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 including compressive strength, workability and setting time. This study has assessed changes in engineering properties of CEM I Portland cement exposed to relative humidities of 60% or 85% for 7 and 28 days. Thin layers of CEM I 42.5R cement, no more than 2 mm thick, were exposed to controlled relative humidities 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% relative humidity, conventional hydration resulted in a negligible reduction in strength development plus a decrease in workability and increase in setting time. At 85% relative humidity, compressive strength was greatly reduced, particularly at early ages. There was also a greater loss of workability and increase in setting time.