Cement

Cement clinker is made by burning limestone and clay at about 1450 °C. It contains four main compounds (Bogue's compounds).

Functions

1. C₃S (alite): hydrates quickly and gives early strength (first 7 days); produces high heat of hydration. More C₃S gives rapid hardening cement.
2. C₂S (belite): hydrates slowly and is responsible for the strength after 7 days, up to a year. Gives low heat and good resistance to chemicals.
3. C₃A: reacts immediately with water and causes the initial flash set, controlled by adding 2 to 3% gypsum. It evolves large heat, adds some early strength, but makes cement weak to sulphate attack.
4. C₄AF: acts as a flux that lowers the burning temperature; gives the grey colour, adds little strength and hydrates moderately fast.

Hydration of C₃S and C₂S produces calcium silicate hydrate (C-S-H gel), which gives the strength, and Ca(OH)₂.

Portland cement is made from raw materials rich in lime, silica, alumina and iron. After burning, the ingredients in cement are:

Cement is a fine grey powder made by burning a mixture of calcareous (limestone) and argillaceous (clay) materials into clinker and grinding it with gypsum. 53 grade OPC (IS 12269) is made by the dry process in modern plants, because it needs a finely controlled, fuel-efficient kiln feed.

Raw materials

• Calcareous: limestone, chalk, marl (supplies CaO, about 60-65%).
• Argillaceous: clay, shale (supplies SiO2, Al2O3, Fe2O3).
• Correctives: bauxite, iron ore, silica sand (to adjust the composition).
• Gypsum (added after burning) to control setting.

Flow diagram

 Limestone + Clay (+ correctives)
            |
      Crushing & Pre-blending
            |
   Raw mill (dry grinding to fine powder)
            |
     Blending / Homogenising silo
            |
   Preheater (cyclones, 800-900 C)
            |
   Rotary kiln (1400-1500 C) -> CLINKER
            |
        Clinker cooler
            |
 Clinker + Gypsum (3-5%) -> Cement mill
            |
         Cement silo
            |
   Packing (50 kg bags) / Bulk dispatch

Steps

1. Quarrying and crushing: limestone and clay are quarried and crushed to about 25 mm.
2. Proportioning and dry grinding: materials are mixed in the right ratio (about 75-80% limestone, 20-25% clay) and ground in the raw mill to a fine powder called raw meal (raw meal is dry, moisture below 1%).
3. Blending: raw meal is homogenised with compressed air in silos so that the chemical composition is uniform (LSF, silica and alumina moduli are checked).
4. Preheating: hot kiln gases heat the meal in cyclone preheaters (calcination begins: CaCO3 gives CaO + CO2).
5. Burning: in the rotary kiln (inclined 3-4%, rotating 1-3 rpm) the meal passes through drying, calcination, and the clinkering zone at about 1450 C, where partial fusion forms nodules of clinker (C3S, C2S, C3A, C4AF).
6. Cooling: clinker is cooled rapidly in a grate cooler.
7. Grinding: clinker with 3-5% gypsum is ground in a ball mill to fineness of at least 225 m²/kg (Blaine).
8. Storage and packing: cement is stored in silos and packed in 50 kg bags.

The 53 grade cement has more C3S and finer grinding, giving a 28-day strength of at least 53 MPa.

In the wet process, raw materials are ground with water into a slurry (35-40% water), which is fed to the rotary kiln. It is suitable when the raw materials are soft and wet, such as chalk and clay.

