Gypsum is added in the manufacture of Portland cement in order to:

Lengthen the setting time of cement

Explanation:
When gypsum is added to Portland cement, it acts as a retarder, which means it slows down the setting time of the cement. This is important because it gives the workers enough time to work with the cement before it becomes rigid. Here's how gypsum helps to lengthen the setting time of cement:

1. Setting Time of Cement: The setting time of cement refers to the time it takes for the cement paste to harden and gain strength. There are two stages of setting: initial setting and final setting. Initial setting occurs when the cement starts to harden, and final setting occurs when the cement paste becomes rigid.

2. Role of Gypsum: Gypsum (calcium sulfate dihydrate) is added to the cement during the grinding process. It is a naturally occurring mineral and is commonly found in sedimentary rock formations. Gypsum is added in small quantities, usually around 3 to 5% by weight of the cement, to control the setting time.

3. Retarding Mechanism: Gypsum acts as a retarder by slowing down the hydration process of cement. Hydration is the chemical reaction between cement and water that results in the formation of cement paste. Gypsum reacts with the tricalcium aluminate (C3A) in cement to form ettringite, which is a hydrated calcium aluminum sulfate compound. This reaction consumes water and slows down the overall hydration process, thereby lengthening the setting time.

4. Benefits: The addition of gypsum provides several benefits in the manufacturing and use of Portland cement. By lengthening the setting time, it allows workers more time to place and finish the cement. This is especially important in large construction projects where there is a need for extended workability. It also helps to prevent flash setting, which is the rapid hardening of the cement paste, and reduces the risk of cracking and shrinkage.

In summary, gypsum is added to Portland cement to lengthen the setting time. By acting as a retarder, it slows down the hydration process and gives workers more time to work with the cement before it becomes rigid.