Solidification of heavy metal elements in waste phosphate acid activated metakaolin geopolymer with environmental and economic benefit

Phosphoric acid-activated geopolymer has become a promising ceramic material due to its superior thermal and mechanical properties. However, the gradual shortage of phosphate resources and the high price of phosphoric acid will be the key factors limiting the wide application of phosphoric acid-activated geopolymers. This paper developed a low-cost and low-carbon geopolymer non-sintered ceramics using industrial waste acid. The compressive strength of the prepared specimens can reach 32.2 MPa. The thermal gravimetric analysis indicated that the mass loss of the geopolymer remained almost unchanged in the temperature range between 400 and 1500 °C, indicating the geopolymer exhibited excellent thermal stability. Additionally, the infiltration test showed that the majority of the heavy metals could be solidified successfully with over 90% solidification efficiency, meaning the leaching of the heavy metals into the environment can be well controlled. Finally, the environmental and economic analysis showed that the carbon intensity (kg/m3/MPa) was reduced by 92.7% and 57.7% compared to cement paste and reported geopolymer. The cost intensity ($/m3/MPa) was negative due to the income of waste acid treatment, meaning that additional value benefit was added to the product. This paves a promising way for producing low-cost and low-carbon ceramic materials with excellent thermal and mechanical properties.