Slurry Consistency and In Situ Synchrotron X‐Ray Diffraction During the Early Hydration of Portland Cements With Calcium Chloride

Class A and H oil well cements are compared at 25° and 50°C with 0%, 1%, 2%, and 4% CaCl 2 . Up to 4% CaCl 2 accelerated Class A thickening, but 4% led to slower thickening than 2% for Class H. C 3 S hydration in the two cements responded differently to CaCl 2 . CaCl 2 always accelerated aluminate hydration. For Class A, CaCl 2 accelerated early Ca(OH) 2 precipitation, but sometimes reduced the amount at longer times. This may be coupled to C–S–H gel composition changes. For Class H, Ca(OH) 2 precipitation changes nonlinearly with CaCl 2 concentration. Ettringite to monosulfate conversion and Friedel's salt formation were sometimes seen.