Admixtures for recycling of waste concrete

Recent developments in admixture technology have made significant progress to control the hydration of cement. This control has enabled concrete producers and users to stop cement hydration for a desired period and be able to restart it at any time, allowing the concrete to set normally, without sacrificing any of the properties of the hardened material. These types of admixtures, defined as extended-set control admixtures,' not only have a significant influence on the production, transportation, and placement of concrete, but have also had a positive impact on the environment. With this technology, in fact, if is possible to eliminate or greatly reduce the amount of waste in the production and use of concrete. The control of the hydration is currently achieved by the use of two admixtures, one defined as 'Stabilizer' is capable of stopping or stabilizing the hydration of cement, when it is mixed in the concrete, while the second named 'Activator,' is used to restart or activate the hydration. The extended set-control admixtures were originally developed for overnight and weekend stabilization of returned plastic concrete, for the long-haul stabilization of concrete that has to be placed at long distances from the batching plant and for overnight or weekend stabilization of concrete wash water from truck drums. The use of this system for the above mentioned applications allows a sensible reduction, or even an elimination of the waste in the production and use of concrete, and as a consequence a cost reduction for the producers and users of concrete. Other potential uses induce in Roller Compacted Concrete (RCC) applications and in shotcrete. This paper deals with an overview of the chemistry and mechanisms of action of these admixtures, their main practical uses, the effects of the use of extended set-control admixtures on the properties and durability of concrete in comparison with reference concrete, and with an overview of some other applications where this system has been successfully used. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.