Cement: Manufacture,
Chemistry & Hydration Science
01 Manufacture of Portland Cement
Raw materials: calcareous material (limestone/chalk — source of CaO) + argillaceous material (shale/clay — source of SiO₂, Al₂O₃, Fe₂O₃).
Core process (common to all methods):
- Grinding and proportioning raw materials as per required chemical composition
- Burning in a rotary kiln at 1300–1500°C — material sinters/partially fuses to form nodular clinker
- Cooling the clinker (rotary cooler, controlled conditions)
- Grinding clinker finely with 3–5% gypsum added — gypsum controls setting time, prevents flash set
1.1 Wet Process
Raw materials are ground with water into a pumpable slurry (35–50% water content, creamy consistency, particles finer than IS Sieve No. 9). Slurry is homogenised in agitated storage/blending tanks, then fed to the kiln against hot hanging chains that recover heat and dry the slurry into flakes before burning.
Advantage: better/easier control of raw mix composition. Disadvantage: very high fuel consumption (drying large amounts of water).
1.2 Dry Process
Raw materials are crushed and ground dry, blended with compressed air into homogeneous raw meal, then pelletised in a rotating disc (granulator) using ~12% water before kiln feed. Requires far less fuel since no bulk water needs evaporating.
Semi-dry process: a variant — raw material ground dry, then mixed with 10–14% water and burnt to clinkering temperature (a compromise between wet and dry).
1.3 Quality Control of Clinker
Two principal direct methods are used to assess clinker quality:
- Reflected-light optical microscopy of polished & etched clinker sections, followed by point-count analysis of the area occupied by each compound.
- X-ray diffraction (XRD) of powdered cement, compared against calibration curves of known pure-compound mixtures.
A simpler, rough-and-ready field indicator is the litre weight (bulk density) of clinker — about 1200 g/litre is considered satisfactory; litre weight is a quick proxy for overall clinker quality.
Effect of cooling rate on strength — data (Enkegaard) shows compressive strength is sensitive to how clinker is cooled:
| Cement type | Cooling rate | 3-day | 7-day | 28-day |
|---|---|---|---|---|
| Normal cement | Quick | 9.9 | 15.3 | 26 |
| Normal cement | Moderate | 9.7 | 21.0 | 27 |
| Normal cement | Slow | 9.7 | 19.3 | 24 |
| Normal cement | Very slow | 8.7 | 18.7 | 23 |
| High early strength | Quick | 10.2 | 18.8 | 29 |
| High early strength | Moderate | 14.2 | 26.7 | 33 |
| High early strength | Slow | 10.2 | 21.0 | 29 |
| High early strength | Very slow | 9.1 | 18.1 | 28 |
02 Chemical Composition & Bogue's Compounds
Oxide composition limits of ordinary Portland cement are controlled within an approximate ratio (Lime Saturation Factor concept): CaO / (2.8·SiO₂ + 1.2·Al₂O₃ + 0.65·Fe₂O₃) is kept close to but not exceeding 1 — too much lime leaves free lime uncombined (causes unsoundness); too much silica (relative to alumina/iron) makes the mix hard to fuse into clinker.
On burning, the oxides recombine into four principal Bogue compounds:
| Compound | Formula | Mineral name (Tornebohm) | Typical % | Role |
|---|---|---|---|---|
| Tricalcium silicate | C3S | Alite | ~45% | Early strength, high heat |
| Dicalcium silicate | C2S | Belite | ~25% | Later strength, low heat |
| Tricalcium aluminate | C3A | Celite | small | Flash set risk, no strength |
| Tetracalcium alumino-ferrite | C4AF | Felite | small | No strength, colour |
C3S + C2S together make up 70–80% of cement and are the compounds mainly responsible for strength. Both Le Chatelier and Tornebohm independently identified four crystal types in clinker thin-sections; Tornebohm's mineral names (Alite/Belite/Celite/Felite) map directly onto Bogue's C3S/C2S/C3A/C4AF.
A high total alumina + ferric oxide content favours high early strength (helps completely combine the available lime into tricalcium silicate).
03 Hydration of Cement
Hydration = the chemical reaction between cement and water; anhydrous cement has no binding property until hydrated.
Two hydration mechanisms are believed to co-exist:
- Through-solution mechanism: cement compounds dissolve to form a supersaturated solution, from which hydrated products precipitate. Dominates in the early stages when plenty of water is available.
- Solid-state (topochemical) mechanism: water attacks solid compounds directly from the surface inward. Dominates in later stages.
