Cement, glass and ceramics industries

In short

Cement, glass and ceramics are high-temperature, mineral-based process industries: solid raw materials (limestone, clay, sand, soda ash, feldspar) are ground, heated to 1,200-1,600 °C to form new crystalline or glassy phases, then shaped, cooled and finished. They are among India's largest employers of chemical engineers in solids handling, combustion, heat transfer, kiln and furnace operation, utilities and environment management — India is the world's second-largest cement producer (installed capacity above 600 MTPA, output roughly 440 Mt a year; figures revise annually). The topic is examined in GATE Chemical Engineering (Chemical Technology), in university "Process Industries / Chemical Technology" papers, and in interviews at UltraTech, Ambuja-ACC, Shree, Dalmia Bharat, JK Cement, Saint-Gobain, Asahi India Glass, HNG, Kajaria and the Morbi ceramic cluster. Unit operations dominate: size reduction, pneumatic conveying, cyclone separation, calcination, sintering, melting, drying and firing, plus bag filters and ESPs.

Explained

Raw materials and the chemistry of Portland cement

• Feed: ~80-85% limestone (CaCO₃), 10-15% clay/shale/laterite (SiO₂, Al₂O₃, Fe₂O₃), plus correctives — bauxite for alumina, iron ore/mill scale for iron, sand for silica. Gypsum (3-5%) is added only at the final grinding stage.
• Roughly 1.3 t of limestone and 1.5-1.6 t of raw meal per tonne of clinker (loss on ignition ≈ 35%, almost all CO₂). Limestone mining is governed by the MMDR Act, 1957 (as amended).
• Four clinker minerals (cement chemists' notation C = CaO, S = SiO₂, A = Al₂O₃, F = Fe₂O₃, H = H₂O):
  • C₃S (alite, 45-65%) — main strength up to 28 days, high heat of hydration.
  • C₂S (belite, 10-30%) — slow, gives late strength, low heat.
  • C₃A (3-12%) — flash set if ungypsumed, very high heat, poor sulphate resistance.
  • C₄AF (6-12%) — little strength, gives the grey colour, acts as flux.
• Control moduli: lime saturation factor (LSF, 0.92-0.98), silica modulus (SM, 2.0-3.0) and alumina modulus (AM, 1.0-2.5) are computed on the raw mix in the control lab every hour from XRF analysis.
• Hydration: C₃S and C₂S + water → C-S-H gel (the binder) + Ca(OH)₂; gypsum reacts with C₃A to form ettringite, which retards setting. Setting is chemical reaction, not drying.

Cement manufacture — the dry preheater/precalciner route

1. Crushing and prehomogenisation: limestone crushed to <25 mm in impact/hammer crushers; stacker-reclaimer blending beds average out composition.
2. Raw grinding and drying: vertical roller mill or ball mill dries the mix with kiln exhaust gases to <1% moisture and grinds to ~10-15% residue on 90 µm; blending silos homogenise.
3. Preheating: meal is fed to a 5- or 6-stage cyclone preheater in counter-current with kiln gases; clay dehydroxylates at 400-600 °C.
4. Calcination: in the precalciner at 850-900 °C, CaCO₃ → CaO + CO₂ (strongly endothermic, ~1,780 kJ/kg CaCO₃); 90-95% calcination is achieved before the kiln.
5. Rotary kiln (sintering): 3-5 m diameter, 50-80 m long, 3-4% slope, 3-4 rpm, refractory-lined. Burning-zone gas ~1,800-2,000 °C, material ~1,450 °C; a liquid phase (from C₃A, C₄AF) forms and alite crystallises — the clinkering reaction.
6. Cooling: grate cooler quenches clinker from ~1,350 °C to below 100 °C; rapid cooling preserves reactive C₃S and prevents C₂S inversion. Hot cooler air is recuperated as secondary/tertiary combustion air.
7. Finish grinding: clinker + gypsum (+ fly ash or slag) ground in a ball mill with closed-circuit separator, VRM or roller press to 280-400 m²/kg Blaine; packed in 50 kg bags or sold bulk.
8. Wet process (slurry feed, long kiln, 5,200-6,700 kJ/kg clinker) is obsolete in India; semi-dry nodulised feed on a travelling grate (Lepol) survives in a few old plants. Modern dry precalciner lines run ~2,900-3,200 kJ/kg (700-760 kcal/kg) clinker.
9. Key hazards and nuisances: alkali-sulphur-chloride cycles cause preheater build-up and coating rings (a chlorine bypass is used); dust, high-temperature surfaces and confined-space silo entry are the main safety risks, now under the Occupational Safety, Health and Working Conditions Code (in force 21 November 2025, replacing the Factories Act, 1948).

