of phosphogypsum is dihydrate gypsum (CaSO₄•2H₂O) with a small amount of phosphoric acid, calcium phosphate, residual acid, soluble P₂O₅, fluoride, heavy metals, organic matter and other impurities, usually in the form of white or dark gray moist fine powder. Around the world, only 15% of phosphogypsum has been recycled for building materials, agricultural soil improvement, cement retarder, etc., and the remaining 85% phosphogypsum is stacked as solid waste. In China, 20% of phosphogypsum has been recycled.

In order to improve the value of comprehensive utilization of phosphogypsum and realize a large amount of phosphogypsum consumption, Changyao has carried out the research on phosphogypsum in highway roadbase. A new type of phosphogypsum cement graded crushed stone stabilized material was prepared by adding an appropriate amount of CaO into the milled phosphogypsum for modification and aging treatment, and mixed with slag powder, alkaline activator, crushed stone, stone chips and water.

Application project: Bailinhe Avenue

Application project: Yinlinglenglian Road

The main innovations of the project are as follows.

lMaterial innovation

The cementitious material system consists of phosphogypsum, slag powder and alkaline activator. The phosphogypsum cementitious material is changed from pneumatic cementitious material to hydraulic cementitious material.

lTechnological innovation of solid waste resource utilization

Phosphogypsum accounts for 45% - 55% of the components of phosphogypsum slag cement. Slag powder is also industrial solid waste. Steel slag can be used as alkaline activator together. This technology could comsume three kinds of solid waste.

lEngineering application technology innovation

The 7-day unconfined compressive strength of graded crushed stone layer is increased to 5~9Mpa, which is higher than 2.5MPa of cement stabilized crushed stone layer. The preparation and construction method of phosphogypsum slag graded crushed stone stabilized layer of the project is simple and can be realized without many professional equipments.

Phosphogypsum slag graded crushed stone stabilized layer

Phosphogypsum slag cement stabilized layer on rolling

This research has been applied in three roadway projects in Yichang City. The first is one road of 148 meters long with consumption of 150 tons of phosphogypsum inside of Sanxia Yichang Pipe gallery Construction Co., Ltd. The second is Xiaohuang Road of 200 meters long with consumption of 200 tons of phosphogypsum. The third road is Bailinhe Road I of 2.14 kilometers long with consumption of 2117 tons of phosphogypsum. As semi-rigid roadbase material, phosphogypsum slage cement plays a great role in consuming the stock of phosphogypsum, and has good social benefits. After calculation, the total cost per cubic meter of phosphogypsum slage cement roadbase is lower than that of ordinary cement, and the project has good economic benefits.

Test road laying

Effect of test road after laying of test road