Key reactions:
- 2 C3S + 6H → C3S2H3 (C-S-H gel) + 3 Ca(OH)2 (by weight: 100 + 24 → 75 + 49)
- 2 C2S + 4H → C3S2H3 (C-S-H gel) + Ca(OH)2 (by weight: 100 + 21 → 99 + 22)
C3S produces less C-S-H but more Ca(OH)2; C2S produces more C-S-H but less Ca(OH)2 — an important reason C2S-rich cements are preferred for hydraulic/sulphate-exposed structures.
Complete hydration is very slow: it cannot be achieved in under a year unless cement is very finely ground and reground with excess water. After 28 days, hydration typically penetrates only about 4 microns into a grain; full hydration is realistically possible only for grains smaller than 50 microns.
04 Heat of Hydration
The cement–water reaction is exothermic. This matters greatly in mass concrete (dams, thick foundations), where internal temperature can rise ~50°C above ambient and stay elevated for a long period — risking thermal cracking.
Rate-of-heat-liberation curve typically shows two peaks:
- Peak A (first few minutes): rapid initial reaction of aluminates and sulphates; subsides quickly once gypsum depresses aluminate solubility.
- Peak B (a few hours later): due to ettringite formation and the main C3S hydration reaction.
Since heat of hydration is an additive property, it can be predicted from: H = aA + bB + cC + dD, where A, B, C, D are the % contents of C3S, C2S, C3A, C4AF and a, b, c, d are each compound's per-percent heat contribution.
| Compound | 3 days | 90 days | 13 years |
|---|---|---|---|
| C3S | 58 | 104 | 122 |
| C2S | 12 | 42 | 59 |
| C3A | 212 | 311 | 324 |
| C4AF | 69 | 98 | 102 |
Order of heat contribution: C3A > C4AF > C3S > C2S. Since retarders (gypsum) control C3A's flash-heat release, the early heat of hydration in practice is mainly contributed by C3S. Fineness affects the rate of heat development, not the total heat released.
Typical normal cement produces about 89–90 cal/g at 7 days and 90–100 cal/g at 28 days.
05 Calcium Silicate Hydrate (C-S-H) Gel
C-S-H makes up 50–60% of the solid volume of a fully hydrated paste — the single most important strength-giving product. It is termed a 'gel' because it is not a well-defined crystalline compound but a poorly-crystalline, fibrous mass (sometimes called tobermorite gel for its resemblance to the natural mineral tobermorite).
Two historical theories on the nature of the hydration product:
- Le Chatelier's crystalline theory: the hydration products are interlocked crystals.
- Michaelis's colloidal theory: the hydration products are a colloidal, gelatinous mass.
Modern understanding accepts elements of both — the product is gel-like but made of extremely small, poorly-formed fibrous crystals, seen as a bundle of fibres with refractive index 1.5–1.55, increasing with age.
Gel porosity ≈ 28%; gel pores are filled with water; specific surface of cement gel ≈ 2 million cm² per gram — can be measured by capillary condensation or mercury porosimetry.
C3S gives a comparatively lower-density, slightly inferior C-S-H (but faster, giving early strength); C2S gives a denser, higher specific-surface C-S-H (slower, giving long-term strength).
06 Calcium Hydroxide — Ca(OH)₂
The second hydration product; unlike C-S-H it forms a distinct hexagonal prism crystal, and constitutes 20–25% of the solid volume of hydrated paste.
- Drawback: soluble in water → leaches out → increases concrete porosity, especially problematic in hydraulic structures.
- Sulphate attack: Ca(OH)2 reacts with sulphates (from soil/water) to form calcium sulphate, which further reacts with C3A causing expansive deterioration of concrete.
- Remedy: blending with pozzolanic materials (fly ash, silica fume, etc.) converts free Ca(OH)2 into additional cementitious C-S-H, reducing its harmful effect.
- Advantage: being alkaline, it maintains concrete pH around 13, which passivates and protects steel reinforcement from corrosion.
07 Calcium Aluminate Hydrates
C3A hydrates extremely fast (can cause flash set) — controlled by adding gypsum at the clinker-grinding stage. The stable cubic compound formed is C3AH6 (stable up to ~225°C). Hydrated aluminates contribute nothing to strength and are harmful to sulphate durability, though they may add a little to very early strength due to their rapid reaction.
In the presence of gypsum, two possible crystalline products form depending on the sulphate/aluminate ratio in solution:
- Ettringite — calcium aluminate trisulphate hydrate (C6AS̄3H32): forms first, as needle-like prismatic crystals, during the first hour of hydration when the sulphate/aluminate ratio in solution is high.
