Testing Of Cement: Chemical Tests (lS: 4032)

Testing Of Cement: 

Chemical Tests (lS: 4032)

Loss on ignition: 1.00 g of the sample is heated for 15 minutes in a weighed and covered platinum crucible of 20 to 25 ml capacity by placing it in a muffle furnace at any temperature between 900 o and 1000 o C. It is then cooled and weighed. Thereafter, the loss in weight is checked by a second heating for 5 minutes and reweighing. The loss in the weight is recorded as the loss on ignition and the percentage of loss on ignition to the nearest 0.1 is calculated (loss in weight × 100). The percentage loss on ignition should not exceed 4 per cent.

Silica: 0.5 g of the sample is kept in an evaporating dish, moistened with 10 ml of distilled water at room temperature to prevent lumping. To this 5 to 10 ml of hydrochloric acid is added, and digested with the aid of gentle heat and agitation until solution is complete. Dissolution may be aided by light pressure with the flattened end of a glass rod. The solution is evaporated to dryness on a steam bath. Without heating the residue any further, it is treated with 5 to 10 ml of hydrochloric acid and then with an equal amount of water. The dish is covered and digested for 10 minutes on a water bath. The solution with an equal volume of hot water is diluted and is immediately filtered through an ashless filter paper, and the separated silica (SiO₂) is washed thoroughly with hot water and the residue is reserved.

The filtrate is again evaporated to dryness, baking the residue in an oven for one hour at 105 o C to 110 o C. Then the residue is added with 10 to 15 ml of hydrochloric acid (1:1) and is heated on a water bath. This solution is then diluted with an equal volume of hot water and the small amount of silica it contains is filtered and washed on another filter paper. The filtrate and washings are reserved for the determination of combined alumina and the ferric oxide.

The papers containing the residues are transferred to a weighed platinum crucible. The papers are dried and ignited, first at a low heat until the carbon of the filter papers is completely consumed without inflaming, and finally at 1100 o C to 1200 o C until the weight remains constant

(say W₁).

The ignited residue thus obtained, which will contain small amounts of impurities is treated in the crucible with a few drops of distilled water, about 10 ml of hydrofluoric acid and one drop of sulphuric acid and evaporated cautiously to dryness. Finally, the small residue is heated at 1050 o C to 1100 o C for a minute or two: cooled and weighed (say W₂). The difference between this weight and the weight of the ignited residue represents the amount of silica (W).

Silica (%) = 200 (W₁ - W₂)

Combined Ferric Oxide and Alumina: 200 ml of the sample from the filtrate reserved in silica test is heated to a boil. A few drops of bromine water or concentrated nitric acid is added during boiling in order to oxidize any ferrous ion to the ferric condition. It is then treated with ammonium hydroxide (1:1), drop by drop, until the solution smells of ammonia. The solution containing the precipitates of aluminium and ferric hydroxides is boiled for one minute. The precipitate is allowed to settle, filtered through an ashless filter paper and washed with two per cent hot ammonium nitrate solution. The filtrate and washings are set aside.

The precipitate and the filter paper is transferred to the same beaker in which the first precipitation was effected. The precipitate is then dissolved in hydrochloric acid (1:3). The solution is diluted to about 100 ml and the hydroxides are reprecipitated. The solution is filtered and precipitated with two 10 ml portions of hot ammonium nitrate solution. The filtrate and washings are then combined with the filtrate set aside and is reserved for the determination of calcium oxide.

The precipitate is placed in a weighed platinum crucible, heated slowly until the papers are charred, and finally ignited to constant weight at 1050 o C to 1100 o C with care to prevent reduction, and weighed (W₁) as combined alumina and ferric oxide.

If silica is suspected to be carried into the filtrate used for this estimation, the residue in the crucible is treated with a drop of water, about 5 ml of hydrofluoric acid and a drop of sulphuric acid and is evaporated cautiously to dryness. Finally, the crucible is heated at 1050 o C to 1100 o C for one or two minutes; cooled and weighed (W₂). The difference between this weight and the weight (W₁), represents the amount of residue silica. This amount is subtracted from the weight of ferric oxide and alumina found as W₁ and the same amount is added to the amount of silica (W). The ratio of percentages of alumina to iron oxide should not exceed 0.66.

Combined ferric oxide and alumina (%) = weight of residue × 200

Ferric Oxide: 40 ml of cold water is added to 1 g of the sample and while the mixture is stirred vigorously, 50 ml of hydrochloric acid is added. If necessary, the solution is heated and cement is ground with flattened end of a glass rod until it is evident that cement is completely decomposed. The solution is heated to a boil and is treated with stannous chloride solution added drop by drop while stirring, until the solution is dicolourized. A few drops of stannous chloride solution is added in excess and the solution is cooled to room temperature. Then, 15 ml of a saturated solution of mercuric chloride and 25 ml of manganese sulphate solution are added and titrated with standard solution of potassium permanganate until the permanent pink colour is obtained. Iron as ferric oxide is calculated.

