Temperature:
It has been observed that the workability decreases with the increase of temperature because, with the increase in temperature both the rates of hydration and evaporation increases, hence the loss of moisture is high.
Thus, during hot weather concreting it is always preferred to use a few workability agents to manage the desired degree of workability.
Others
All the factors, which are, in common, responsible for the loss of plasticity of concrete or the loss of moisture from the concrete mix are also responsible for reducing the workability of concrete mix. For example, high working temperature (i.e. more than 21oC), long time taken in handling the concrete mix, high heat of hydration, low initial setting time of cement, etc. are few of them which increases the loss of moisture from the concrete.
Also, one can enhance the workability of concrete mix by using certain specific types of admixtures for example, plastisizers, super-plasticizers, air-entraining admixtures, etc.