- Monosulphate — calcium aluminate monosulphate hydrate (C4AS̄H18): as sulphate in solution depletes and aluminate concentration rises again (renewed C3A/C4AF hydration), ettringite becomes unstable and converts to monosulphate — the final stable product in cements with more than 5% C3A.
The aluminate-to-sulphate (A/S̄) ratio balance governs normal setting behaviour; an imbalance can cause abnormal setting phenomena (false set, flash set) — of real practical importance in mix design.
C4AF hydrates to a CaO–Fe2O3–H2O system (stable form ~C3FH6); also contributes nothing to strength, but its hydrates show better sulphate resistance than calcium aluminate hydrates.
08 Structure of Hydrated Cement Paste & Transition Zone
Concrete is generally modelled as a two-phase material: the paste phase and the aggregate phase. The paste phase controls most engineering properties — strength, permeability, durability, shrinkage, elastic behaviour, and creep — far more than the aggregate phase does.
At the microscopic scale, a third phase appears around large aggregate particles: the Transition Zone — the interfacial region between coarse aggregate and hardened paste.
- Quality of paste here is poorer than bulk paste, mainly due to internal bleeding (water accumulating under elongated/flaky/large aggregate particles), which weakens the paste–aggregate bond.
- Crystalline products (Ca(OH)2, ettringite) grow larger here than in bulk paste.
- Drying shrinkage/temperature change causes microcracks in the transition zone even before any load is applied; under stress these propagate into larger cracks, causing bond failure.
- Because of this, the transition zone is the weakest link and is usually the strength-limiting phase — concrete typically fails at a lower stress than either the bulk paste or the aggregate would individually withstand.
Fresh paste is a plastic mix of water + cement. As hydration proceeds, unhydrated cement decreases while hydration products increase; after roughly a month, hydrated paste is typically ~85–90% hydration products and ~10–15% unreacted cement. Continuous-monitoring techniques used to study this evolution include setting-time and strength measurement, heat-of-hydration (conduction calorimetry), optical/electron microscopy, and X-ray diffraction.
09 Water Requirement for Hydration
Estimated water demand for the two main silicate compounds:
- C3S requires ~24% water by weight of cement for chemical reaction
- C2S requires ~21% water by weight of cement
- Average for Portland cement: ~23% bound water (chemically combined water)
- An additional ~15% gel water is needed to fill the gel pores
- Total ≈ 38% water by weight of cement for complete hydration + full gel-pore filling (in a sealed/closed system)
If water supplied equals exactly 38%, the paste fully hydrates with no undesirable capillary cavities. If water exceeds 38%, the excess remains as capillary cavities/pores — the more the excess, the greater the capillary porosity, and the weaker/more permeable the hardened concrete.
w/c ratio and pore structure: at lower w/c, cement particles are closer together, so hydration products can more easily bridge the gaps and fill the space once occupied by water. This works well up to w/c ≈ 0.6. Above w/c ≈ 0.7, the volume of hydration product is never enough to fill all the voids created by the original mixing water — such concrete remains permanently porous.
Modern High-Performance Concrete (HPC) is typically made with w/c around 0.25 — again leaving a considerable unhydrated core, with only surface hydration occurring; the unreacted cement core functions as very fine aggregate within the hydrated matrix.
10 Quick Revision
| Parameter | Value / Fact |
|---|---|
| Portland cement patented | 21 Oct 1824 — Joseph Aspdin |
| Kiln burning temperature | 1300–1500 °C |
| Gypsum added at grinding | 3–5% |
| Wet process slurry water | 35–50% |
| Dry process granulator water | ~12% |
| Clinker size | 3–20 mm nodules |
| Clinker litre weight (quality) | ~1100–1300 g/l (1200 g/l = good) |
| Coal use — dry process | ~100 kg / ton cement |
| Coal use — wet process | ~350 kg / ton cement |
| C3S content (avg.) | ~45% |
| C2S content (avg.) | ~25% |
| C3S + C2S | 70–80% of cement (strength givers) |
| C-S-H gel — % of solid volume | 50–60% |
| Ca(OH)2 — % of solid volume | 20–25% |
| Gel porosity | ~28% |
| Gel specific surface | ~2,000,000 cm²/g |
| Refractive index of C-S-H | 1.5–1.55 |
| Bound (chemical) water needed | ~23% by weight of cement |
| Gel-pore water needed | ~15% by weight of cement |
| Total water for full hydration | ~38% by weight of cement |
| Max w/c for pores to fully fill | ~0.6 (never fills fully above 0.7) |
| pH of concrete pore solution | ~13 (from Ca(OH)2) — protects rebar |
| Depth of hydration after 28 days | ~4 microns |
| Max grain size for full hydration | <50 microns |
| Heat contribution ranking | C3A > C4AF > C3S > C2S |
| Normal cement heat of hydration | 89–90 cal/g (7d), 90–100 cal/g (28d) |
| Abrams' extreme strength result | 280 MPa at w/c = 0.08 |
| Typical HPC water/cement ratio | ~0.25 |
11 Sample Viva / Exam Questions
Why is gypsum added while grinding clinker?