Types of cement, Indian standards and tests

• IS 269:2015 now specifies OPC 33, 43 and 53 grades in one standard (the earlier separate IS 8112 and IS 12269 were withdrawn/merged); the grade number is the minimum 28-day compressive strength in MPa on standard mortar cubes.
• IS 1489 (Part 1) — Portland Pozzolana Cement, fly-ash based (fly ash 15-35%, conforming to IS 3812); IS 455 — Portland Slag Cement (GGBS up to 70%, slag to IS 12089/IS 16714); IS 16415 — composite cement (clinker + fly ash + slag).
• Special cements: IS 8041 rapid hardening, IS 12330 sulphate-resisting (C₃A limited), IS 12600 low-heat, IS 6452 high-alumina cement (titled "for structural use"; also the binder in refractory castables), IS 8042 white cement (iron kept below ~0.4%), oil-well cement.
• Blended cements dominate India (PPC + PSC + composite are roughly three-quarters of despatches) because they cut clinker factor to ~0.7, lower heat of hydration, improve durability and reduce CO₂.
• Tests (IS 4031 series, physical; IS 4032, chemical): fineness by Blaine air-permeability (min 225 m²/kg for OPC, 300 m²/kg for PPC) or 90 µm sieve; consistency of standard paste (Vicat); setting time (initial ≥30 min, final ≤600 min for OPC); soundness by Le Chatelier (≤10 mm) and autoclave expansion (≤0.8%); compressive strength at 3, 7 and 28 days on 70.6 mm cubes of 1:3 cement:standard sand mortar.
• Cement is under mandatory BIS certification (ISI mark) by Quality Control Order under the BIS Act, 2016. GST on cement is 18% since the 22 September 2025 rationalisation (earlier 28%).

Glass — composition, melting and forming

• Glass is an inorganic product of fusion cooled to a rigid state without crystallising: no sharp melting point, only a glass transition range; it is a supercooled liquid with short-range order.
• Constituents: network formers (SiO₂, B₂O₃, P₂O₅); fluxes/modifiers (Na₂O from soda ash, K₂O, Li₂O) that break Si-O-Si bridges and lower melting temperature; stabilisers (CaO from limestone, MgO from dolomite, Al₂O₃ from feldspar) that restore chemical durability; minors — salt cake (Na₂SO₄) as a refining agent, carbon as reducer, Se/Co/Fe oxides for colour, cullet (recycled glass, 20-80% of batch) to save energy.
• Typical soda-lime glass: SiO₂ 70-74, Na₂O 12-15, CaO 8-12, MgO 0-4, Al₂O₃ 0.5-2 wt%. Borosilicate (SiO₂ ~80, B₂O₃ ~13) has α ≈ 3.3 × 10⁻⁶/K — labware and Borosil. Others: lead crystal (PbO, high refractive index), aluminosilicate (fire-resistant), fused silica, glass-ceramics, E-glass fibre.
• Melting: continuous regenerative end-port or side-port furnace, 1,500-1,580 °C, 4-6 GJ per tonne of glass; zones are melting → fining (bubble removal) → conditioning/working end at ~1,100-1,200 °C. Refractories: silica crown, AZS (alumina-zirconia-silica) sidewalls, fused-cast throat.
• Forming routes: float process (ribbon poured on a molten tin bath under reducing atmosphere, 1,100 → 600 °C, thickness 0.4-25 mm set by ribbon speed and top rollers — all flat glass today); blow-and-blow / press-and-blow on IS machines for bottles and jars; drawing for tubes and fibres; pressing for tableware; rolling for patterned glass.
• Annealing (lehr) removes residual stress by holding above the annealing point (viscosity 10¹² Pa·s) then cooling slowly — skipping it leaves glass that cracks on cutting. Tempering instead chills the surface rapidly to put it in compression (~70-100 MPa), raising strength 4-5× and giving harmless dice on breakage; laminated glass bonds plies with PVB. Safety glazing is specified in IS 2553 (Part 1 architectural, Part 2 automotive); float glass in IS 14900.
• Glass is 100% recyclable without quality loss; 1 kg of cullet replaces ~1.2 kg of batch and saves melting energy and CO₂.

