Technical calcium nitrate as set accelerator for cement pastes at low temperatures

Technical calcium nitrate (CN) is tested as a set accelerator for five French cements by Vicat needle and isothermal calorimetry at both 5°C and 20°C. All five cements were characterized by standard test methods and X-ray diffraction. CN works as a set accelerator for all five cements tested (but poorly for cement no. 2). The CN dosages tested at 5°C were 0·00%, 1·55%, 3·86% and 7·73% of the cement weight. The optimum dosage was found to be 1·55%, 1·55%, 3·86%, 3·86% and 1·55% CN for cements nos 1, 2, 3, 4 and 5, respectively. However, optimum CN dosages less than 1·55% cannot be excluded for cements nos 1, 2 and 5. The accelerating efficiency of CN is higher at 5°C than at 20°C. In addition to shortening the overall setting time, the CN accelerator often reduced the time gap between initial and final set, indicating a more rapid strength build-up. Isothermal calorimetry revealed a good correlation between the extra heat output during the first five hours and the effectiveness of 1·55% CN (fixed dosage) in accelerating initial set at 5°C. The efficiency of CN as set accelerator for the different cements seems to increase with increasing β-C_sS, as partly confirmed by X-ray diffraction, or other cement characteristics promoting belire formation in the clinker process.