To retard the very fast hydration of C3A and prevent flash set, giving workable setting time.
Why does C2S give better long-term durability than C3S?
C2S hydrates slowly, releases less heat, and produces a denser C-S-H gel with less Ca(OH)2, which is less prone to leaching/sulphate attack — hence better suited for mass and hydraulic structures.
What is the transition zone and why is it important?
It is the interfacial region between coarse aggregate and bulk paste, weakened by internal bleeding and larger crystal growth; it is usually the weakest link, governing overall concrete strength and failure.
Why is Ca(OH)2 considered undesirable, yet also useful?
It is soluble and leaches out (causing porosity) and reacts with sulphates to cause sulphate attack — undesirable. But being alkaline, it maintains pH ~13, which protects reinforcement from corrosion — useful.
What is the difference between ettringite and monosulphate?
Both are calcium aluminate sulphate hydrates. Ettringite (trisulphate) forms first when sulphate is abundant relative to aluminate; as sulphate depletes, it converts to monosulphate, the final stable product in cements with >5% C3A.
Why does wet process consume more fuel than dry process?
The wet process slurry contains 35–50% water that must be evaporated in the kiln, requiring far more fuel (~350 kg coal/ton) than the dry process (~100 kg coal/ton), where raw meal is already dry.
What happens if water used is more than 38% by weight of cement?
The paste still hydrates fully, but the excess water remains as capillary cavities/pores once it is not consumed for hydration or gel-pore filling — leading to greater porosity and lower strength/durability.
Why can concrete never be 100% hydrated at practical w/c ratios above 0.7?
Because the volume of hydration product formed is insufficient to fill all the space originally occupied by the mixing water once w/c exceeds ~0.7, leaving the paste permanently porous.
What are Alite, Belite, Celite and Felite?
Tornebohm's microscopic mineral names for the four Bogue compounds: Alite = C3S, Belite = C2S, Celite = C3A, Felite = C4AF.
How is the heat of hydration of a cement predicted from its compound composition?
Using the additive relation H = aA + bB + cC + dD, where A, B, C, D are percentage contents of C3S, C2S, C3A, C4AF, and a, b, c, d are the respective heat-contribution coefficients per 1% of each compound.
What are the two raw material types used in cement manufacture?
Calcareous (limestone/chalk — source of CaO) and argillaceous (shale/clay — source of SiO₂, Al₂O₃, Fe₂O₃).
What temperature range does the rotary kiln operate at?
1300–1500°C.
Why is gypsum added at the grinding stage?
To control setting time and prevent flash set caused by rapid C₃A hydration. Added at 3–5%.
Compare wet vs dry process on water content and fuel use.
Wet: 35–50% slurry water, ~350 kg coal/ton. Dry: ~12% water in granulator, ~100 kg coal/ton.
What is the semi-dry process?
Raw material ground dry, then mixed with 10–14% water before burning to clinkering temperature.
What does litre weight of clinker indicate, and what's a good value?
Clinker quality — ~1200 g/litre (range 1100–1300) is considered satisfactory.
Name two direct methods of clinker quality control.
Reflected-light optical microscopy (point-counting mineral areas) and X-ray diffraction (against calibration curves).
How does cooling rate affect strength?
Moderate cooling generally gives the best 28-day compressive strength, especially for high-early-strength cement; both quick and very slow cooling reduce strength.
What are the four Bogue compounds and their approximate percentages?
C₃S (~45%, Alite), C₂S (~25%, Belite), C₃A (small, Celite), C₄AF (small, Felite).
Which compounds are mainly responsible for strength, and what % do they form?
C₃S and C₂S, together 70–80% of cement.
What happens if lime content is too high in raw mix?
Free lime remains uncombined in clinker → causes unsoundness in cement.
How does high alumina + ferric oxide content affect cement?
