Codes referenced
Standard Indian mix design — supports SCM use, accommodates ternary blends and LC3 in trials.
The three SCMs covered for cement substitution. IS 3812 (fly ash), IS 16714 (GGBS), IS 15388 (silica fume).
Covers natural aggregates; recycled aggregates fall under experimental provisions until a dedicated standard emerges.
Product Category Rules for concrete — the European reference for Environmental Product Declarations.
GRIHA credits for low-carbon concrete and LCA documentation; MoEFCC Construction & Demolition Waste Management Rules drive RCA availability.
What is embodied carbon?
Embodied carbon is the sum of greenhouse-gas emissions (expressed as kgCO₂e — carbon dioxide equivalent) released in producing a material, cradle-to-gate. For concrete, it covers raw-material extraction (limestone, clay, fuel), cement manufacturing, aggregate processing, mixing water, admixtures, and transport to site. A typical Indian M30 mix with 100% OPC has embodied carbon of 280-320 kgCO₂e/m³; higher grades (M50, M60) range 350-450. The single largest contributor is cement — typically 70-80% of the total.
Cement's carbon footprint
OPC has an embodied carbon of about 0.83-0.95 kgCO₂e per kg of cement (India average ~0.85). Two processes drive this: calcining limestone (CaCO₃ → CaO + CO₂) which releases chemically-bound CO₂, and burning the kiln at 1450°C on fossil fuel. Per-tonne emissions: ~0.55 kgCO₂e from calcination, ~0.30 kgCO₂e from fuel and electricity. Switching to PPC (fly ash blended) drops the figure to 0.55-0.65; PSC (slag blended) to 0.40-0.55; LC3 to 0.35-0.45.
SCM substitution — the cheapest first move
Substituting SCMs for OPC is the most accessible carbon reduction. Three common SCMs and their per-kg savings vs OPC:
Saves ~0.6 kgCO₂e per kg substituted. Widely available from thermal power plants; IS 3812 covers specification. Typically 25-35% replacement.
Saves ~0.5 kgCO₂e per kg substituted. From steel plants; IS 16714 covers specification. IS 10262:2019 Table 9 recommends 25-50% by mass of total cementitious materials; a chloride-exposed project specification usually asks for more. Contributes to long-term strength.
Saves ~0.4 kgCO₂e per kg substituted. By-product of silicon metal production; IS 15388. 5-10% replacement, used in HPC and durability-critical mixes.
Alternative cements — LC3 and CSA
Beyond SCM substitution at the concrete plant, two cement technologies lower the clinker factor:
- LC3 (Limestone Calcined Clay Cement): combines clinker, calcined clay, limestone, and gypsum. Clinker factor drops from ~0.95 (OPC) to ~0.50-0.60, with comparable strength. Reduces CO₂ per kg cement by 30-40%. Pioneered by IIT Madras and TARA, now piloted globally.
- Calcium Sulfoaluminate (CSA) cement: ye'elimite-based, lower kiln temperature (1250°C vs 1450°C), lower limestone content. Embodied carbon ~30% of OPC. Used for shrinkage-compensating and rapid-hardening applications; emerging for general structural concrete.
- Geopolymers: alkali-activated fly ash / GGBS / slag, no clinker at all. Research-stage for structural concrete; not yet a routine RMC plant input.
Recycled concrete aggregate (RCA)
C&D waste in India is estimated at 150-200 million tonnes/year. MoEFCC's Construction & Demolition Waste Management Rules (2016, amended 2024) push recycling. Processed RCA can replace natural coarse aggregate at 20-30% in structural concrete with minimal strength penalty, saving ~10 kgCO₂e/m³ (the avoided extraction and transport). Higher replacements reduce strength 10-20% and increase water demand. IS 383 covers natural aggregates; RCA use is covered under experimental provisions pending a dedicated Indian standard.
GRIHA & green building credits
GRIHA (Green Rating for Integrated Habitat Assessment) is India's national green-building rating, with Manual Volume 5 the latest criteria set. Credits exist for low-embodied-carbon concrete (typically 30% reduction vs OPC baseline), for EPD submission, for use of recycled materials, and for life-cycle assessment. For GRIHA 5-star certification, EPD documentation per EN 16757 is increasingly expected. LEED v4.1 and IGBC also reward low-carbon concrete, but GRIHA remains the most widely adopted Indian rating.
