Chapter 2 of 3

Cement & Admixtures

In the GATE Civil syllabus under Construction Materials · 2 parts

Cement — Manufacture, Composition, Types & Tests · Concrete Mix Design & Admixtures

CIVILGYAN TEST SERIES · civilgyantests.com — free study notes for civil engineering exams

📑 Contents (16 sections)

Part 1 of 2

Cement — Manufacture, Composition, Types & Tests

Last reviewed 16 Sept 2026 · 11 min read

Portland cement

Cement is a finely ground hydraulic binder that sets and hardens by chemical reaction with water and remains stable under water. Joseph Aspdin patented Portland cement in 1824, naming it after its resemblance to Portland stone.

Raw materials

Material	Examples	Supplies
Calcareous	Limestone, chalk, marl, shells	Lime (CaO)
Argillaceous	Clay, shale, slate	Silica (SiO₂), alumina (Al₂O₃), iron oxide (Fe₂O₃)
Corrective materials	Bauxite, iron ore, sand	Adjust alumina, iron or silica
Gypsum (added to clinker during grinding)	Calcium sulphate	Retards setting — prevents flash set due to C₃A
Fuel	Coal, petcoke, alternative fuels	Heat

Manufacture

Processes

Wet process	Dry process
Raw materials ground with water to form a slurry (about 35–40% water)	Raw materials dried and ground to fine raw meal
High fuel consumption (evaporation of water)	Lower fuel consumption — used by modern plants with preheaters and precalciners
Better homogenisation (older technology)	Needs efficient blending; faster
Longer kilns	Shorter kilns with suspension preheaters

Burning in the rotary kiln

The kiln is a long inclined steel cylinder lined with refractory bricks, rotating slowly; material moves down towards the hot end.

Zone	Temperature (approx.)	Reactions
Drying zone	Up to about 100–200 °C	Evaporation of free water
Preheating / calcination zone	About 600–900 °C	Dehydration of clay; calcination of limestone (CaCO₃ → CaO + CO₂)
Burning (clinkering) zone	About 1400–1500 °C	Lime combines with silica, alumina and iron oxide to form clinker compounds; partial fusion
Cooling zone	Clinker cooled rapidly	Rapid cooling improves quality (glassy phases, less free lime)

The clinker (dark nodules, a few mm to 25 mm) is cooled, stored and ground with about 3–5% gypsum (and other additions for blended cements) to a fine powder, then stored in silos and packed (commonly 50 kg bags) or dispatched in bulk.

Composition of cement

Oxide composition (typical ranges for OPC)

Oxide	Approx. %	Remarks
Lime (CaO)	60–67	Excess causes unsoundness (free lime); deficiency reduces strength
Silica (SiO₂)	17–25	Strength (forms silicates)
Alumina (Al₂O₃)	3–8	Quick setting; lowers clinkering temperature
Iron oxide (Fe₂O₃)	0.5–6	Colour, fluxing, contributes to C₄AF
Magnesia (MgO)	0.1–4	Excess causes unsoundness (limited by standards)
Sulphur trioxide (SO₃)	1–3	From gypsum — controls setting; excess causes unsoundness
Alkalis (Na₂O, K₂O)	0.2–1.3	Efflorescence, alkali–aggregate reaction

Bogue compounds

Compound	Formula (cement notation)	Approx. % in OPC	Properties
Tricalcium silicate (alite)	C₃S — 3CaO·SiO₂	40–60	Hydrates rapidly — early strength (first 7–14 days); high heat of hydration (about 500 J/g)
Dicalcium silicate (belite)	C₂S — 2CaO·SiO₂	15–30	Hydrates slowly — later strength; low heat (about 260 J/g); better resistance to chemical attack
Tricalcium aluminate	C₃A — 3CaO·Al₂O₃	5–10	Reacts very fast — flash set (controlled by gypsum); highest heat (about 870 J/g); vulnerable to sulphate attack
Tetracalcium aluminoferrite (celite)	C₄AF — 4CaO·Al₂O₃·Fe₂O₃	8–12	Low strength contribution; gives grey colour; moderate heat

FormulaBogue's equations (percentages by mass; no free lime)

C_{3} S = 4.071 C a O - 7.600 S i O_{2} - 6.718 A l_{2} O_{3} - 1.430 F e_{2} O_{3} - 2.852 S O_{3}

C_{2} S = 2.867 S i O_{2} - 0.7544 C_{3} S

C_{3} A = 2.650 A l_{2} O_{3} - 1.692 F e_{2} O_{3}

C_{4} A F = 3.043 F e_{2} O_{3}

Part 2 of 2

Concrete Mix Design & Admixtures

Last reviewed 16 Sept 2026 · 9 min read

Concrete mix design

Mix design (mix proportioning) is the process of selecting suitable ingredients and determining their relative proportions to produce concrete of required strength, workability and durability as economically as possible.