Ceramics — whitewares, tiles and refractories

• Ceramics are inorganic non-metallic solids consolidated by firing. Classes: traditional (clayware, whiteware, tiles, sanitaryware, refractories, cement) and advanced/technical (Al₂O₃, ZrO₂, SiC, Si₃N₄, piezo and bio-ceramics).
• Triaxial whiteware body: ~50% clay (kaolin + ball clay — plasticity and green strength), ~25% feldspar (flux: forms glass at 1,100-1,250 °C), ~25% quartz (filler: controls shrinkage, gives skeleton). Varying the ratio gives earthenware → stoneware → vitreous china → porcelain (decreasing porosity).
• Process sequence: raw-material beneficiation → wet ball milling to slip → magnetic separation and screening → spray drying (for tile presses) or filter pressing/pugging → shaping by slip casting (sanitaryware), dry pressing (tiles), extrusion (pipes, honeycombs), jiggering/jollying (plates, cups) → drying (control shrinkage to avoid warping and cracks) → biscuit firing → glazing (spray/dip) → glost firing in a roller-hearth or tunnel kiln → sorting and grading.
• Firing chemistry: 100-200 °C free water; 450-650 °C kaolinite → metakaolin (dehydroxylation); ~980 °C spinel/mullite nucleation; 1,100-1,400 °C mullite (3Al₂O₃·2SiO₂) needles grow in a feldspathic glass — this gives strength; quartz inversions at 573 °C (α-β) demand slow cooling to avoid dunting.
• Ceramic tiles — IS 15622:2017, classified by shaping (B = dry pressed) and water absorption: Group BIa E ≤ 0.5% (vitrified/porcelain), BIb 0.5-3%, BIIa 3-6%, BIIb 6-10%, BIII > 10% (wall tiles). Test methods in IS 13630 series; vitreous-china sanitary appliances in IS 2556 series.
• Refractories — materials that resist heat, load and chemical attack. By chemistry: acid (silica, fireclay, zircon), basic (magnesite, dolomite, chrome-magnesite), neutral (alumina, chromite, carbon, SiC). By form: shaped bricks vs monolithics (castables, gunning mixes, ramming masses — now most of the market).
• Refractory properties tested (IS 1528 series): PCE / refractoriness (pyrometric cone equivalent), RUL (refractoriness under load), permanent linear change on reheating, apparent porosity and bulk density, cold crushing strength, thermal spalling resistance, slag resistance, thermal conductivity (low for insulating, high for recuperators).
• A refractory must be chemically compatible with its slag — a silica (acid) brick fails fast against basic slag, and magnesite against acid slag.

Energy, emissions and Indian regulation

• Cement energy: thermal 700-760 kcal/kg clinker, electrical 65-80 kWh/t cement (grinding is ~70% of power). Cement plants are designated consumers under the PAT scheme of the Energy Conservation Act, 2001 (amended 2022) and now come under the Carbon Credit Trading Scheme.
• CO₂: ~0.51 t/t clinker from calcination alone plus 0.2-0.3 t from fuel; India's average is roughly 0.55-0.60 t CO₂ per tonne of cement, among the world's lowest, mainly through blending. Levers: clinker-factor reduction, waste heat recovery (WHRS) power, alternative fuels and raw materials (AFR), and future CCUS and LC3 (limestone calcined clay cement).
• Co-processing: cement kilns destroy hazardous and plastic waste at >1,400 °C with long residence time, approved under CPCB guidelines and the Hazardous and Other Wastes Rules; thermal substitution rate in India is still only a few per cent against 30-60% in Europe.
• Air pollution control: ESPs and bag houses on kiln, cooler, raw mill and cement mill stacks. Under the MoEFCC cement-industry standards, particulate limits are about 30 mg/Nm³ for newer plants (older vintages 100-150 mg/Nm³), with NOx and SO₂ limits that depend on plant age and feed sulphur — the notified figures have been amended more than once, so check the current notification before quoting.
• Glass and ceramic furnaces face NOx (oxy-fuel firing, low-NOx burners) and fluoride/boron emissions; the Morbi cluster's shift from coal gasifiers to natural gas/PNG was a direct air-quality intervention. Silica dust exposure in ceramic and refractory plants is a notified occupational disease and is regulated under the OSH Code, 2020 (in force 21 November 2025).

