Recycling : Reduce, Reuse and Recycle

A new type of material PAdeCS®, derived from concrete sludge has been developed and its removal performances for heavy metals and toxic anions such as arsenate, boron were examined. Concrete sludge is waste of fresh concrete, which is rich in calcium and strong alkaline. Concrete sludge is generated when unused concrete is returned from construction sites or excess concrete in the concrete-using industries. In Japan, the annual emission rate of concrete sludge is as large as several millions metric tons and it requires an acid treatment to neutralize the alkaline components before disposal. We have developed a method for manipulating concrete sludge to prepare a solid material through partial removal of calcium contents to avoid hardening of concrete. After solid-liquid separation, the solid residue was obtained, which is still rich in calcium and alkaline. PAdeCS®, so prepared have versatile usage including heavy metal removal and toxic anion removal from waste water.

In addition, calcium and alkaline in PAdeCS® would form hydroxyapatite (HAP) when mixed with dissolved phosphorus ions. Thus, PAdeCS® can be used for phosphorous recovery process from sewage stream. We first tested the removal performance of boron and fluorine in water. Since PAdeCS® is derived from hydrated cement, it would contain a certain amount of ettringite, a crystalline material with the chemical formula, Ca6Al2 (SO4)3(OH)12•26H2O of which sulfate ions have anion exchange capacity with ambient anions. The experimental results showed that boron as well as fluorine in water can be removed efficiently. In addition, removal of heavy metals and arsenate in water was examined and showed excellent removal performances. Thus PAdeCS® is a versatile solid material for environmental-related applications.