Flow chart

  Limestone/Chalk        Clay
        |                  |
   Crushing          Washing in wash mill
        |                  |
        +-----> Wash mill / Ball mill <--- water
                     |
               Slurry (35-40% water)
                     |
             Slurry tanks (correction)
                     |
           Rotary kiln (1400-1500 C)
                     |
                  CLINKER
                     |
              Cooler, then mix
              with 3-5% gypsum
                     |
                Ball/tube mill
                     |
               Cement silo
                     |
                Packing/bags

Explanation

1. Preparation of slurry: limestone is crushed and added to the wash mill/ball mill with clay and water. Grinding gives a creamy slurry of about 35-40% water.
2. Correction: slurry is stored in tanks, tested for CaCO3 (about 75-78%) and corrected by adding limestone or clay until the composition is right. Compressed air keeps it agitated.
3. Burning: the slurry enters the upper end of an inclined rotary kiln (diameter 2.5-3.5 m, length up to 150-200 m). In the drying zone water evaporates (up to 400 C), in the calcination zone CaCO3 breaks into CaO and CO2 (about 900 C), and in the burning zone (1400-1500 C) the materials fuse into clinker nodules of 5-10 mm.
4. Cooling and grinding: hot clinker is cooled, mixed with gypsum (about 3-5%) and ground to a fine powder in tube mills.
5. Storage and packing: cement is stored in silos and packed into 50 kg bags.

Merits: easy mixing and uniform composition; less dust. Demerits: more fuel consumption, longer kiln, slow and costly.

The compressive strength of cement is found by crushing standard mortar cubes made from cement and standard sand (IS 4031 Part 6, IS 650).

Apparatus

Cube moulds of 70.6 mm side (area 5000 mm²), vibrating machine, standard (Ennore) sand, graduated cylinder, balance, curing tank, compression testing machine.

Procedure

1. Take 200 g cement and 600 g standard sand for each cube, a 1:3 mix (for 3 cubes: 600 g cement + 1800 g sand). Mix dry for about one minute.
2. Add water equal to (P/4 + 3)% of the combined weight of cement and sand, where P is the percentage of water for standard consistency. Mix for 3-4 minutes until uniform in colour.
3. Place the mortar in the mould immediately and vibrate for 2 minutes at the specified speed; level and smooth the top.
4. Keep the moulds in a moist chamber at 27 ± 2 C and at least 90% relative humidity for 24 hours.
5. Remove the cubes and cure them in clean water at 27 ± 2 C until the test date.
6. Test the cubes at 3, 7 and 28 days after casting, on their sides, with a load rate of 35 N/mm² per minute.
7. Compressive strength = failure load / cross-section area (5000 mm²). The average of three cubes is the result.

Minimum strength (53 grade OPC, IS 12269)

For 43 grade (IS 8112) the values are 23, 33 and 43 MPa.

Harmful constituents are the compounds in cement that, if present in excess, reduce strength, cause cracking or damage concrete in the long run.

1. Excess free lime (CaO): unburnt, it hydrates slowly after setting and expands, causing unsoundness and cracks.
2. Excess magnesia (MgO): above 6% (IS 269) it also hydrates late and causes expansion and cracking.
3. Excess sulphur trioxide (SO3): more than 2.5% (3% when C3A is over 5%) forms ettringite, causing unsoundness and expansion.
4. Alkalis (Na2O and K2O): limited to 0.6% (as Na2O equivalent); they cause efflorescence and alkali-aggregate reaction.
5. Insoluble residue and loss on ignition: limited to 4% each; they show poor burning or adulteration.
6. Excess alumina/iron oxide and uncombined silica: upset the compound balance and lower strength.

Their limits are fixed in IS 269 and checked in chemical analysis.

Clinker is the hard, dark grey, nodular (5-25 mm) material obtained by burning a finely ground mixture of limestone and clay at about 1400-1500 C in a rotary kiln, until partial fusion occurs. It consists mainly of the four compounds C3S (tricalcium silicate), C2S (dicalcium silicate), C3A (tricalcium aluminate) and C4AF (tetracalcium aluminoferrite). Cement is obtained by grinding clinker with 3-5% gypsum.

Admixtures are materials, other than cement, water and aggregate, added to concrete or mortar just before or during mixing to modify its properties in fresh or hardened state. They are used in small doses (usually under 5% of cement weight).

Engineering applications

A cement water proofer is a material added to cement mortar or concrete to make it impervious (water-tight) by reducing the water absorption and permeability of the hardened mass.

