Efflorescence Resulting from Pyrite in Clay Raw Materials

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Abstract
In opening this investigation on the effect of pyrite on the efflorescence of structural clay products, the oxidation behavior of this sulfide was studied. The slow oxidation rate and the evolution of 2 moles of sulfur dioxide for every mole of pyrite present combined to make this impurity the major source of efflorescing salts. The retention of part of the sulfur dioxide by the clay material and the subsequent efflorescence tendency were found to depend on the composition, on the concentration of sulfurous gases, on the temperature of firing, on the time of firing, and on the duration of exposure to sulfur dioxide in the manufacturing process. The conditions necessary to reduce efflorescence from this source are described. The use of ammonium chloride to assist in the control of efflorescence was studied in detail.