Cement Account: Complete Technical Guide

Cement account is the quality laboratory’s daily accounting of the finished cement: it balances the components of every cement batch — clinker, gypsum, limestone filler and fly ash — against the chemical analysis of the product, and it verifies that the cement composition complies with the type definition and the standards it is sold under. This article is based on the widely used engineering workbook “Cement Account” (the Calculation Analysis of Cement tool), which performs the complete accounting cycle: the selection of the cement type (CEM I, CEM II/A 42.5 or CEM II/B 32.5), the theoretical recipe of each type (gypsum, limestone and fly ash contents), the laboratory determinations that feed the account (loss on ignition at 550 °C and 950 °C, the carbon dioxide content, the limestone content by the CO&sub2; equivalent, the sulfur trioxide and gypsum contents, and the fly ash loss on ignition correction), and the final balancing of the cement into clinker, gypsum, limestone and fly ash percentages. The complete workbook is part of the Complete Cement Technical Package, the 931-file cement engineering library from cementequipment.org.

The cement account sits at the intersection of quality control, standards compliance and production economics: it is the document that proves the cement meets its declared type, the basis of the clinker factor calculation (the tonnes of clinker per tonne of cement, which decides the cost and the CO&sub2; footprint of the product), and the reconciliation point between the production silos and the laboratory. This guide explains every test, every formula and every balance of the workbook, with the numerical structure of the example reproduced throughout.

1. The Purpose of the Cement Account

Finished cement is not a single material: it is a mixture of portland cement clinker with a small amount of calcium sulfate (gypsum or anhydrite, added to regulate the setting time), and — for the modern blended cements — with mineral additions such as limestone filler and fly ash (pulverized fuel ash). Each cement type defined by the European standard EN 197-1 (and by the equivalent national and international standards) fixes the permitted percentage ranges of the constituents, and the manufacturer must declare and prove the type of every delivered batch. The cement account is the calculation system that performs this proof: from the laboratory tests of the finished cement and the known composition of its components, it back-calculates the percentage of each constituent and checks the result against the type limits.

The account serves three distinct purposes. First, compliance: the declared type (for example CEM II/A-LL 42.5) requires the limestone content to be between 6 and 20 percent, and the account verifies that the produced cement actually lies in that range. Second, production control: the daily account tells the mill operator whether the additive dosing (gypsum, limestone, fly ash) matches the setpoints of the proportioning feeders, and any deviation between the setpoints and the accounted composition is investigated immediately. Third, economic accounting: the clinker is the expensive component and the CO&sub2;-intensive component of cement, so the clinker factor of the account feeds the production cost calculation, the sales mix decisions and the plant’s carbon accounting, and the additive account is the direct measure of the plant’s clinker efficiency.

2. The Cement Types and Their Theoretical Compositions

The workbook is built around the three cement types of its selection matrix — CEM I, CEM II/A 42.5 and CEM II/B 32.5 — and the user selects the type being produced by marking the corresponding column. Each type carries its theoretical recipe in the workbook: the gypsum percentage (5 percent for CEM I, 3 percent for CEM II/A and CEM II/B), the limestone percentage (0 for CEM I, 7 percent for CEM II/A, 10 percent for CEM II/B) and the fly ash percentage (5 percent for CEM I, 7 percent for CEM II/A and CEM II/B), with the clinker percentage closing the balance to 100 percent. These theoretical values are the accounting reference: they are compared with the measured composition, and the differences drive the corrections of the proportioning system.

The composition ranges follow the logic of the standards. CEM I is the portland cement with 95–100 percent clinker, where the balance is the calcium sulfate and minor additional constituents; the workbook’s theoretical CEM I recipe (90 percent clinker, 5 percent gypsum, 5 percent fly ash, 0 percent limestone) represents a plant practice that uses a small fly ash addition within the minor constituent allowance. CEM II/A allows 6–20 percent of additions and the workbook’s recipe (3 percent gypsum, 7 percent limestone, 7 percent fly ash, 83 percent clinker) sits inside the range; CEM II/B allows 21–35 percent and the workbook’s recipe (3 percent gypsum, 10 percent limestone, 7 percent fly ash, 80 percent clinker) sits at the conservative edge. The type also fixes the strength class (42.5, 32.5), which is a quality requirement of the same batch documentation, and the account’s composition verification is the partner of the strength testing program.

