a) Accelerate initial setting of concrete:
Admixtures, such as accelerators, can be added to concrete to speed up the initial setting time. This is particularly useful in cold weather conditions or when there is a need for rapid construction. Accelerators work by increasing the rate of hydration, which leads to faster setting and hardening of the concrete.

b) Increase the strength of concrete:
Certain admixtures, known as strength enhancers, can be used to improve the strength characteristics of concrete. These admixtures typically work by enhancing the hydration process or by reducing the water content in the mix, resulting in a denser and stronger concrete. This is especially beneficial in applications where high strength is required, such as in the construction of high-rise buildings or heavy-duty structures.

c) Improve workability:
Workability refers to the ease with which concrete can be mixed, placed, and finished. Admixtures called plasticizers or water reducers can be added to concrete to improve its workability. These admixtures reduce the amount of water required to achieve the desired consistency, making the concrete more fluid and easier to handle. Improved workability allows for better compaction and placement, resulting in a more durable and aesthetically pleasing finished product.

d) All of the above:
Admixtures can be used to achieve any combination of the above benefits. They provide flexibility in concrete mix design, allowing engineers and contractors to tailor the properties of concrete to meet specific project requirements. By using the appropriate admixtures, it is possible to accelerate setting time, increase strength, and improve workability simultaneously.

In conclusion, admixtures are valuable additives that offer a range of benefits in concrete construction. They can accelerate initial setting, increase strength, and improve workability, thereby enhancing the overall performance and durability of concrete structures.