Types and action

1. Pore fillers (pore-blocking): very fine inert materials such as fly ash, talc, bentonite, hydrated lime, chalk or silica fume. They fill the voids between particles, so water cannot enter.
2. Water repellents (hydrophobic): soaps, stearates, fatty acids, oils, wax emulsions. They line the pores and make them repel water.
3. Crystalline waterproofers: react with water and free lime to grow crystals in the pores.
4. Integral liquid waterproofers: ready-made liquid compounds mixed into the mixing water.

Uses

• Basements, water tanks, swimming pools, retaining walls.
• Roof slabs, terraces, bathrooms and toilets.
• Plaster for damp walls; damp-proof courses.

Points

• Dosage is usually 2-3% by weight of cement, as per the maker.
• Good compaction, low water-cement ratio and curing are needed alongside; the water proofer alone is not enough.

The wet process is preferred when the raw materials are soft, naturally wet and easily made into slurry, for example chalk and clay (or marl and clay), because grinding with water gives an easy, uniform mix. It is also preferred where a very uniform raw mix is needed, where dust nuisance is a problem, and where cheap fuel is available, as wet-process plants use more fuel (to evaporate the water).

In the dry process the raw materials are ground and blended dry, and the dry powder (raw meal) is burnt in the kiln. It is used when the raw materials are hard (limestone and shale) and fuel is costly, so it is the modern standard process.

 Limestone + Clay/Shale (+ correctives)
              |
        Crushing (jaw/impact)
              |
      Drying (if moisture > 1%)
              |
   Raw mill: fine dry grinding (raw meal)
              |
    Blending & storage silo (air agitation)
              |
    Preheater cyclones (800-900 C)
              |
    Rotary kiln (1400-1500 C) -> Clinker
              |
        Clinker cooler
              |
   + Gypsum (3-5%) -> Cement mill
              |
         Cement silo
              |
          Packing

Features: short kiln, fuel use of 3-3.5 GJ/t clinker (much lower than the wet process), higher output, but more dust and a need for good blending.

For 53 grade OPC (IS 12269) the minimum compressive strength of 70.6 mm mortar cubes is:

• 7 days: 37 MPa (N/mm²)
• 28 days: 53 MPa (N/mm²)

(The 3-day minimum is 27 MPa.)

1. Ordinary Portland Cement (OPC)

Made by grinding clinker with 3-5% gypsum; available in 33, 43 and 53 grades (28-day strength in MPa). Used for general RCC, plain concrete, plaster, flooring and masonry.

2. Portland Pozzolana Cement (PPC, IS 1489)

Made by grinding clinker with 15-35% pozzolana (fly ash, calcined clay) or by blending. It has lower heat of hydration, better resistance to sulphates and chemicals, a denser structure and lower cost, but gains strength more slowly. Used in mass concrete, marine works, sewers, dams and plaster.

3. Rapid Hardening Cement

Finer grinding and a higher C3S content than OPC; it gains in 3 days the strength OPC reaches in 7 days. Used for early formwork removal, road repairs and cold-weather work.

Others: low heat cement, sulphate resisting cement, white cement, high alumina cement, blast furnace slag cement.

Rapid hardening cement (also called high early strength cement, IS 8041) is cement that develops strength much faster than OPC in the first few days, though its setting time is about the same as OPC.

How it is made

• More C3S (about 55-65%) and less C2S.
• Clinker is ground finer (specific surface at least 325 m²/kg compared with 225 for OPC).

Properties

• 1-day strength about equal to OPC at 3 days; 3-day strength equal to OPC at 7 days (minimum 16 MPa at 1 day and 27 MPa at 3 days).
• Generates more heat of hydration, so it is not suited to mass concrete.
• Higher cost, and slightly lower long-term strength gain than OPC.
• Initial setting time not less than 30 min; final not more than 600 min.

Uses

• Where formwork must be removed early (precast work, prestressed members).
• Road repair and airfield works needing quick opening to traffic.
• Cold-weather concreting, as the heat produced prevents freezing.
• Urgent repairs and emergency structures.

