The effect of prehydration on the engineering properties of CEM I Portland cement

Prehydration – the reaction between anhydrous cement and water vapour – has deleterious effects on engineering properties such as compressive strength, workability and setting time. This study assessed changes in the engineering properties of CEM I Portland cement exposed to relative humidities (RHs) of 60% or 85% for 7 and 28 days. Thin layers (no more than 2 mm thick) of CEM I 42.5R cement were exposed to controlled RHs of 60% and 85%, followed by assessment of compressive strength, setting time and workability. These measurements were complemented by characterising the prehydrated anhydrous cement using isothermal calorimetry, scanning electron microscopy, X-ray diffraction and thermal gravimetric analysis. Following prehydration at 60% RH, conventional hydration resulted in a negligible reduction in strength development plus a decrease in workability and increase in setting time. At 85% RH, compressive strength was greatly reduced, particularly at early ages. There was also a greater loss of workability and increase in setting time.