What the Mill Certificate Doesn't Tell You

A cement tanker reaches the dam site. The mill test certificate in the driver's folder is clean. The fineness is in range, the initial and final setting times are in range, the soundness passes, the 28-day strength is comfortably over grade, the chemical limits are all within IS 269. The site lab signs it in. Three lifts later, the embedded thermocouples are reading hotter than the last pour from the same plant, and nobody changed the mix design. The cement changed. The certificate did not show it, because the two properties that govern how much heat this concrete will generate are not on a standard Indian cement certificate.

The first is the tricalcium aluminate content, written C3A. The second is the heat of hydration itself. Neither is a required line on a mill test certificate for ordinary Portland cement under IS 269. C3A is reported and capped only for sulphate resisting Portland cement, where IS 12330 limits it to 5 per cent. Heat of hydration is tested only for low heat cement, where IS 12600 caps it at 272 kilojoules per kilogram at 7 days and 314 at 28 days, measured by the calorimetry method of IS 4031 Part 9. For the ordinary OPC that goes into most pours, neither number is measured, and neither appears on the paper that says the cement is acceptable.

That matters because those two numbers are most of the answer to how hot a lift will run. The four main compounds in cement clinker release very different amounts of heat as they hydrate. C3A releases the most, on the order of 850 to 870 joules per gram on full hydration; C3S, the compound that drives strength, releases roughly 500; C4AF around 420; and C2S, the slow strength-builder, only about 260. The exact figures vary by source and by the degree of hydration assumed, but the ranking does not. C3A and C3S are the heat. A cement batch that runs a few per cent higher in C3A, or that is ground finer so it reacts faster, will generate more early heat at the same 53-grade strength, and nothing on the certificate will warn you.

Use of low alkali ordinary Portland cement having total alkali content not more than 0.6 percent (as Na2O equivalent).

IS 456:2000, Clause 8.2.5.4, on measures to be taken when reactive aggregate is used.

The same invisible variability sits on the alkali line. The alkali content of cement is reported as the sodium oxide equivalent, Na2O plus 0.658 times the potassium oxide, and it drives alkali-silica reaction when the aggregate is reactive. The widely used low-alkali threshold is 0.6 per cent, written into IS 456 for reactive aggregate and offered as an optional limit in ASTM C150. India's textbook case of what happens when this is missed is Rihand Dam, where high-alkali cement combined with reactive aggregate to crack the structure from the inside, the story we told in issue 9. The cement at Rihand was measured well above the threshold. The lesson there was not that Indian cement is uniformly high in alkali. It is the opposite.

A survey of 26 Indian cement brands, reproduced by Kaushal Kishore, found that none exceeded 1 per cent alkali as Na2O equivalent, and that Indian cements were not as alkali-rich as many in the United States or the United Kingdom. So the risk is not a high national baseline. The risk is variability. A 2021 peer-reviewed study of clinker from four zones of India found the eastern and northern zones higher in alkali and the southern zone higher in C3S and C3A, the reactive, high-heat compounds. The same brand from a different plant, or the same plant in a different month, can hand you a measurably different cement, and the certificate will pass every time.

The lesson: A mill test certificate tells you the cement is compliant. It does not tell you the cement is the same as last month's, and for a mass pour that difference is the difference between staying under the thermal-cracking threshold and crossing it. The two levers that matter most for heat, the C3A content and the heat of hydration, are not on a standard IS 269 certificate, and the alkali figure that governs AAR risk is only tested when the purchaser asks for it. Before a major pour, ask for the oxide analysis and compute C3A yourself from the Bogue equation, request heat-of-hydration data or specify low heat cement where the section warrants it, confirm the alkali equivalent against the aggregate, and lock the source so the cement does not change underneath the mix. The certificate certifies the batch. It does not certify that the next batch will behave like this one.

Read more: Cement Optimisation in Mass Concrete →