The setting time of cement is lowered by adding

The correct answer is Option 1: oxides of aluminium.

In cement chemistry, the setting time refers to the period it takes for the cement paste to stiffen. The speed of setting is primarily governed by the chemical composition of the clinker:

• Oxides of Aluminium (Alumina): Alumina imparts the property of quick setting to the cement. It acts as a flux and lowers the clinkering temperature. An excess of alumina hastens the chemical reaction with water, thereby lowering (decreasing) the setting time.
• Gypsum (Option 2): Gypsum is added specifically to increase the setting time (act as a retarder) to prevent "flash set," allowing workers time for mixing and placing.
• Silica (Option 4): Silica imparts strength but makes the cement set slowly.
• Magnesium Oxide (Option 3): This is primarily responsible for hardness and color; in excess, it causes unsoundness.

Therefore, to reduce the time it takes for cement to set, the concentration of aluminium oxides is increased.