Key points

• Clinker - the nodular intermediate from the kiln; cement = ground clinker + gypsum (+ pozzolana/slag).
• Dry precalciner process - the only route built today: 5-6 stage preheater, precalciner, short kiln, grate cooler.
• Calcination - CaCO₃ → CaO + CO₂ at 850-900 °C; the single largest heat demand and CO₂ source.
• Clinkering/sintering - 1,450 °C material temperature where liquid phase forms and C₃S crystallises.
• Bogue equations - estimate C₃S, C₂S, C₃A, C₄AF from oxide analysis; they assume ideal equilibrium, so they are estimates.
• LSF - lime saturation factor, 0.92-0.98; too high leaves free lime (unsoundness), too low loses strength.
• Gypsum - retards flash set of C₃A by forming ettringite; 3-5% addition at the cement mill.
• Blaine fineness - specific surface in m²/kg; higher fineness = faster strength but more grinding power.
• Grade of cement - minimum 28-day mortar-cube strength in MPa; IS 269:2015 covers 33, 43 and 53 grade OPC.
• Clinker factor - tonnes of clinker per tonne of cement; ~0.70 in India thanks to PPC/PSC.
• Glass - fusion product cooled without crystallising; has a transition range, not a melting point.
• Former / flux / stabiliser - SiO₂ builds the network, Na₂O lowers melting point, CaO restores durability.
• Float process - flat glass formed on molten tin, giving both faces optically parallel.
• Annealing vs tempering - annealing removes stress; tempering deliberately creates surface compression.
• Triaxial body - clay + feldspar + quartz ≈ 50:25:25 for whiteware.
• Mullite - needle-like 3Al₂O₃·2SiO₂ formed above ~1,100 °C; source of fired strength.
• Water absorption - the classifier for tiles (IS 15622); ≤0.5% = vitrified.
• Acid/basic/neutral refractory - must match the slag chemistry or it is chemically eaten away.
• PCE and RUL - refractoriness without load (cone equivalent) and under load; RUL is the service-relevant one.

Formulas and examples

Example 1 — clinker phases and moduli. Clinker analysis: CaO 65.0, SiO₂ 21.5, Al₂O₃ 5.2, Fe₂O₃ 3.0 wt%.

• C₃S = 4.0710(65.0) − 7.6024(21.5) − 6.7187(5.2) − 1.4297(3.0) = 264.62 − 163.45 − 34.94 − 4.29 = 61.9%
• C₂S = 2.8675(21.5) − 0.7544(61.9) = 61.65 − 46.70 = 14.9%
• C₃A = 2.6504(5.2) − 1.6920(3.0) = 13.78 − 5.08 = 8.7%; C₄AF = 3.0432(3.0) = 9.1%
• LSF = 65.0/(2.80×21.5 + 1.18×5.2 + 0.65×3.0) = 65.0/68.29 = 0.952 (in range); SM = 21.5/8.2 = 2.62; AM = 5.2/3.0 = 1.73. A sound, high-alite clinker suitable for 53-grade OPC.

Example 2 — kiln material and fuel balance. A 5,000 t/day clinker line, raw-meal LOI 35%, heat consumption 730 kcal/kg clinker, coal GCV 5,500 kcal/kg, clinker CaO 65%.

