Accelerating admixtures for concrete. State of the art

Abstract

Accelerating admixtures affect the rates of reactions between cement and water to give an overall increase in the hydration rate. Thus, the use of accelerators in concrete provides a shortening of setting time and/or an increase in early strength development. This report gives a historical and technical overview of liquid accelerators used for concrete, their mode of actions and the characteristics oftoday’s commercial accelerators. Accelerators for both ‘normal’ and sprayed concrete are covered. Antifreeze admixtures for winter concreting are also discussed. Calcium chloride, the beste accelerator ever, is no longer an option due to severe corrosion hazzards in steel reinforced concrete. Inorganic salts of nitrate (for setting) and thiocyanate (for hardening) are common alternatives, although they are less efficient than calcium chloride. Many commercial admixtures today are made of mixtures of nitrate and thiocyanate. Some also contain alkanolamines and carboxylic acids or their salts. Combinations of chemicals may give interesting synergistic effects. Alkali-free shotcrete accelerators are based on aluminum sulphate as main ingredient, often combined with diethanolamine and organic acids (oxalic, formic) and/or inorganic acids (phosphorous, hydrofluoric).