This change in its physical properties is due to the chemical reaction between cement and water, a process known as hydration. Hydration involves chemical changes, not just a drying out of the material. Hydration is irreversible. The reaction is gradual, first causing stiffening of the concrete, and then development of strength, which continues for a very long time. Under certain ideal conditions it is probable that concrete would continue to increase in strength indefinitely.

Air temperature, ground temperature and weather conditions all play major roles in the rate with which cement hydrates. The setting time of concrete decreases with a rise in temperature, but above 30^oC a reverse effect could be observed. At low temperatures setting time is retarded. Proper curing techniques and site preparation will aid the setting time. However when concrete is being used in times of temperature extremes, i.e. colder weather or in the middle of summer, several admixtures may be used in the concrete mix to aid in the placement of the final product. These admixtures are Accelerators and Retarders.

Accelerators have been designed to significantly boost the early setting times and increase the early age strengths of concrete. Setting times of non accelerated concrete are significantly slower as temperatures get colder. This obviously affects finishing times. The action of the accelerator counters this set retardation and shortens setting times back to what is considered a normal set time.

Retarders are designed for use in areas where early setting of concrete is not preferable, e.g. high ambient temperatures, long travel times between concrete plant and job site, large slow pours – to prevent formation of cold joints etc. The chemical composition of the retarder is formulated to temporarily stop the action of hydration, delaying the initial set of the concrete. This delay is proportional to the dose of retarder used. Once the effect of the retarder wears off initial set will take place and hardening will develop at an accelerated rate.

The two graphs below demonstrate the effects of accelerators and retarders on the setting time of concrete.

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DISCLAIMER: Allied Concrete has endeavoured to present the best possible information. However we accept no responsibility for the application of the principles discussed.