The basic purpose of a retarder in concrete is A. To increase the initial setting time of cement paste in concrete B. To decrease the initial setting time of cement paste in concrete C. To render the concrete more water tight D. To improve the workability of concrete mix

The correct answer is A. To increase the initial setting time of cement paste in concrete.

A retarder is a substance that is added to concrete to slow down the setting time of the cement paste. This allows for more time to place and finish the concrete before it sets. Retarders can also be used to improve the workability of concrete, making it easier to place and finish.

Option B is incorrect because a retarder actually increases the initial setting time of cement paste.

Option C is incorrect because a retarder does not render the concrete more water tight.

Option D is incorrect because a retarder does not improve the workability of concrete mix.