Cement, Mortar and Concrete: Composition and Forensic Analysis
UGC-NET Paper 2 Unit VII notes on cement, mortar and concrete. Clinker phases C3S/C2S/C3A/C4AF, OPC/PPC/PSC, XRD, SEM-EDX, FTIR, and Indian building-collapse cases.
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Cement, mortar and concrete sit as a single bullet under UGC-NET Forensic Science Unit VII (Trace Evidence). The syllabus asks you to recall what these construction materials are made of, the chemistry of clinker and hydration, the Indian Standard cement types, and the instrumental workflow a forensic lab runs on a cement fragment. NTA likes this topic because every sub-part has a clean MCQ hook: alite is C3S, ettringite has three sulphates, OPC 53 is IS 12269, M20 means 20 MPa characteristic strength at 28 days.
Treat this as a short, fact-dense bullet bound by one story line. Clinker (four phases) hydrates to give C-S-H gel, portlandite and ettringite, that paste binds aggregate to make mortar or concrete, and the forensic lab fingerprints the resulting material by phase, morphology and bulk elements. The book companion on construction-materials forensics has not been written yet, so the inline links here cross over to the instrumental-techniques tree and to the explosion / arson sibling NET topic that most often pulls cement debris into a case.
- Portland cement
- Hydraulic cement patented by Joseph Aspdin in 1824. Made by sintering limestone and clay at around 1450°C to give clinker, then grinding with about 5 percent gypsum.
- Clinker
- Greyish nodules leaving the kiln, made of four crystalline phases: alite (C3S), belite (C2S), aluminate (C3A) and ferrite (C4AF).
- Alite (C3S)
- Tricalcium silicate, 3CaO.SiO2. Dominant clinker phase (50 to 70 percent). Hydrates fast, gives early strength up to 28 days.
- Belite (C2S)
- Dicalcium silicate, 2CaO.SiO2. 15 to 30 percent. Hydrates slow, gives long-term strength beyond 28 days.
- Aluminate (C3A)
- Tricalcium aluminate, 5 to 10 percent. Reacts violently with water, attacked by sulphate. SRC cement is low in C3A.
- C-S-H gel
- Calcium silicate hydrate, the main hydration binder. Amorphous, broad hump on XRD.
- Portlandite
- Calcium hydroxide Ca(OH)2, crystalline hydration by-product. Dehydrates at about 450°C on TGA.
- Ettringite
- Calcium trisulphoaluminate hydrate 3CaO.Al2O3.3CaSO4.32H2O. Needle-shaped under SEM, formed in the first hours from C3A and gypsum.
What cement, mortar and concrete actually are
One binder, two mixes, three different evidence types.
The three materials are a hierarchy, not three alternatives. Cement is the binder powder. Mortar is cement plus fine sand plus water, used to lay bricks and finish walls. Concrete is cement plus fine and coarse aggregate plus water plus optional admixtures, used as the structural matrix of buildings.
Portland cement (Aspdin 1824). Joseph Aspdin's 1824 patent describes burning a slurry of limestone and clay to a hard mass, grinding it, and selling it as "Portland cement" because the set mortar looked like Portland stone from Dorset. Modern Indian cement plants run essentially the same process at industrial scale.
Raw materials. A typical kiln feed is about 80 percent limestone (CaCO3, source of CaO), 15 percent clay or shale (source of SiO2, Al2O3, Fe2O3), small amounts of iron ore to balance the ferrite phase, and about 5 percent gypsum (CaSO4.2H2O) added after burning to control setting time.
Dry-process kiln (~1450°C). In the dry process used by most modern Indian plants, the raw meal is preheated in cyclone towers, calcined at about 900°C (CaCO3 to CaO + CO2), then sintered in a rotary kiln at around 1450°C. The product, clinker, is cooled, ground with gypsum, and bagged as cement.
Mortar. The site mix is one part cement, four to six parts sand, water to a workable consistency. Lime mortar (older buildings, conservation work) replaces some or all of the cement with hydrated lime.
