The occurrence and distribution of various forms of silica and zeolites in soils developed from wastes of iron production

Young soils developed in blast furnace slag, slag sand and ash are often alkaline. Under these conditions the solubility of silica increases greatly. In four soil profiles investigated (two in France and two in Germany) we found not only that dissolved silicic acid and amorphous silica occurred, but also that zeolites had developed. We identified several soil properties that may influence the formation of amorphous silica and zeolites. In the blast furnace slag, the formation of amorphous silica seemed to be enhanced by decreasing pH and increasing Mg content, while the amount of zeolites grew with increasing contents of water soluble Na and K and with decreasing content of CaCO3. In the slag sand, Mg also appeared to promote the formation of amorphous silica. In addition, there was a weaker correlation between rising CaCO3 content and an increasing amount of amorphous silica. Furthermore, in the slag sand the Sio : Alo ratio decreased with increasing salinity, suggesting that the zeolites were Al enriched. Longer periods of water saturation of the soil, it is suggested, enhance the development of zeolites and inhibit the formation of amorphous silica.