• Raw meal = 1/(1 − 0.35) = 1.538 t/t clinker → 5,000 × 1.538 = 7,690 t/day of meal (≈ 6,500 t/day limestone at 85% of mix).
• Coal = 730/5,500 = 0.1327 kg/kg clinker = 132.7 kg/t → 5,000 × 0.1327 = 664 t/day coal.
• Calcination CO₂ = 0.65 × 44.01/56.08 = 0.510 t/t clinker; coal CO₂ at ~2.3 t CO₂/t coal ≈ 0.133 × 2.3 = 0.305 t/t → total ≈ 0.82 t CO₂/t clinker. At a clinker factor of 0.70, that is ≈ 0.57 t CO₂ per tonne of cement.

Example 3 — glass batch calculation. Compute the batch for 100 kg of simplified soda-lime glass, SiO₂ 74, Na₂O 14, CaO 12 wt% (minors ignored). Sand is 99.5% SiO₂; soda ash Na₂CO₃ (M = 105.99) → Na₂O (61.98); limestone CaCO₃ (100.09) → CaO (56.08).

• Sand = 74/0.995 = 74.4 kg
• Soda ash = 14 × 105.99/61.98 = 14 × 1.710 = 23.9 kg (releases 9.9 kg CO₂)
• Limestone = 12 × 100.09/56.08 = 12 × 1.785 = 21.4 kg (releases 9.4 kg CO₂)
• Batch total = 74.4 + 23.9 + 21.4 = 119.7 kg for ~100 kg glass → batch-to-glass yield ≈ 83.6%, CO₂ released 19.3 kg. Replacing 50% of this with cullet cuts both the gas release and melting energy.

Example 4 — refractory brick properties. A fireclay brick: dry mass D = 2,000 g, soaked mass S = 2,090 g, suspended mass W = 1,180 g.

• Apparent porosity = (2,090 − 2,000)/(2,090 − 1,180) × 100 = 90/910 = 9.9%
• Bulk density = 2,000/910 = 2.20 g/cm³; water absorption = 90/2,000 = 4.5%
• Low porosity like this resists slag penetration; an insulating brick would deliberately show 45-75% porosity and ~0.8 g/cm³.

Example 5 — shrinkage and thermal shock. A tile test bar is 100.0 mm plastic, 94.0 mm after drying, 90.0 mm after firing.

• Drying shrinkage = 6/100 = 6.0%; firing shrinkage = (94 − 90)/94 = 4.3%; total = 10.0% — so the die must be oversized by ~11.1% (100/90) to give a 90 mm fired piece.
• Thermal shock, soda-lime (σ = 50 MPa, E = 70 GPa, α = 9 × 10⁻⁶/K, ν = 0.22): ΔT = 50×10⁶ × 0.78/(70×10⁹ × 9×10⁻⁶) = 62 K.
• Borosilicate (same σ and ν, E = 64 GPa, α = 3.3 × 10⁻⁶/K): ΔT = 39×10⁶/(2.112×10⁵) = 185 K — three times better, which is why lab glassware is borosilicate.

Where it is used

• Cement plants (UltraTech, Ambuja-ACC, Shree, Dalmia Bharat, Ramco, JK Cement, Nuvoco): process engineer roles in kiln/CCR operation, raw-mix design, heat and mass balance, WHRS, AFR co-processing and pollution control.
• Flat and container glass (Saint-Gobain Sriperumbudur, Asahi India Glass, Gujarat Guardian, HNG, Borosil, Piramal/PGP Glass): furnace and batch-house engineering, melting-energy optimisation, cullet handling, forming-line yield.
• Tiles and sanitaryware (Kajaria, Somany, Orient Bell, Cera, Hindware and the ~1,000-unit Morbi cluster in Gujarat): body and glaze formulation, spray dryer and kiln control, fuel conversion to PNG.
• Refractories (Dalmia-OCL, IFGL, Vesuvius India, RHI Magnesita's Orient Refractories): design and supply of linings for cement kilns, steel ladles, glass tanks and incinerators.
• Technical ceramics and supporting industries: catalyst supports and honeycombs, ceramic membranes, grinding media, abrasives, insulators (BHEL, Aditya Birla Insulators), and solar/auto glass processing.
• Allied roles: environment and safety officers handling consent to operate, stack monitoring and PAT/carbon reporting; equipment makers (FLSmidth, thyssenkrupp Polysius, Walchandnagar, L&T) in project engineering and commissioning.