Precautions

Needs more curing care, avoid in thick sections and large pours.

• Initial setting time: the time from adding water to cement until the paste begins to lose plasticity, that is, when the Vicat needle (1 mm square) stops 5 ± 0.5 mm from the bottom of the mould. For OPC it must be at least 30 minutes.
• Final setting time: the time from adding water until the paste has completely lost plasticity and become hard enough to resist a definite pressure, that is, when the needle with annular attachment makes an impression but the attachment fails to. For OPC it must not exceed 600 minutes (10 hours).

Significance

• The initial setting time gives enough time to mix, transport, place and compact the concrete or mortar before it stiffens; a short value (less than 30 min) causes difficulty in work.
• The final setting time shows when the concrete has hardened enough to bear loads, which fixes formwork removal, curing start and moving of the work.
• It checks quality: abnormal setting (flash set or false set) indicates deterioration, wrong gypsum content or storage defects.
• It decides if the cement suits the work (such as hot-weather work).

Test (Vicat apparatus, IS 4031 Part 5)

1. Take 400 g cement and mix with water equal to 0.85 P (P is the standard consistency percentage); gauge for 3-5 minutes from the moment of adding water (start the stopwatch at this time).
2. Fill the Vicat mould resting on a glass plate and level the top.
3. Initial setting time: lower the 1 mm square needle gently on the paste at regular intervals. When the needle penetration stops 5 ± 0.5 mm from the bottom, the time elapsed from adding water is the initial setting time.
4. Final setting time: replace the needle with the needle having the annular collar (attachment). Final setting is when the needle makes an impression but the collar does not. The time is noted from adding water.
5. Keep the apparatus at 27 ± 2 C and 90% humidity, and do not let the needle fall on the same spot twice.

The consistency test finds the water percentage (P) that gives cement paste of standard consistency (IS 4031 Part 4), using the Vicat apparatus with a 10 mm diameter, 50 mm long plunger.

Procedure

1. Take 400 g of cement and mix with a trial water percentage (for example 26% by weight of cement), gauging for 3-5 minutes.
2. Fill the Vicat mould (resting on a glass plate) with paste, then smooth the top surface level.
3. Place the mould under the plunger, lower it until it just touches the paste surface and tighten the clamp; set the pointer to zero.
4. Release the plunger quickly, letting it sink into the paste.
5. Read the depth of penetration. If the plunger stops 5-7 mm from the bottom of the mould (that is 33-35 mm from the top), the paste has standard consistency.
6. Otherwise repeat with a fresh sample and a different amount of water (increase by 1% each time) until this penetration is obtained.
7. Consistency P = (weight of water / weight of cement) x 100, typically 26-33%.

P is then used for the setting time, soundness and strength tests.

Importance

Soundness means the ability of the hardened cement paste to keep its volume without expansion. Unsound cement (excess free lime, magnesia or sulphates) expands after setting and causes cracking, distortion and failure of concrete. So the test makes sure the cement is safe before use.

Le Chatelier test (IS 4031 Part 3)

1. Mix 100 g cement with 0.78 P of water (P = standard consistency), gauging well.
2. Place the paste in the Le Chatelier mould (a brass split cylinder 30 mm diameter by 30 mm high with two pointers 165 mm long), keeping it on a glass plate and covering it with another glass plate.
3. Store the assembly in a humid chamber at 27 ± 2 C for 24 hours.
4. Measure the distance between the pointer ends (A) to the nearest 0.5 mm.
5. Submerge the mould in water, bring it to boiling in 25-30 minutes and boil for 3 hours.
6. Cool the mould, and measure the distance again (B).
7. Expansion = B - A. For OPC it must not exceed 10 mm; if it does, retest after aeration of cement for 7 days.

The autoclave test (expansion limit 0.8%) is also used for magnesia.

The soundness test is carried out to make sure that the cement does not undergo a harmful change of volume (expansion) after it has set.

