Retarders for Concrete and Their Effects on Setting Time and Shrinkage : Interim Report

Abstract

Sixty-five pure chemicals and three proprietary retarders were tested in the penetration test for setting time at concentrations at or around 0.1% of the cement. The most effective retarders had several, closely-grouped oxygen atoms in hydroxyl, carboxyl , or carbonyl groups.Some of the retarders were used in the fabrication of 0.2 x 0.2 x 4-in. bars of cement paste, which were subsequently oven-dried and the shrinkage, non-evaporable water content, and specific surface of the paste was determined. The retarders caused a change in the shrinkage that was paralleled by a change in the specific surface of the paste.The contents do not necessarily reflect the official views or policies of the Federal Highway Administration. This report does not constitute a standard, specification, or regulation. Determinations of specific surface area, non-evaporable and evaporable water contents, and scanning electron microscopic observation were made on cement paste samples to explore whether or not some changes were induced in the cement hydrates by the addition of retarders.The following are main findings in this research.

Effects of selected retarders on workability window and other properties of cement-based sprayed mortars

“…The exact mechanism of their action when introduced to the hydrating cement is not completely known, but it is believed that a combination of several processes (such as formation of diffusion barrier on a surface of cement grains, "poisoning" portlandite (CH) nuclei, complexation of RET with surrounding ions etc.) is induced and that a major role play Ca 2+ ions released in the pore solution [10][11][12].…”