Interview questions

• Q: Why has the dry process replaced the wet process for cement? A: The wet process evaporates 30-40% slurry water, needing roughly twice the fuel (5,200-6,700 vs ~3,000 kJ/kg clinker); preheaters and precalciners recover kiln gas heat instead.
• Q: What exactly happens in the rotary kiln that does not happen in the precalciner? A: The precalciner only decarbonates the meal at 850-900 °C; the kiln raises it to ~1,450 °C, where a liquid phase forms and alite (C₃S) crystallises — the actual clinkering reaction.
• Q: Why is gypsum added, and what if it is omitted or overdosed? A: It retards C₃A hydration via ettringite; without it the cement flash-sets, and with excess SO₃ delayed ettringite formation causes expansion and unsoundness — hence the SO₃ cap in IS 269.
• Q: Which clinker phase gives early strength and which gives late strength? A: C₃S (alite) up to 28 days with high heat of hydration; C₂S (belite) beyond 28 days with low heat.
• Q: How would you lower the CO₂ intensity of a cement plant? A: Cut the clinker factor with fly ash, slag or calcined clay (LC3); raise the thermal substitution rate with AFR; install WHRS and higher-efficiency grinding; longer term, oxy-fuel or CCUS.
• Q: Why is clinker cooled rapidly? A: To freeze in reactive C₃S and fine C₃A, avoid C₂S converting to the inert γ form, and recover hot air as secondary combustion air.
• Q: Why is glass floated on tin and not on another metal? A: Tin is molten and unreactive over 232-1,100 °C, denser than glass and has low vapour pressure; a nitrogen-hydrogen reducing atmosphere prevents tin oxide forming on the ribbon.
• Q: Difference between annealed, toughened and laminated glass? A: Annealed is slow-cooled and stress-free but breaks in shards; toughened is rapidly chilled so the surface is in compression, 4-5× stronger, breaking into blunt dice; laminated bonds plies with PVB so fragments stay on the interlayer.
• Q: Why does soda-lime glass contain lime at all? A: Soda alone gives a water-soluble glass ("water glass"); CaO and MgO act as stabilisers and restore chemical durability.
• Q: What is the role of each component of a triaxial ceramic body? A: Clay gives plasticity and green strength, feldspar fluxes and forms the glassy bond on firing, quartz is a non-plastic filler that limits shrinkage and provides the skeleton.
• Q: Would you line a steel ladle slag zone with silica brick? A: No — steelmaking slag is basic and would react with acid silica; the slag zone needs magnesia-carbon or doloma refractory.
• Q: What does "vitrified tile" mean technically? A: A dry-pressed tile with water absorption ≤0.5% (Group BIa in IS 15622), i.e. fired to near-zero open porosity, so it is frost-resistant and stain-resistant without glaze protection.

Common mistakes

• Applying the standard Bogue equations when AM < 0.64 (ferrite-rich clinker) — a different set applies, and free lime must be deducted from CaO first.
• Confusing calcination (850-900 °C, CO₂ release) with clinkering/sintering (1,450 °C, phase formation), or quoting gas temperature as material temperature.
• Writing "cement sets by drying" — hydration is a chemical reaction; concrete must be cured wet, not dried.
• Treating glass as having a melting point; it has a transition range, and viscosity (not temperature alone) decides every forming step.
• Mixing up annealing and tempering, or forgetting that toughened glass cannot be cut or drilled after treatment.
• Forgetting that batch mass exceeds glass mass by the CO₂ lost from soda ash and limestone (~15-20%) — batch calculations must be done on an oxide basis.
• Quoting PCE as the service limit for a refractory: PCE is measured without load, and RUL under load is always much lower.

Related topics

Fertilisers and inorganic chemicals · Size reduction and mechanical operations · Heat transfer in furnaces and kilns · Fuels, combustion and waste heat recovery · Air pollution control and environmental regulation · Materials science: phase diagrams and ceramics