The determination of low levels of the quartz form of crystalline silica in ferrous slags and silica fume samples was carried out using differential scanning calorimetry (DSC). Such quantitative information is required for the classification of these and other cementitious materials according to the Workplace Hazardous Materials Information System (WHMIS) and other environmental as well as health and safety provisions. The International Agency for Research on Cancer (IARC) now requires Materials Safety Data Sheets for products containing 0.1% or more crystalline silica. A calibration curve correlating the quartz content with the height of the endothermic peak of the alpha/beta quartz polymorphic transformation was constructed using the standard addition method. The technique and the methodology were found to be reliable and convenient, and enabled the detection of quartz levels below 0.1% in the samples.