Requirements

Fresh concrete: workability suited to placing and compaction; no segregation or excessive bleeding.
Hardened concrete: specified characteristic compressive strength; durability for the exposure condition (maximum w/c, minimum cement content, cover).
Economy: minimum cement content consistent with the above (lower cost, less heat and shrinkage).

Nominal versus design mixes

Nominal mix	Design mix
Fixed proportions (e.g. 1 : 1.5 : 3) irrespective of materials	Proportions determined from properties of actual materials
May be uneconomical and variable in strength	Economical and reliable
Permitted for concrete up to M20	Required for M25 and above

Factors governing mix design

Grade (characteristic strength) and degree of quality control (standard deviation); type and grade of cement; exposure conditions; maximum nominal size, shape and grading of aggregates; workability (slump); type of mixing and placing; use of admixtures; maximum and minimum cement content.

IS 10262:2019 method

Step 1 — Target strength

FormulaTarget mean strength

f_{c k}^{'} = f_{c k} + 1.65 s or f_{c k}^{'} = f_{c k} + X

whichever is greater.

Grade	Assumed standard deviation $s$ (N/mm²)	Factor $X$ (N/mm²)
M10, M15	3.5	5.0
M20, M25	4.0	5.5
M30 to M60	5.0	6.5
M65 and above	6.0	8.0

(Actual standard deviation from site data is used where available.)

Step 2 — Water–cement ratio

Selected from established relationships (curves in the code or the producer's experience) between free w/c ratio and 28-day compressive strength for the cement used.
Checked against durability — must not exceed the maximum w/c ratio for the exposure condition in IS 456; the lower value governs.

Step 3 — Water content

For 20 mm nominal maximum size (angular coarse aggregate) and 50 mm slump, the base free water content is about 186 kg/m³ (about 208 kg/m³ for 10 mm and 165 kg/m³ for 40 mm aggregate).
Increase about 3% for every additional 25 mm slump above 50 mm.
Reduce for rounded/sub-angular aggregates and for water-reducing admixtures (plasticisers about 5–10%, superplasticisers about 20% or more, as established by trials).

Step 4 — Cement (binder) content

Cement content = \frac{water content}{w / c}

Check against the minimum cement content for exposure (IS 456) and the maximum (commonly 450 kg/m³ OPC); if below the minimum, increase cement and recompute w/c. With fly ash or slag, the total cementitious content and replacement limits are considered.

Step 5 — Proportion of coarse aggregate

The volume of coarse aggregate per unit volume of total aggregate is taken from the code table for the maximum size of coarse aggregate and the grading zone of fine aggregate, for a w/c ratio of 0.50 — e.g. for 20 mm aggregate: about 0.60 (Zone I), 0.62 (Zone II), 0.64 (Zone III) and 0.66 (Zone IV).

Adjust by ±0.01 for every ∓0.05 change in w/c ratio (lower w/c → more coarse aggregate).
Reduce (e.g. by about 10%) for pumpable concrete.

Step 6 — Mix calculations (absolute volume method)

Per cubic metre of concrete:

FormulaAbsolute volume calculations

Volume of entrapped air: about 1.0% for 20 mm aggregate (about 1.5% for 10 mm, 0.8% for 40 mm)
Volume of cement $= \frac{mass of cement}{G _{c} \times 1000}$
Volume of water $= \frac{mass of water}{1000}$
Volume of admixture $= \frac{mass of admixture}{G _{a d m} \times 1000}$
Volume of all-in aggregate $= 1 - (air + cement + water + admixture)$
Mass of coarse aggregate $=$ volume of aggregate × CA fraction × $G_{C A}$ × 1000
Mass of fine aggregate $=$ volume of aggregate × FA fraction × $G_{F A}$ × 1000

Step 7 — Trial mixes and adjustments

Trial mix 1 checked for workability (slump) and absence of segregation; water/admixture adjusted keeping w/c constant.
Further trials at w/c ratios varying by about ±10% of the chosen value; cubes tested at 7 and 28 days.
The final mix is selected from the strength–w/c relationship.
Field adjustments for moisture content and absorption of aggregates (aggregate masses in design are in SSD condition).

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