Determination of limestone and gypsum contents in cements

Appropriate amounts of limestone and gypsum in cement are crucial because they control aluminate reactions, sulfate balance, hydration kinetics, and ultimately setting behavior and performance of the cementitious system. However, their amounts are not always disclosed in product information. This work employed different analytical techniques, including quantitative x-ray diffraction (QXRD), x-ray fluorescence (XRF), loss on ignition (LOI), and thermogravimetric analysis (TGA), to determine the unreported amounts of limestone and gypsum in five cements collected from different regions in the U.S. The advantages and disadvantages of each technique for the work are discussed. Equations and correction factors to account for assumptions are proposed to refine the estimated values from these techniques. Estimation of calcite and limestone is relatively straightforward using all methods. XRF and TGA were not able to properly estimate gypsum content since dehydrated forms of calcium sulfate are usually present; but QXRD is appropriate for gypsum estimation.