Why it is necessary

• Cement with too much free lime (CaO), free magnesia (MgO) or excess sulphates hydrates slowly even after setting. The slow reaction increases volume, which causes cracks, distortion, disintegration and loss of strength in concrete and mortar.
• It cannot be noticed at the time of use, as the effect appears days or months later, when repair is costly. Hence it is checked in advance.
• It confirms that the clinker was properly burnt and ground and that the cement is not adulterated.
• It checks that the cement has not changed through bad storage.

How it is tested

• Le Chatelier test: detects expansion due to free lime. The paste is made with 0.78 P water in the mould, cured for 24 hours, then boiled for 3 hours; expansion between pointers must not exceed 10 mm.
• Autoclave test: detects expansion due to magnesia; a bar is heated with steam at 2 MPa for 3 hours, with expansion limited to 0.8%.

Remedy

If the cement fails, it may be spread out in layers 150 mm thick to air for 3-4 weeks and retested; free lime then slakes by absorbing moisture and CO2. Otherwise it is rejected.

Conclusion on use

Only sound cement is used in structural and hydraulic works, to ensure durability and safety.

Field tests (quick checks)

• Colour: uniform grey with a light greenish shade.
• Feel: smooth when rubbed between fingers, free from lumps.
• Hand test: a hand pushed into the bag feels cool; a handful thrown in water should float for some time before sinking.
• Lump test: no hard lumps (lumps mean the cement has absorbed moisture).
• Smell and setting (block) test: a cake made with water, kept under water 24 hours, must remain intact without cracks.

Laboratory tests (IS 4031)

1. Fineness (sieve or Blaine air permeability): at least 225 m²/kg.
2. Standard consistency (Vicat apparatus).
3. Setting time: initial at least 30 min; final at most 600 min.
4. Soundness: Le Chatelier (at most 10 mm) and autoclave.
5. Compressive strength: 70.6 mm mortar cubes at 3, 7, 28 days.
6. Specific gravity: about 3.15 (Le Chatelier flask).
7. Heat of hydration and chemical analysis (MgO, SO3, insoluble residue, loss on ignition).

Hydration takes place because the anhydrous compounds of cement (C3S, C2S, C3A, C4AF) are chemically unstable in the presence of water. They react with water to form stable, insoluble hydrated products (mainly calcium silicate hydrate, C-S-H gel, and calcium hydroxide), which set and harden and give cement its binding strength. The reaction is exothermic and continues as long as water and unhydrated cement are available.

The four clinker compounds contribute to strength at different rates:

• C3S (tricalcium silicate): hydrates fast, responsible for early strength (up to 7 days) and continues up to 28 days.
• C2S (dicalcium silicate): hydrates slowly, contributes little in the first weeks and is mainly responsible for later strength (after 28 days to one year).
• C3A: reacts rapidly, gives some strength in the first 1-3 days only.
• C4AF: contributes very little to strength.

 Strength
   ^
   |                     ___________ C3S
   |                ___--
   |            _--         _______ C2S
   |          /          _-'
   |        /        __-'
   |      /     __--'
   |    /  _--'                 ... C3A
   |  /_-'''''''''''''''''''''''''
   | ......................... C4AF
   +-------------------------------> Age
     1d  3d  7d   28d   90d   1yr

Hence the early strength of cement depends on C3S and C3A, and the long-term strength on C2S (together with C3S).

Grade 43 cement (IS 8112) is ordinary Portland cement whose 28-day compressive strength, tested on standard 70.6 mm mortar cubes (1:3 cement-sand), is not less than 43 MPa (N/mm²). Its 3-day and 7-day strengths are at least 23 MPa and 33 MPa.

Pozzolana (such as fly ash or calcined clay) is a siliceous material that, with moisture, reacts with the calcium hydroxide released by hydrating cement to form extra C-S-H gel. It is used in PPC to:

• improve durability and resistance to sulphates, chlorides and chemicals;
• reduce heat of hydration and cracking in mass concrete;
• give a denser, less permeable concrete and lower efflorescence;
• reduce cost and save clinker and energy (reducing CO2), and use industrial waste.