Cement is responsible for a large share of the world’s industrial CO2, and most of it comes from one ingredient: clinker. LC3, short for limestone calcined clay cement, replaces up to half of that clinker with two cheap and widely available materials, calcined clay and ordinary limestone. India gave it its own standard in 2023, IS 18189, and it is now sold commercially. This guide explains what LC3 is, why the chemistry works, what IS 18189 actually requires, and where you should and should not use it.
| Cement | Main supplementary material | Clinker share |
|---|---|---|
| OPC (IS 269) | None (up to 5% additives) | about 95% with gypsum |
| PPC (IS 1489-1) | Fly ash 15 to 35% | about 65 to 85% |
| PSC (IS 455) | Slag 25 to 70% | about 30 to 75% |
| LC3 (IS 18189) | Calcined clay 10 to 35% + limestone 5 to 20% | 50 to 80% |
Why clinker is the problem
To make clinker, limestone is heated in a kiln to about 1,450 °C. Two things release CO2. The fuel burned to reach that temperature is one. The bigger one is chemistry: calcium carbonate breaks down into lime and CO2, and no amount of clean fuel stops that reaction. The only direct way to cut it is to use less clinker per tonne of cement. That is what fly ash in PPC and slag in PSC already do. The difficulty is that good fly ash and slag are limited by how much coal power and steel a region produces, and their supply will shrink as those industries decarbonise. Clay and limestone, by contrast, are available almost everywhere.
How LC3 works: the chemistry in plain words
LC3 relies on two reactions that help each other. The foreword to IS 18189 calls this “dual hydration”.
- Calcined clay is a pozzolan. Kaolinite clay heated to roughly 750 to 950 °C loses its bound water and turns into a disordered, reactive aluminosilicate called metakaolin. It reacts with the calcium hydroxide released by the clinker to form extra binding gel. Because clay contains no carbonate, calcining it releases water, not CO2, and it needs a much lower temperature than clinker.
- Limestone reacts with the alumina. Calcined clay is rich in alumina. Finely ground limestone reacts with that alumina to form carboaluminate phases, which fill pores and add strength. On its own, limestone is mostly a filler. Together with calcined clay, it becomes a reactive ingredient.
The result is a cement with a denser, finer pore structure than OPC at the same age, from about half the clinker. The 2:1 ratio of calcined clay to limestone seen in the common LC3-50 blend comes from balancing these two reactions.
What IS 18189:2023 requires
IS 18189:2023, “Portland Calcined Clay Limestone Cement – Specification”, was published by the Bureau of Indian Standards in June 2023. The main requirements are below. The numbers are from the standard’s tables. Always check your own copy before writing them into a specification.
| Requirement | IS 18189:2023 |
|---|---|
| Clinker (when clinker is used) | 50 to 80% |
| OPC (when 43 grade OPC is used instead of clinker) | 55 to 85% |
| Calcined clay | 10 to 35% |
| Limestone | 5 to 20% |
| Raw clay quality | At least 40% kaolinite |
| Calcination temperature | 750 to 950 °C |
| Limestone quality | At least 75% CaCO3 + MgCO3 |
| SO3, max | 3.5% |
| MgO, max | 6.0% |
| Loss on ignition, max | 10.0% |
| Chloride, max | 0.1% (0.05% for prestressed concrete) |
| Fineness, min | 400 m²/kg |
| Initial setting time, min | 30 minutes |
| Soundness (Le Chatelier), max | 10 mm |
| Compressive strength, min | 23 MPa (3 days), 33 MPa (7 days), 43 MPa (28 days) |
| Drying shrinkage, max | 0.15% |
| Bag marking | Must state the percentage of calcined clay and limestone |
Two points matter on site. First, the 40% kaolinite rule is the key quality control. Clays with too little kaolinite simply do not react enough, which is why plant location depends on finding the right clay. Second, the higher fineness requirement (400 m²/kg, compared with 225 m²/kg minimum for OPC under IS 269) reflects how finely calcined clay must be ground to react. LC3 often needs a little more water or admixture for the same workability, so do not change your mix without trial batches. Our guide to the water–cement ratio explains why adding water to fix workability costs you strength.
Enter a blend by mass. The default is the common LC3-50 mix.
