Relation of Chemical Properties to the Bloating of Clays

Two conditions necessary for bloating which have long been recognized are (1) that a material must produce a high temperature glassy phase with a viscosity high enough to trap a gas and (2) that some substance must be present that will liberate a gas at a temperature at which a glassy phase has formed. By utilizing a large number of chemical analyses of bloating and nonbloating clays, the author was able to define the limits of bloating on a composition diagram. The “area of bloating” on this diagram showed the desirable compositions of clays which satisfy the first necessary condition for bloating. As a test of the area of bloating, silica and alumina were added to nonbloating clays to give them compositions conforming to paints within this area. When fired these mixtures bloated. Tests showed that of all the accessory minerals of the Decorah shale, hematite, pyrite, and dolomite are the only ones which could dissociate and produce a gas at the proper temperature. Fractions of the shale which apparently contain only illite also bloated. It was discovered that many igneous rocks whose compositions fall within the area of bloating on the composition diagram produced good bloats when ground and cast into briquettes. Aphanitic rocks such as rhyolites, trachytes, and dacites should be investigated as possible raw materials for lightweight aggregate.