Cement

OPC (Ordinary Portland Cement) vs PPC (Portland Pozzolana Cement)

So the key difference is that PPC contains a pozzolanic additive, which makes it slower-setting but more durable and cheaper than OPC.

(i) Portland cement

Portland cement is a finely powdered hydraulic binder made by burning a mixture of calcareous material (limestone) and argillaceous material (clay) at about 1400–1500 °C, and then grinding the resulting clinker with a small amount of gypsum. It is called "hydraulic" because it sets and hardens by reacting with water, and it sets hard like Portland stone.

(ii) Major components present in Portland cement

The major components (compounds) present in Portland cement are:

• Tricalcium silicate, $\mathrm{3CaO.SiO_{2}}$ (alite)
• Dicalcium silicate, $\mathrm{2CaO.SiO_{2}}$ (belite)
• Tricalcium aluminate, $\mathrm{3CaO.Al_{2}O_{3}}$
• Tetracalcium aluminoferrite, $\mathrm{4CaO.Al_{2}O_{3}.Fe_{2}O_{3}}$

The oxides from which these are formed are lime ($\mathrm{CaO}$), silica ($\mathrm{SiO_{2}}$), alumina ($\mathrm{Al_{2}O_{3}}$) and iron oxide ($\mathrm{Fe_{2}O_{3}}$).

(iii) Why gypsum is used in clinker during cement production

Gypsum ($\mathrm{CaSO_{4}.2H_{2}O}$) is added to the clinker (about 2–3%) to control the setting time of cement. Without gypsum, tricalcium aluminate reacts with water very fast and the cement sets within minutes (flash set), which makes it impossible to handle. Gypsum slows down this hydration and gives the cement a workable setting time.

(iv) Two instruments used for quality control of cement

1. Vicat apparatus – used to determine the setting time and consistency of cement paste.
2. Le Chatelier apparatus – used to test the soundness (volume stability) of cement.

Flowsheet diagram for the manufacture of Portland cement:

        Limestone (CaCO₃)          Clay (Al₂O₃·SiO₂·Fe₂O₃)
              |                              |
              +--------------+---------------+
                             |
                    Crushing & Grinding
                             |
                    Mixing (in correct proportion)
                             |
              ┌──────────────┴──────────────┐
              |                             |
        Wet Process                   Dry Process
        (slurry with water)          (dry powder)
              |                             |
              +──────────────┬──────────────+
                             |
                     Rotary Kiln
              (heating at 1400–1600 °C)
                             |
                     Clinker (2CaO·SiO₂, 3CaO·SiO₂,
                      3CaO·Al₂O₃, 4CaO·Al₂O₃·Fe₂O₃)
                             |
                     Cooling (in rotary cooler)
                             |
                     Grinding (with 2–3% gypsum)
                             |
                     Storage in silos
                             |
                     Packing in bags
                             |
                     Portland Cement

Key points:

• Raw materials: Limestone (source of CaO) and clay (source of SiO₂, Al₂O₃, Fe₂O₃), mixed in the ratio of about 3:1.
• Rotary kiln: The mixture is heated to 1400–1600 °C, where it fuses and forms clinker.
• Gypsum: 2–3% gypsum (CaSO₄·2H₂O) is added during grinding to regulate the setting time of cement.

Portland cement is a finely ground grey powder made by heating a mixture of calcareous material (limestone) and argillaceous material (clay) to about 1400–1500 °C in a rotary kiln, which produces clinker; the clinker is then cooled and ground with a small amount of gypsum. It is a hydraulic binder — it sets and hardens by reacting with water — and is the main binding material in concrete and mortar.

Gypsum (CaSO₄·2H₂O) is added (about 2–3% by mass) to the clinker during grinding to retard the setting time of the cement. Without gypsum, cement sets very quickly (flash set) and becomes unworkable before it can be placed. Gypsum slows down the hydration of tricalcium aluminate (C₃A), giving enough time for mixing, transporting and placing the concrete. It also slightly improves the strength and shrinkage properties of the set cement.

The four main components of cement and their functions are:

1. Tricalcium silicate (3CaO·SiO₂ or C₃S) — Provides early strength (within about 7 days) and is the main strength-giving compound of cement.

2. Dicalcium silicate (2CaO·SiO₂ or C₂S) — Provides strength over a longer period (after about 7 days) and contributes to the ultimate strength of cement.

3. Tricalcium aluminate (3CaO·Al₂O₃ or C₃A) — Causes the initial setting of cement (flash set) and accelerates the setting and hardening process.

4. Tetracalcium aluminoferrite (4CaO·Al₂O₃·Fe₂O₃ or C₄AF) — Helps in the formation of the solid mass and imparts colour (grey) to the cement; it also contributes to the strength of cement.

Quality control in the manufacture of Portland cement [2]

Quality control ensures that the raw materials and the finished cement meet the required chemical and physical standards. The main points are:

1. Control of raw materials: The limestone (calcium source) and clay (silica, alumina, iron oxide source) are analysed for their oxide composition before use. The ratio of these oxides is adjusted so that the raw mix has the correct lime saturation factor (LSF) and silica ratio, which decide the quality of the clinker.

2. Control during burning and grinding: The raw mix is finely ground and fed to the rotary kiln, where the burning temperature (about 1400–1500 °C) is carefully controlled to get well-burnt clinker. A small amount of gypsum (2–3%) is then added during grinding of the clinker to control the setting time of the cement.

3. Testing of the finished product: The final cement is tested for fineness, setting time, soundness, and compressive strength to confirm it meets the standard (e.g. IS specification) before it is packed and marketed.

(a) Retards setting action

Calcium sulphate (gypsum) is added to cement clinker in about 2–3% by mass to slow down the setting of cement; without it, cement would set too quickly to be workable.

On the basis of hardening and setting behaviour, cement is classified into two types:

1. Quick-setting cement — sets and hardens rapidly (e.g. quick-setting Portland cement, used for repair work and where early strength is needed).
2. Slow-setting cement — sets and hardens slowly over a longer period (e.g. ordinary Portland cement, used for general construction work).