The setting time of cement is lowered by adding

Join Our Telegram Channel

oxides of aluminium

gypsum

oxides of magnesium

silica

Answer is Right!

Answer is Wrong!

This question was previously asked in

UPSC NDA-2 – 2016

Download PDF Attempt Online

The correct answer is A) oxides of aluminium. The question asks what is added to lower the setting time of cement, which means making it set faster. Gypsum (option B) is added to *increase* the setting time (retard setting). While standard accelerators are typically calcium chloride or alkali aluminates, reactive forms of aluminium oxides or phases rich in alumina like Calcium Aluminate Cements (CAC) or specific additives containing activated alumina can act as accelerators, causing rapid setting. Therefore, among the given options, oxides of aluminium (in a suitable form or context as present in cement phases like C₃A) are the most plausible candidates for contributing to or accelerating the setting process compared to the other options which are either retarders (gypsum) or primary components/cause of expansion (magnesium oxide).

– Setting time refers to the time required for cement paste to lose its plasticity.
– Additives are used to control setting time: accelerators decrease setting time, retarders increase it.
– Gypsum is a common retarder, preventing flash set by reacting with C₃A.
– Oxides of aluminium are present in cement clinker primarily as tricalcium aluminate (C₃A), which hydrates rapidly and contributes to early setting. Certain forms of reactive alumina can act as accelerators.

– Calcium chloride (CaCl₂) is a very common and effective accelerator used in cement.
– Oxides of magnesium (MgO), if present in excess in portland cement, can cause delayed expansion and soundness issues, not primarily affecting initial setting time in a controlled manner.
– Silica (SiO₂) is a main component of cement (silicates C₃S and C₂S), which are responsible for strength development, but adding pure silica is not a method to control setting time.
– The phrasing “lowered by adding” implies acceleration. While gypsum is a crucial additive related to setting time, its effect is the opposite (increasing/retarding). This points towards A as the intended answer, assuming a context where an aluminium oxide based additive is used for acceleration.

More similar MCQ questions for Exam preparation:-