Which substance is added to cement clinker during grinding to retard the setting of cement?
- A.Gypsum
- B.Fly ash
- C.Silica
- D.Alumina
Correct answer
Explanation
The correct answer is A, gypsum. About two or three per cent of gypsum, which is calcium sulphate dihydrate, is ground with the clinker because it slows down the hydration of tricalcium aluminate, the compound that would otherwise set almost the moment water was added and leave no time to place the concrete. Option B is wrong because fly ash is added in much larger quantity to make pozzolana cement, where its purpose is to reduce the heat of hydration and improve long-term strength, not to control the setting time. Option C is wrong because silica is one of the main raw oxides of the cement itself and is present from the clay. Option D is wrong because alumina, also from the clay, is a constituent oxide rather than an additive. Gypsum heated to about 120 degrees Celsius gives plaster of Paris.
Read the full article: Industrial Chemistry: Glass, Cement, Fertilisers and PYQs
Practice Questions
View allWhich among the following is not a nitrogenous fertilizer?
- A.Ammonium Sulphate
- B.Calcium Cyanamide
- C.Superphosphate of Lime
- D.Urea
Show answer
Superphosphate of lime is made by treating rock phosphate with sulphuric acid, and the nutrient it supplies is phosphorus in the form of available phosphate, not nitrogen; it is therefore a phosphatic fertiliser.
Read the full explanation →What is a fertiliser that supplies two or more than two primary nutrients called?
- A.Straight fertiliser
- B.Organic manure
- C.Green manure
- D.Complex fertiliser
Show answer
Nitrogen, phosphorus and potassium are the three primary nutrients, and a fertiliser that carries two or more of them in one chemical compound is called a complex or mixed fertiliser; diammonium phosphate, which supplies both nitrogen and phosphate, and the NPK grades sold by number are the familiar examples.
Read the full explanation →What is the percentage of nitrogen present in farm yard manure (FYM) in general?
- A.0.5
- B.0.6
- C.0.8
- D.0.7
Show answer
Farmyard manure is the decomposed mixture of cattle dung, urine and bedding litter, and in general it carries only about half a per cent of nitrogen, along with roughly the same order of phosphate and potash.
Read the full explanation →The main raw materials used in the manufacture of Portland cement are:
- A.Limestone and clay
- B.Gypsum and sand
- C.Bauxite and coke
- D.Dolomite and graphite
Show answer
Cement needs one calcareous material, limestone, for its lime, and one argillaceous material, clay or shale, for its silica, alumina and iron oxide; the mixture is burnt in a rotary kiln at about 1450 degrees Celsius to give clinker, which is then ground with gypsum.
Read the full explanation →The blue colour in glass is produced by adding which oxide to the melt?
- A.Chromium oxide
- B.Cobalt oxide
- C.Cuprous oxide
- D.Manganese dioxide
Show answer
Metal oxides dissolved in molten glass absorb part of the visible spectrum, and cobalt oxide is the standard colourant for the deep blue of bottles and decorative glass, effective in very small quantities.
Read the full explanation →