Checks the composition ranges only, against clinker 50 to 80%, calcined clay 10 to 35% and limestone 5 to 20%. Check Table 1 of IS 18189 for exactly how gypsum is counted. A real cement must also pass the chemical and physical tests in the standard.
How much CO2 and money does LC3 save?
RMI, citing ClimateWorks, reports that LC3 can typically achieve emission reductions of up to 40% compared with OPC, and up to 25% lower overall cost for the LC3-50 blend. Real savings depend on the clinker factor, the kiln fuel used to calcine the clay and how far the clay is transported. A 2017 full-scale trial by JK Lakshmi Cement with IIT Delhi showed potential CO2 reductions of up to 30% and energy reductions of 20% in cement production. A fair way to state it: LC3 roughly matches the clinker savings of a good PPC, but it does not depend on fly ash supply.
LC3 vs OPC vs PPC: practical comparison
| Property | OPC 43 | PPC | LC3 |
|---|---|---|---|
| Clinker | About 95% | About 65 to 85% | 50 to 80% |
| Early strength | Highest | Lower at 3 and 7 days | Close to OPC 43 at 3 and 7 days by IS minimums |
| Chloride resistance | Lowest of the three | Good | Very good (fine pore structure) |
| Carbonation | Best resistance | Lower | Lower, needs good curing and cover |
| Heat of hydration | Highest | Lower | Lower |
| Raw material risk | Limestone, fuel | Fly ash quality and supply | Clay with 40%+ kaolinite |
The durability rows reflect what research on LC3 generally reports, not limits in the code. The main caution is carbonation. Because LC3 uses up much of the calcium hydroxide that protects steel from carbonation, it needs proper curing and the full cover that your code asks for. That is the same rule as for any blended cement. For a refresher on cover, see our guide on types of cement and their uses and how blended cements compare with fly ash in concrete.
Where to use LC3, and where to be careful
- Good fits: general RCC in houses and buildings, foundations, masonry mortar and plaster, precast blocks, and marine or coastal work where chloride resistance helps.
- Be careful: work that needs very high early strength for fast form stripping, and precast or prestressed elements, unless the supplier gives test data for your mix. Remember the stricter 0.05% chloride limit for prestressed concrete.
- Always: run trial mixes, check slump retention with your admixture, and cure properly for at least the period your specification asks for blended cement.
LC3 in India and around the world
LC3 was developed by EPFL in Switzerland with partners including IIT Delhi, IIT Madras and IIT Bombay. In India, RMI notes more than 25 applications since 2014, including a model house in Jhansi built with 98% LC3, and JK Lakshmi Cement and TARA began calcined clay production in 2022. LC3 products are now on the market from Indian manufacturers. Outside India, Cuba built a pilot plant in 2020, and Holcim reports a calcined clay cement from its Saint-Pierre-la-Cour plant in France with a CO2 footprint 50% lower than CEM I. Europe’s EN 197-5 (2021) added new low-clinker cement types, and in the US, ternary blended cements are covered by ASTM C595.
LC3 vs geopolymer: which is the future?
They solve the same problem in different ways. Geopolymer concrete removes Portland clinker completely but needs alkaline activators, careful handling and, often, heat curing, so it is mostly used in precast. LC3 is still a Portland-based cement, is made in an ordinary cement plant and is used on site exactly like PPC. For most builders, that makes LC3 the easier step today. Read our comparison of geopolymer vs traditional concrete for the other side, and our explainer on carbon capture concrete for a third route.
Composition, chemical and physical limits come from the text of IS 18189:2023 as published by BIS. We read a scanned copy, so a few table values came through OCR; confirm them against your official copy before specifying. CO2, cost and deployment facts come from RMI’s 2023 LC3 article and the sources it cites. Fly ash and slag ranges are from IS 1489-1 and IS 455. The durability comparison summarises published research trends and is not a code requirement. This article is general information. Your structural engineer and cement supplier should approve any change of cement in a design mix. Last reviewed November 2026.
Sources: IS 18189:2023 (BIS, via Public.Resource.Org); RMI, Unleashing the potential of LC3; RMI, LC3 deployment in India; RILEM TC-282 CCL review of standards; CemNet, JK Lakshmi calcined clay production.
Frequently asked questions
What is LC3 cement?
What is the IS code for LC3 cement?
Is LC3 as strong as OPC?
How much CO2 does LC3 save?
Is LC3 better than PPC?
Can LC3 be used for RCC?
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