Loss on Ignition Test for Cement
One of the crucial chemical tests to determine the quality of cement is the Loss on Ignition test. This test measures the amount of volatile materials present in the cement sample, which can affect its overall quality.
Here’s how the test is conducted:
Preparation: A 1.00 g sample of cement is carefully measured and placed in a platinum crucible with a capacity of 20 to 25 ml. The crucible is covered and weighed accurately.
Heating Process: The covered crucible with the cement sample is then subjected to heat in a muffle furnace, maintained at a temperature ranging between 900°C and 1000°C. The sample is heated for 15 minutes, ensuring the complete elimination of volatile substances.
Cooling and Weighing: After heating, the crucible with the sample is allowed to cool in a desiccator and then reweighed accurately.
Secondary Heating: To ensure the thorough removal of all volatile components, a second heating is performed for 5 minutes, followed by another round of weighing.
Calculating Loss on Ignition: The loss in weight between the initial and final weighings is recorded as the loss on ignition. The percentage of loss on ignition is calculated by multiplying the weight loss by 100 and dividing it by the initial weight of the sample.
Quality Benchmark: The acceptable standard for the percentage loss on ignition in cement is a maximum of 4 per cent. Any percentage exceeding this threshold might indicate an excessive presence of volatile materials, potentially affecting the cement’s quality and performance.
Ensuring that the loss on ignition remains within this specified limit is crucial to maintaining the desired quality and properties of cement for construction purposes.
Determination of Silica Content in Cement
To assess the silica content in cement, a meticulous process is followed:
Sample Preparation: Begin with 0.5 g of the cement sample in an evaporating dish, moistened with 10 ml of distilled water to prevent clumping. Add 5 to 10 ml of hydrochloric acid and gently heat the mixture, ensuring complete dissolution. Light agitation or gentle pressure with a glass rod aids in the dissolution process. Once dissolved, evaporate the solution to dryness on a steam bath.
Residue Treatment: Without further heating, add 5 to 10 ml of hydrochloric acid and an equal amount of water to the dried residue. Cover the dish and digest the solution on a water bath for 10 minutes. Dilute the solution with hot water and filter it through ashless filter paper to separate the silica (SiO2). Thoroughly wash the separated silica residue with hot water and reserve it.
Second Residue Processing: Evaporate the filtrate to dryness, bake the residue in an oven at 105°C to 110°C for an hour. Treat this residue with hydrochloric acid and heat it on a water bath. Filter and wash the small amount of silica present on a separate filter paper. Reserve the filtrate and washings for the determination of combined alumina and ferric oxide.
Calculation of Silica: Transfer the filter papers containing the residues to a weighed platinum crucible. Dry and ignite the papers gradually until the weight stabilizes (W1). The resulting ignited residue will contain minor impurities.
Silica Quantification: Treat the ignited residue in the crucible with distilled water, hydrofluoric acid, and a drop of sulfuric acid. Cautiously evaporate it to dryness and heat it at 1050°C to 1100°C for a brief period. After cooling, weigh the residue (W2). The difference between W1 and W2 represents the silica content (W).
Calculation Formula: Silica (%) = 200 × (W1 – W2)
This process allows for the accurate determination of silica content in cement, a crucial factor influencing its properties and quality.
Determining Combined Ferric Oxide and Alumina Content in Cement
Precipitation: Begin with 200 ml of the filtrate reserved from the silica test. Heat this to a boil and add a few drops of bromine water or concentrated nitric acid to ensure the oxidation of any ferrous ions to the ferric state. Gradually add ammonium hydroxide (1:1) drop by drop until the solution smells of ammonia. Boil the solution for a minute to precipitate aluminium and ferric hydroxides.
Filtration and Washing: Allow the precipitate to settle, filter it through ashless filter paper, and wash it with hot two per cent ammonium nitrate solution. Set aside the filtrate and washings for further use.
Reprecipitation: Transfer the precipitate and filter paper back to the original beaker. Dissolve the precipitate in hydrochloric acid (1:3), dilute the solution to about 100 ml, and reprecipitate the hydroxides. Filter the solution and precipitate it twice with 10 ml portions of hot ammonium nitrate solution. Combine the filtrate and washings with the previously reserved filtrate for the determination of calcium oxide.
Weighing: Place the resulting precipitate in a weighed platinum crucible. Heat it gradually until the papers are charred and finally ignite it to a constant weight at temperatures between 1050°C and 1100°C. Weigh this (W1) as combined alumina and ferric oxide.
Correction for Silica Contamination: If there’s a suspicion of silica in the filtrate used for this estimation, treat the residue in the crucible with hydrofluoric acid and sulphuric acid. Heat it cautiously at 1050°C to 1100°C for a minute or two, then cool and weigh it (W2). The difference between W1 and W2 represents the residue silica. Subtract this amount from W1 and add it to the silica content (W).
Calculation Formula: Combined ferric oxide and alumina (%) = weight of residue × 200
Determining Ferric Oxide Content
Sample Preparation: Add 1 g of the cement sample to 40 ml of cold water. While vigorously stirring the mixture, add 50 ml of hydrochloric acid. If needed, heat the solution and grind the cement with a glass rod until complete decomposition is evident.
Treatment and Titration: Heat the solution to a boil and slowly add stannous chloride solution drop by drop while stirring until the solution becomes colorless. A slight excess of stannous chloride solution is added, and the solution is then cooled to room temperature.