EPD — Environmental Product Declaration
An EPD is a third-party-verified document reporting a product's environmental impact per a defined Product Category Rule (PCR). For concrete in Europe, EN 16757 is the PCR; for India, BIS is aligning a national PCR but most projects accept EN 16757 EPDs. An EPD for a concrete mix reports kgCO₂e/m³ plus acidification, eutrophication, and ozone-depletion indicators. EPDs are increasingly required in government and large private tenders; suppliers without EPDs will be filtered out at the bid stage.
M30 mix embodied carbon — worked example
For an M30 mix with 350 kg OPC + 100 kg GGBS = 450 kg binder, 720 kg sand, 1180 kg coarse, 175 L water, 4 L admixture — cradle-to-gate emissions: OPC 350 × 0.85 = 297 kgCO₂e; GGBS 100 × 0.40 = 40; aggregate 1900 × 0.005 = 9.5; water 175 × 0.001 = 0.2; admixture 4 × 1.5 = 6. Total ~252 kgCO₂e/m³. For comparison, a 100% OPC mix gives 290-310; 50% GGBS + 30% fly ash blend drops to ~180.
Source: per-kg emission factors from ICE database v3, India-specific electricity grid mix, and supplier-published cement EPDs.
Worked example — three M30 mixes compared
| Mix | Binder | kgCO₂e/m³ | 28-day strength |
|---|---|---|---|
| A — OPC only | 400 kg OPC 53 | ~290 | 38 MPa |
| B — 30% fly ash | 280 OPC + 120 FA | ~225 (-22%) | 36 MPa |
| C — 50% GGBS | 220 OPC + 220 GGBS | ~205 (-29%) | 40 MPa at 28 d (52 at 90 d) |
| D — LC3 + 30% RCA | LC3 450 kg + 30% RCA coarse | ~150 (-48%) | 38 MPa — lowest carbon |
Mix D is the lowest embodied carbon achievable today for an M30 in India. Mix B is the most common "first step" — fly ash availability is the highest among SCMs.
Common deviations seen on Indian sites
Many Indian tenders do not yet require EPDs, but government and large private projects increasingly do. Submitting a mix design with an EPD gives a competitive edge even when not mandatory — and prepares the supplier for future regulations.
RCA is often rejected on quality concerns without proper trial mixes. 20-30% coarse RCA replacement works in M25-M40 with negligible strength loss — trials prove it; assumption does not.
Even when low-carbon mixes are used, no record is kept. Without cradle-to-gate tracking, the carbon reduction cannot be claimed for GRIHA / LEED credits. Capture mix design, supplier EPDs, and volumes delivered.
LC3 is in pilot production. Specifying LC3 without confirming local supply leads to project delays. Verify availability with regional cement suppliers before tender.
Field-checklist blocks
A short questionnaire, an inspector's checklist, and three worked examples that show how a low-carbon target turns into a mix you can actually buy, batch and claim.
Questionnaire — ask before the mix design is frozen for tender
- What is the baseline the reduction is measured against — a named reference mix with its binder composition, or the 280–320 kgCO₂e/m³ band for an M30 in 100% OPC? A percentage without a baseline is not a claim.
- Which binder can the plant near the site actually supply, and at what volume? Fly ash, GGBS and LC3 are not interchangeable on availability, and LC3 is still in pilot production.
- Has the actual blend been trial-mixed at the actual replacement level, with the strength results in hand — 28-day, and 90-day where the blend is GGBS-heavy?
- Is the exposure class fixed for the member? IS 456 Cl 5.2 permits GGBS as a part replacement of cement and states no percentage, and IS 10262:2019 Table 9 recommends 25–50% by mass of total cementitious materials; the chloride-exposed project specification sets the level above that. Durability sets the carbon options, not the budget.
- Where does the carbon figure stop — at the plant gate, or after transport to site? EN 16757 PCR defines the boundary, and a number quoted outside it is not comparable.
- Do you hold the supplier EPDs for the cement, the aggregates and the admixture that the claim depends on?
- Where is the volume record kept that ties each delivered mix to a per-m³ figure? Without it the reduction is a design value, not a project value.
