Question

Gypsum is added to portland cement to (a) fasten the process of setting (b) slow down the process of setting (c) improve the colour of the cement (d) increase the melting point of cement.

Short answer

Gypsum is added to portland cement to (b) slow down the process of setting.

Step-by-step solution

Understanding the Purpose of Gypsum in Cement

Gypsum is a mineral added to portland cement to regulate the setting time. Without gypsum, cement would set too quickly for practical use. The correct understanding of gypsum's role will help determine the appropriate answer.

Identifying the Function of Gypsum

Review the options presented in the exercise to determine which one corresponds with the known function of gypsum in portland cement. Recall that gypsum acts to extend the time it takes for cement to set.

Choosing the Correct Option

Match the function of delaying the setting process to the options given. Option (b) directly states that gypsum is used to 'slow down the process of setting,' which aligns with the established purpose of gypsum in cement setting.

Key concepts

Cement Setting Time

The setting time of cement is one of the most critical properties in the construction process. It determines how long workers have to manipulate the material before it hardens. If the setting time is too swift, there may not be enough time to mix, transport, and place the cement before it begins to set, which can lead to complications in the construction work and weaker structural integrity.

Conversely, a setting time that is too slow can delay construction projects, increasing costs and extending timelines. Therefore, controlling the setting time is essential for efficiency and strength in construction projects. It is here that gypsum plays a vital role, interacting chemically with the other components of cement to manage the timing of the setting process accurately.

Purpose of Gypsum in Cement

The purpose of gypsum in cement is multifaceted. Primarily, it is used to regulate the setting time of cement, providing a predictable and extendable period for construction workers to work with the material. But how does gypsum do this? When mixed with water and cement, gypsum reacts to form calcium sulfate dihydrate. This reaction prolongs the hydration of the other components, effectively delaying the setting.

Gypsum is also vital in controlling the early hydration of tricalcium aluminate (C3A) in cement, which is known for its rapid reaction with water that can lead to early stiffening, known as 'flash setting'. By controlling the rate at which C3A reacts, gypsum ensures the durability and workability of the cement paste.

Chemistry of Cement

Understanding the chemistry of cement is crucial for grasping why certain additives like gypsum are necessary. Cement comprises various ingredients, including calcium, silicon, aluminum, and iron, which react with water in a complex process called hydration. During this chemical reaction, the components of cement form compounds that gradually harden and gain strength.

Cement's principal components include tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A), and tetracalcium aluminoferrite (C4AF). The reactions of these compounds with water are exothermic and evolve over time, contributing to the different stages of setting and hardening. For instance, the C3S and C2S phases primarily grant strength to the set cement over time, whereas C3A reacts rapidly and can lead to premature setting without the regulation provided by additives like gypsum.