Concrete. IS 456:2000 fixes design mixes by characteristic strength at 28 days in MPa. The common nominal mixes a candidate should memorise: M10 (1:3:6, 10 MPa, levelling course), M15 (1:2:4, 15 MPa, mass concrete), M20 (1:1.5:3, 20 MPa, workhorse RCC), M25 (1:1:2, 25 MPa, slabs and columns). Anything beyond M25 is design mix only, proportioned per IS 10262.
Clinker phases and hydration
Four phases in, four hydration products out.
The single most testable block in this topic is the four clinker phases and what they do on contact with water.
Hydration products. Water on ground clinker gives four products on overlapping timescales:
- Ettringite (AFt) in the first minutes to hours. C3A plus gypsum plus water gives needle-shaped 3CaO.Al2O3.3CaSO4.32H2O. Gypsum is there precisely to slow C3A; without it the cement flash-sets in seconds.
- C-S-H gel over hours to weeks. Alite and belite hydrate to amorphous calcium silicate hydrate, the actual glue holding concrete together. XRD shows it as a broad hump.
- Portlandite, Ca(OH)2, as a by-product of silicate hydration. Hexagonal plates, sharp XRD peaks, dehydrates at about 450°C on TGA. Portlandite gives fresh concrete its pH around 12 to 13, which protects rebar from rust.
- Monosulphate (AFm) after a day or so, as remaining C3A reacts with the early ettringite and gives plate-like calcium monosulphoaluminate.
Indian cement types and BIS standards
Five cements, five IS numbers, memorise them as a set.
BIS publishes a standard for every commercial cement type sold in India. NTA loves the IS-number questions.
| Cement | IS number | Composition (broad) | Key feature | Typical use |
|---|---|---|---|---|
| OPC 33 / 43 / 53 | IS 269 / 8112 / 12269 | Clinker (~95%) + gypsum (~5%) | Grade = 28-day compressive strength MPa | General RCC, plastering |
| PPC | IS 1489 | Clinker + 15-35% fly ash + gypsum | Pozzolanic, lower heat, more durable | Mass concrete, hydraulic structures |
| PSC | IS 455 | Clinker + 25-70% blast-furnace slag | Sulphate resistant, low heat | Marine works, sewage, mass pours |
| SRC | IS 12330 | Low-C3A clinker + gypsum |
Forensic significance
Building collapses, burglary fragments, blast debris, body-disposal cases.
Cement and concrete enter casework in four distinct fact patterns.
Building collapses. Cores from collapsed structures are tested to check whether the as-built mix met the design grade. The Mumbra building collapse near Thane in April 2013 killed 74 people and the investigation flagged substandard concrete and unauthorised additional floors. The Kanpur housing collapse (2018) and Greater Noida structural failures (2023) followed the same script: cores extracted, compressive strength tested at a NABL lab, mix design and W/C ratio reconstructed by petrographic thin section, and the result compared against the contractor's design submittal. Charges typically include BNS Section 109 (attempt to murder) when negligence amounts to dolus, and BNS Section 196 (criminal force / hurt).
Burglary and forced entry. A burglar drilling or chiselling through a wall picks up mortar and cement fragments on tools and clothing. Stereomicroscopy plus XRD can link a recovered tool or shoe to a particular wall by matching aggregate type, mortar composition and cement clinker fingerprint.
Explosion debris. A blast against a concrete structure produces fine cement particles which embed in clothing, hair and victim wounds. Recovery of these particles helps reconstruct the seat-of-blast and is read alongside chemical residue work covered in the investigation of explosion and arson cases NET bullet.
Concreted-body disposals. A recurring case type involves disposal of human remains inside a freshly cast concrete pillar, tank or slab. Forensic work covers the age of the concrete (carbonation depth, portlandite content, ettringite versus monosulphate ratio), the source of the cement, and the link to materials at the suspect's possession.
Examination workflow in an Indian forensic lab
From stereomicroscope to TGA in seven steps.
A fragment received in a sealed exhibit packet runs through a standard sequence at CFSL Chandigarh, the National Council for Cement and Building Materials (NCCBM) at Ballabhgarh, and state SFSL trace-evidence divisions.
- Stereomicroscopy
- Petrographic thin section (ASTM C856)
- X-ray diffraction (XRD)
- SEM-EDX
- FTIR / ATR-FTIR