Producing environment-conscious building materials from contaminated phosphogypsum

Abstract

It is known that dozens millions tones of contaminated phosphogypsum (by-product of fertilizers industry) in Morocco, Togo, USA, Canada, Russia, Korea, China, India, Israel, Jordan and other countries are disposed annually. The review of the suggested processes of purification shows that no effective technologies are known for phosphogypsum processing and utilization in construction industry. The central problem of phosphogypsum is slightly elevated radioactivity of the phosphogypsum (radium content is usually 16-22 pCi/g), because other chemical impurities can be extracted relatively easy, for example, by using phase transformations between different kinds of calcium sulfate hydrate and filtering the solution. Traditional technologies of purification of phosphogypsum from radium are not effective, because of the similarity of chemical properties of radium sulfate and calcium sulfate, when the contaminant salt is isomorphously introduced in the gypsum crystals. The authors suggest a principally new technological approach to purify phosphogypsum from radioactive and other contaminants, which make possible producing environment-conscious building binders. As preliminary laboratory tests showed, the radium content in phosphogypsum decreased from 20 to 0.4 pCi/g, and from gray contaminated phosphogypsum having 1.5% of P2O5 and 0.7% of sulfate/phosphate/silicofluoride salts of number of metals pure gypsum of white color containing the traces only of these impurities and of P2O5 was obtained. The economical, geographical and environmental aspects of processing phosphogypsum for environment-conscious construction are discussed. 1. A problem of utilization of phosphogypsum in construction One of the important features of the modern trends in engineering sciences is the destruction of the barriers between separate fields of engineering science and creation of new technologies in their interfaces. An accumulation of huge stocks of industrial by-products in developed countries plays the role of catalyzer of this process. Israel has recently joined the countries accumulating a significant amount of industrial by-products, including those containing slightly elevated amounts of radioactive impurities. The most known example is phosphogypsum by-product from phosphorous fertilizers production. This by-product is usually stockpiled within the special areas. The problem of phosphogypsum will become soon an international ecological problem, because a huge amount of it are accumulated in Florida, USA (more than 1 milliard (!) tons), in Europe, where the contaminated phosphogypsum is discharged into the Rhine river close to the North Sea, and in other parts of the world. Many valuable materials are lost in this by-product. Moreover, it must be emphasized that many of them will serve future generations as good raw materials in producing electrical energy, chemical, building, and other useful products. The building materials industry seems to be the largest among all the industries, which is able to reprocess the greatest amount of this industrial by-product and benefit man. However, the key problem restraining the utilization of phosphogypsum in construction is radiological effect on the human population. 2. Radiological constraints of using phosphogypsum and other industrial by-products in construction As can be seen from Table 1 adopted from [1], radioactivity concentrations found in phosphogypsum and in some other industrial by-products are often significantly higher in comparison with most common building materials. Table 1 : Typical and maximum activity concentrations in common building materials and industrial by-products used for building materials in the EU Typical activity concentration (Bq/kg) Radium equivalent Maximum activity concentration (Bq/kg) Radium equivalent Material Ra Th K Bq/kg pCi/g Ra Th K Bq/kg pCi/g Most common building materials (may include by-products) Concrete 40 30 400 114 3.1 240 19