Alumina: The calculated weight of ferric oxide and the small amount of silica is subtracted from the total weight of oxides (W_l). The remainder is the weight of alumina and of small amounts of other oxides reported as alumina.

Calcium Oxide: The combined filtrate reserved in the combined ferric oxide and alumina test is acidified with hydrochloric acid and evaporated to a volume of about 100 ml. 40 ml of saturated bromine water is added to the hot solution and ammonium hydroxide is added until the solution is distinctly alkaline. The solution is boiled for 5 minutes or more, making certain that the solution is at all times distinctly alkaline. Then the precipitate is allowed to settle, filtered and washed with hot water.

The beaker and filter is washed once with nitric acid (1:33) and finally with hot water. Any precipitate (of manganese dioxide) that may be left on the tunnel is discarded. The filtrate is mixed with hydrochloric acid and boiled until all the bromine is expelled. 25 ml of boiling ammonium oxalate solution is added to the boiling solution. The solution is made alkaline with ammonium hydroxide and brought to boiling, the boiling being continued until the precipitated calcium oxalate assumes a well-defined, granular form. The precipitate is allowed to stand for about 20 minutes or until it has settled, filtered and washed moderately with ammonium oxalate solution (one gram per litre). The filtrate and washings (W₃) are set aside for estimating magnesia.

The precipitated lime after ignition and heating at 1100 o C-1200 o C is weighed. The percentage of CaO = weight of residue × 200. Also CaO 0.7SO₃

2.8 SiO ₂   1.2 Al ₂ O ₃   0.65 Fe ₂ O₃

in per cent should not be less than 0.66.

Magnesia: The filtrate (W₃) is acidified with hydrochloric acid and is concentrated to about l50 ml. To this solution, about 10 ml of ammonium hydrogen phosphate solution (250 g per litre) is added and the solution is cooled by placing in a beaker of ice water. After cooling, ammonium hydroxide is added drop by drop, while stirring constantly, until the crystalline magnesium ammonium phosphate begins to form, and then the reagent is added in moderate excess (5 to 10 per cent of the volume of the solution), the stirring being continued for several minutes. The solution is set aside for at least 16 hours in a cool atmosphere and then filtered. The precipitate is washed with ammonium nitrate wash solution (100 g of ammonium nitrate dissolved in water, 200 ml of ammonium hydroxide added and diluted to one litre). It is then charred slowly and the resulting carbon is burnt carefully. The precipitate is ignited at 1100 o C to 1200 o C to constant weight, taking care to avoid bringing the pyrophosphate to melting.

From the weight of the magnesium pyrophosphate obtained, the magnesia content of the material taken for the test is calculated.

The percentage of MgO = weight of residue × 72.4. Free magnesia in cement should be less than 4 per cent.

Sulphuric Anhydride: To one gram of the sample, 25 ml of cold water is added and while the mixture is stirred vigorously 5 ml of hydrochloric acid is added. If necessary, the solution is heated and the material is ground with the flattened end of a glass rod until it is evident that the decomposition of cement is complete. The solution is diluted to 50 ml and digested for 15 minutes. The residue is filtered and washed thoroughly with hot water. The filter paper with the residue (W₄) is set aside. The filtrate is diluted to 250 ml and heated to boiling. 10 ml of hot barium chloride (100 g per litre) solution is added drop by drop and the boiling is continued until the precipitate is well formed. The solution is digested on steam bath for 4 hours or preferably overnight. The precipitate is filtered and the precipitate is washed thoroughly. The filter paper and the contents are placed in a weighed platinum or porcelain crucible and slowly the paper is incinerated without inflaming. Then it is ignited at 800 o C to 900 o C, cooled in a desiccator and the barium sulphate is weighed. From the weight of the barium sulphate obtained, the sulphuric anhydride content of the material taken for the test is calculated. The percentage of SO₃ = weight of residue × 34.3. Sulphur in cement should be less than 2.5 per cent.

Insoluble Residue: The filter paper containing the residue (W₄) is digested in 30 ml of hot water and 30 ml of 2 N sodium carbonate solution maintaining constant volume, the solution being held for 10 minutes at a temperature just short of boiling. It is then filtered and washed with dilute hydrochloric acid (1:99) and finally with hot water till free from chlorides. The residue is ignited in a crucible at 900 o C to 1000 o C, cooled in a desiccator and weighed. The insoluble residues should not exceed 1.5 per cent.