Favors high early strength — helps combine all available lime into tricalcium silicate.
What are the two mechanisms of cement hydration?
Through-solution (dissolve → precipitate, dominant early) and solid-state (surface-inward attack, dominant later).
Write the hydration reaction of C₃S.
2C₃S + 6H → C₃S₂H₃ (C-S-H gel) + 3Ca(OH)₂.
Write the hydration reaction of C₂S.
2C₂S + 4H → C₃S₂H₃ (C-S-H gel) + Ca(OH)₂.
Which compound produces more Ca(OH)₂ — C₃S or C₂S?
C₃S produces more Ca(OH)₂ and less C-S-H; C₂S produces more C-S-H and less Ca(OH)₂.
How deep does hydration typically penetrate a cement grain after 28 days?
About 4 microns.
What is the maximum grain size for complete hydration under normal conditions?
Less than 50 microns.
Why is heat of hydration important in mass concrete?
Can raise internal temperature ~50°C above ambient for a prolonged period, risking thermal cracking.
What causes Peak A and Peak B in the heat liberation curve?
Peak A — initial aluminate/sulphate reaction (minutes). Peak B — ettringite formation + C₃S reaction (hours later).
Rank the four compounds by ultimate heat of hydration.
C₃A > C₃S > C₄AF > C₂S.
What's the typical heat of hydration of normal cement at 7 and 28 days?
~89–90 cal/g at 7 days, 90–100 cal/g at 28 days.
What % of solid volume does C-S-H gel occupy?
50–60%.
Name the two historical theories of gel structure.
Le Chatelier's crystalline theory vs Michaelis's colloidal theory.
What is the refractive index of C-S-H gel?
1.5–1.55, increasing with age.
What is the porosity and specific surface of cement gel?
Porosity ≈ 28%; specific surface ≈ 2 million cm²/gram.
What % of solid volume does Ca(OH)₂ occupy, and what shape does it form?
20–25%; forms a distinct hexagonal prism crystal.
What is the main drawback of Ca(OH)₂ in concrete?
Soluble, leaches out causing porosity; reacts with sulphates to cause sulphate attack (attacks C₃A).
What is the one advantage of Ca(OH)₂?
Keeps pore solution alkaline (pH ≈ 13), protecting reinforcement from corrosion.
How is excess Ca(OH)₂ managed in modern concrete?
By blending with pozzolanic materials (fly ash, silica fume) that convert it into additional C-S-H.
What stable compound forms from C₃A hydration, and up to what temperature is it stable?
C₃AH₆, stable up to ~225°C.
What is ettringite and when does it form?
Calcium aluminate trisulphate hydrate (C₆AS̄₃H₃₂); forms first as needle-like crystals when sulphate/aluminate ratio in solution is high (first hour).
What is monosulphate and when does it form?
Calcium aluminate monosulphate hydrate (C₄AS̄H₁₈); forms as sulphate depletes and aluminate rises again — final stable product in cements with >5% C₃A.
What governs normal setting behavior in cement?
The aluminate-to-sulphate (A/S̄) ratio balance.
How does C₄AF's sulphate resistance compare to C₃A hydrates?
C₄AF hydrates show better sulphate resistance than calcium aluminate hydrates.
What are the two (or three) phases of concrete?
Paste phase and aggregate phase (bulk model); transition zone is a third, thinner interfacial phase.
Why is the transition zone weaker than bulk paste?
Internal bleeding causes water accumulation under aggregate particles, and crystalline products (Ca(OH)₂, ettringite) grow larger there.
Why is the transition zone called the "weakest link"?
Microcracks form there from shrinkage/temperature even before loading; under stress they propagate first, so concrete fails at lower stress than either paste or aggregate alone.
How much water does C₃S need vs C₂S for hydration?
C₃S ~24%, C₂S ~21% (by weight of cement).
What is the total water requirement for full hydration + gel-pore filling?
~38% (23% bound/chemical water + 15% gel-pore water).
What happens if water exceeds 38% by weight of cement?
Excess remains as undesirable capillary cavities — more excess = more porosity = weaker concrete.
Up to what w/c ratio can hydration products fully fill voids?
Up to about w/c ≈ 0.6; above w/c ≈ 0.7, voids can never be fully filled.
What did Abrams' extreme low w/c experiment show?
At w/c = 0.08 (with very high compaction), strength up to 280 MPa was achieved — only surface hydration occurs, unhydrated core acts as fine aggregate.
What w/c ratio is typical for High Performance Concrete?
~0.25.