title: Chemical vs. Physical Acceleration of Cement Hydration.
abstract: Cold weather concreting often requires the use of chemical accelerators to speed up the hydration reactions of the cement, so that setting and early-age strength development will occur in a timely manner. While calcium chloride (dihydrate - CaCl2·2H2O) is the most commonly used chemical accelerator, recent research using fine limestone powders has indicated their high proficiency for physically accelerating early-age hydration and reducing setting times. This paper presents a comparative study of the efficiency of these two approaches in accelerating hydration (as assessed via isothermal calorimetry), reducing setting times (Vicat needle), and increasing early-age mortar cube strength (1 d and 7 d). Both the CaCl2and the fine limestone powder are used to replace a portion of the finest sand in the mortar mixtures, while keeping both the water-to-cement ratio and volume fractions of water and cement constant. Studies are conducted at 73.4 °F (23°C) and 50 °F (10 °C), so that activation energies can be estimated for the hydration and setting processes. Because the mechanisms of acceleration of the CaCl2and limestone powder are different, a hybrid mixture with 1 % CaCl2and 20 % limestone powder (by mass of cement) is also investigated. Both technologies are found to be viable options for reducing setting times and increasing early-age strengths, and it is hoped that concrete producers and contractors will consider the addition of fine limestone powder to their toolbox of techniques for assuring performance in cold weather and other concreting conditions where acceleration may be needed.
authors.affiliation: National Institute of Standards and Technology, Materials and Structural Systems Division, 100 Bureau Drive, Stop 8615, Gaithersburg, MD 20899 USA.
authors.forename: Dale P
authors.initials: DP
authors.lastname: Bentz
authors.affiliation: National Institute of Standards and Technology, Materials and Structural Systems Division, 100 Bureau Drive, Stop 8615, Gaithersburg, MD 20899 USA; Pontificia Universidad Católica de Chile, School of Engineering, Department of Construction Engineering and Management, Vicuña Mackenna 4860, Macul, Santiago, CHILE.
authors.forename: Franco
authors.initials: F
authors.lastname: Zunino
authors.affiliation: Sika Technology AG- Central Research, Tueffenwies 16 CH-8048, Zurich, SWITZERLAND.
authors.forename: Didier
authors.initials: D
authors.lastname: Lootens
keywords: Acceleration; calcium chloride; cement hydration; early-age strength; limestone powder; setting time