Inspector's checklist — what to verify on site and in the file
- Baseline mix document: binder composition in kg/m³ and its stated kgCO₂e/m³, with the source noted (ICE database v3, the India-specific electricity grid mix, supplier-published cement EPDs).
- Emission factor sheet: the per-kg factors actually used — OPC at 0.85, and the substitution savings of 0.6 for fly ash, 0.5 for GGBS, 0.4 for silica fume — each with its source and date.
- Mix design with the SCM split and the trial-mix strength results, at 28 days and at 90 days where the blend calls for it.
- Written supply confirmation from the regional cement supplier for the specific grade and volume, with the plant named. For LC3 this check comes before the specification, not after.
- RCA trial-mix record where recycled aggregate is proposed, with the processed-source evidence (crushed, sieved, washed). RCA sits under the experimental provisions of IS 383 until a dedicated Indian standard exists.
- Supplier EPDs on file, referenced to the EN 16757 PCR.
- Delivery record: mix code, volume in m³ per delivery note, tied to the mix design version that was batched.
- Carbon summary sheet: the per-m³ figure for each mix actually used and the total, signed and dated by the person who owns the GRIHA or LEED submission.
What happens if… three worked examples
What happens. Measured against Mix A at ~290 kgCO₂e/m³, a 30% reduction means 290 × 0.70 = 203 kgCO₂e/m³. Mix C at ~205 sits 2 kgCO₂e/m³ above that line, so it misses a 30% target. Mix D at ~150 clears it by 53. Mix B at ~225 is a 22% reduction and lands short as well.
Likely outcome. Either the target moves to 25% and Mix C is specified with confidence, or Mix D is specified and the LC3 supply and the RCA trial both have to be closed out first. The dangerous version of this decision is Mix D specified on paper while the plant quietly batches Mix B, because LC3 availability is the open item.
Preventive correction. Fix the availability position before the target is negotiated. A 30% number agreed in a tender meeting and an OPC mix delivered at the gate is a dispute waiting for the volume records to expose it.
What happens. The same 120 kg of substitution against 120 kg of OPC removes 120 × 0.6 = 72 kgCO₂e/m³ with fly ash, and 120 × 0.5 = 60 kgCO₂e/m³ with GGBS. The gap is 12 kgCO₂e/m³. Fly ash is also the easier material to source, since thermal power stations produce it in volume, while GGBS ties the plant to steel-plant slag.
Likely outcome. Fly ash for the general case, and GGBS where chloride or sulphate exposure rules the decision. Exposure is not a preference here: IS 456 Cl 5.4 requires GGBS at 50% or more for chloride exposure, and GGBS brings a lower chloride diffusion coefficient with it. A fly ash blend chosen for carbon alone will fail the durability review.
Preventive correction. Set the exposure class first, then compare the carbon options that survive it. Silica fume at 0.4 kgCO₂e/kg saved is the smallest saving of the three and belongs in HPC and durability-critical mixes, not in a bulk volume carbon play.
What happens. Mix C delivers ~205 kgCO₂e/m³ against ~290 for Mix A, a 29% reduction on the mix design. But the mix design is not a project record. With no delivery volume tied to the mix code, no supplier EPD on file and no cradle-to-gate tracking, there is nothing to hand the assessor, and the GRIHA or LEED credit for the low-carbon concrete cannot be claimed.
Likely outcome. The reduction happened and cannot be evidenced. This is the commonest failure of all four, because it costs nothing on site and costs the credit later. The remedy is procedural rather than technical: mix design version, delivery volumes and EPDs captured as the concrete is placed.
Preventive correction. Start the record with the first truck. Every later month is reconstruction; the first month is a download from the plant and the site.
Frequently Asked Questions
What is the lowest-embodied-carbon concrete available in India today?
Is recycled concrete aggregate (RCA) durable enough for structural concrete?
What is the commercial availability of LC3 cement in India?
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Amit Haridas
Founder & Proprietor, ConcreteInfo. 25+ years experience in construction QA/QC, concrete technology, RMC plant operations, consulting and technical training across India. For low-carbon mix design review, EPD documentation support or LC3 / RCA trial-mix supervision, contact amit@concreteinfo.in.
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