Titration Process: Add 15 ml of a saturated mercuric chloride solution and 25 ml of manganese sulfate solution. Titrate the solution with a standard potassium permanganate solution until a permanent pink color is achieved. This titration allows for the calculation of iron as ferric oxide.
Determining Alumina
The weight of ferric oxide and any trace amount of silica is subtracted from the total weight of oxides (W1)( Ferric Oxide and Alumina Content ). The resulting value represents the combined weight of alumina and residual traces of other oxides, which is reported as the alumina content.
Calcium Oxide Test in Cement
Preparation of Solution:
- Combine the reserved filtrates from the ferric oxide and alumina test. Acidify this combined filtrate with hydrochloric acid and reduce it to a volume of about 100 ml.
Bromine Water Addition:
- Add 40 ml of saturated bromine water to the heated solution. Gradually introduce ammonium hydroxide until the solution becomes distinctly alkaline. Boil the solution for a minimum of 5 minutes, ensuring it retains its alkalinity throughout. Allow the resulting precipitate to settle, filter it, and thoroughly wash it with hot water.
Final Treatment:
- Wash the beaker and filter once with nitric acid (1:33) followed by a final rinse with hot water. Discard any manganese dioxide precipitate that may remain on the filter.
Bromine Expulsion:
- Mix the filtrate with hydrochloric acid and boil until all traces of bromine are expelled.
Calcium Oxalate Precipitation:
- Add 25 ml of boiling ammonium oxalate solution to the boiling mixture. Adjust the solution to an alkaline state using ammonium hydroxide, and continue boiling until the calcium oxalate precipitate achieves a distinct, granular form. Allow the precipitate to settle for approximately 20 minutes, then filter it.
Washing Procedure:
- Wash the precipitate moderately with ammonium oxalate solution (at a concentration of one gram per liter).
Estimating Magnesia:
- Set aside the filtrate and washings (W3) for the estimation of magnesia.
Calculation of Percentage:
- Weigh the precipitated lime after ignition and heating at 1100°C-1200°C. Calculate the percentage of CaO using the formula: Percentage of CaO = (weight of residue) × 200.
Additionally, esure that (CaO-0.7SO3) /(2.8SiO2+1.2Al2O3+0.65Fe2O3) is not less than 0.6% for accurate assessment.
Determining Magnesia
Filtrate Preparation (W3):
- Acidify the filtrate (W3) obtained earlier with hydrochloric acid and concentrate it to approximately 150 ml.
Magnesium Ammonium Phosphate Precipitation:
- Add approximately 10 ml of ammonium hydrogen phosphate solution (250 g per liter) to the concentrated solution. Cool the mixture in an ice water bath. While stirring continuously, add ammonium hydroxide drop by drop until the formation of crystalline magnesium ammonium phosphate begins. Continue adding the reagent in moderate excess (5 to 10% more than the solution’s volume), stirring for several minutes.
Precipitate Collection:
- Let the solution sit undisturbed for a minimum of 16 hours in a cool environment. Afterward, filter the precipitate and wash it with an ammonium nitrate wash solution (prepared by dissolving 100 g of ammonium nitrate in water, adding 200 ml of ammonium hydroxide, and diluting to one liter).
Char and Ignition Process:
- Slowly char the precipitate and carefully burn off the resulting carbon. Ignite the precipitate at temperatures ranging from 1100°C to 1200°C until a constant weight is achieved, avoiding the pyrophosphate from melting.
Magnesia Calculation:
- Calculate the magnesia content of the material used for the test based on the weight of the magnesium pyrophosphate obtained, using the formula: Percentage of MgO = (weight of residue) × 72.4.
Determining Sulphuric Anhyride
Sample Preparation:
- Mix one gram of the sample with 25 ml of cold water. While vigorously stirring, add 5 ml of hydrochloric acid. If necessary, heat the solution and grind the material using a glass rod until complete decomposition of the cement is observed.
Solution Digestion and Filtration:
- Dilute the solution to 50 ml and allow it to digest for 15 minutes. Filter the residue thoroughly and wash it extensively with hot water. Set aside the filter paper with the residue (W4).
Precipitate Formation:
- Dilute the filtrate to 250 ml and bring it to a boil. Add 10 ml of hot barium chloride solution (100 g per liter) drop by drop, continuing boiling until a well-formed precipitate is obtained.
Digestion and Filtration of Precipitate:
- Digest the solution on a steam bath for 4 hours or preferably overnight. Filter the resulting precipitate and thoroughly wash it.
Incineration and Weighing:
- Place the filter paper and contents in a weighed platinum or porcelain crucible. Slowly incinerate the paper without causing it to ignite. Then ignite the residue at 800°C to 900°C, cool it in a desiccator, and weigh the resulting barium sulfate.
Calculation:
- Calculate the sulfuric anhydride content of the material using the formula: Percentage of SO3 = (weight of residue) × 34.3.
Insoluble Residue Determination
Preparation of Solution:
- Digest the filter paper containing the residue (W4) in 30 ml of hot water and 30 ml of 2 N sodium carbonate solution, maintaining a constant volume. Hold the solution for 10 minutes at a temperature just short of boiling.
Filtration and Washing:
- Filter the solution and wash the residue with dilute hydrochloric acid (1:99) followed by thorough rinsing with hot water until free from chlorides.
Residue Incineration:
- Ignite the residue in a crucible at temperatures ranging from 900°C to 1000°C. After incineration, cool the crucible in a desiccator.
Percentage of Insoluble Residues:
- Weigh the resulting residue to determine the percentage of insoluble residues. Ensure that the insoluble residues do not exceed 1.5% for compliance with quality standards.
