Title: Performance of Calcium Nitrate as Accelerator for Cement Blended with Blast Furnace Slag

Author(s): Harald Justnes, Tobias Danner, Elise Marie Rong Anfinsen, Aleksandra Marie Høye, Margrethe Munch-Ellingsen, and Mehrdad Torabzadegan

Publication: Symposium Paper

Volume: 362

Issue:

Appears on pages(s): 331-343

Keywords: accelerator, blastfurnace slag, calcium nitrate, calcium hydroxide, compressive strength, ettringite, isothermal calorimetry, thermogravimetry, X-ray diffraction

DOI: 10.14359/51740894

Date: 6/6/2024

Abstract:
The objective is to elucidate the effect of calcium nitrate CN on cement replaced with 50% blast furnace slag (ggbs). The influence of 2 and 4% CN accelerator on hydration development of these binders was investigated by isothermal calorimetry, X-ray diffraction, and thermogravimetry. The strength development of mortar with two different ggbs blended binders cured at 20°C was followed and the influence of CN was discussed. Concrete with 50% of a third ggbs, 0, 2, and 4% CN and w/c = 0.46 was cured at +5°C and compressive strength development was measured. CN led to lower mortar strength at 1 day at 20°C, but higher strength from 3 days onwards. Mortar with two different ggbs achieved the same strength at 7 and 28 days when added 4% CN. However, CN increased the 1-day strength of concrete cured at +5°C. Since ggbs is accelerated by high pH, the somewhat lower 1-day strength in mixes with CN is probably caused by the immediate precipitation of hydroxyl ions by calcium to portlandite leading to a lowering of the pH. CN led to more ettringite formation and AFm phases at 1-day sealed curing and the calcium hydroxide content was reduced. Reasons for calcium hydroxide reduction are discussed.