Constituent	CEM I (workbook)	CEM II/A 42.5 (workbook)	CEM II/B 32.5 (workbook)	EN 197-1 reference
Clinker	90 %	83 %	80 %	95–100 / 80–94 / 65–79
Gypsum (CaSO&sub4;)	5 %	3 %	3 %	CaSO&sub4; set regulator
Limestone	0 %	7 %	10 %	6–20 % (CEM II/A-LL), 21–35 % (CEM II/B)
Fly ash	5 %	7 %	7 %	Within total addition limits

The selection matrix also carries a water/cement style parameter per type (0.36 for CEM I, 0.30 for CEM II/A, 0.25 for CEM II/B), used by the laboratory as the consistency reference of the account tests. The account is run per batch or per production shift, and its results accumulate into the monthly quality report that the plant files with its certification body.

3. The Loss on Ignition at 550 °C

The first laboratory determination of the account is the loss on ignition at 550 °C, the test that measures the organic matter and the water released by the sample at this temperature. The test procedure is the classical gravimetric cycle: a clean crucible (tare) is weighed, a sample of the cement is added and weighed, the crucible is placed in the muffle furnace at 550 °C until constant mass, and the cooled crucible is weighed again (the last weighing). The loss on ignition follows from the mass balance:

LOI 550 °C (%) = (sample + crucible before − crucible + sample after) / (sample mass) × 100

In the workbook the sample mass is about 0.99 g in a crucible of about 20.79 g tare, and the result of this determination is the first input of the account. The 550 °C loss is the signature of the fly ash and organic constituents: fly ash itself contains residual carbon (its own loss on ignition is typically 2–6 percent, and the workbook uses a fly ash loss on ignition of 0.3 percent for the accounting), so the measured cement LOI at 550 °C must be interpreted together with the fly ash content to avoid double-counting the fly ash carbon in the limestone determination that follows.

4. The Loss on Ignition at 950 °C and the Carbon Dioxide Calculation

The second determination is the loss on ignition at 950 °C on a fresh sample, the test that measures the carbonate decomposition: at this temperature the calcium and magnesium carbonates of the limestone filler decompose, releasing their carbon dioxide, and the mass loss of the 950 °C cycle is therefore the sum of the volatile losses (the 550 °C loss) plus the carbon dioxide from the carbonates. The carbon dioxide content of the cement is obtained by difference:

CO&sub2; (%) = LOI 950 °C (%) − LOI 550 °C (%)

which is exactly the workbook’s calculation structure: the result of the 950 °C determination (the loss of the sample between the tare and the last weighing) is combined with the 550 °C result to isolate the CO&sub2;. The subtraction is valid because the volatile components measured at 550 °C are also lost at 950 °C, so the difference isolates the carbonate decomposition. The CO&sub2; content of the finished cement is the direct measure of the limestone filler in the product: it is the trace element that the whole limestone determination rests on, and its value is small for a CEM II/A with 7 percent limestone (roughly 2.5–3 percent CO&sub2;, since the limestone itself contains 44 percent CO&sub2;) and larger for higher limestone additions.

The 950 °C test requires the same gravimetric discipline as the 550 °C cycle: constant-mass drying of the sample, controlled furnace temperature, complete cooling in the desiccator before each weighing, and the use of the same balance for all three weighings of the cycle. The workbook records the tare, the sample and the last weighing of the 950 °C cycle exactly as it records the 550 °C cycle, and both results feed the account with their uncertainties understood.

5. The Limestone Content from the CO&sub2;

The limestone (calcium carbonate) content of the cement follows from the carbon dioxide content by the stoichiometry of calcium carbonate: one molecule of CaCO&sub3; contains one molecule of CO&sub2;, and the molecular weight ratio is 100.09/44.01 = 2.27, so:

Limestone (%) = 2.27 × CO&sub2; (%) − (correction terms)

The workbook applies the factor 2.27 to the carbon dioxide result and corrects the value for the CO&sub2; contributions of the other constituents (for example, the carbon dioxide that the fly ash itself contains, and the carbonate contribution of the clinker, which is small but not zero for freshly made clinker), using the loss on ignition data already measured. The result is the limestone filler percentage of the cement, and it is compared directly with the theoretical recipe of the selected type: a CEM II/A produced with a 7 percent limestone setpoint must account a limestone percentage near 7 percent, and a significant deviation triggers the correction of the limestone proportioning feeder.

The 2.27 factor is the fixed reference of the carbonate determination, and its accuracy depends on the purity of the carbonate: for a limestone with a small dolomite content, the factor is adjusted toward the mixed carbonate ratio (dolomite contributes 47.8 percent CO&sub2; and 52.2 percent MgO+CaO, so its conversion factor is 2.09), and the plant’s limestone quality certificate supplies the actual factor for the accounting. The account therefore records the limestone quality of the batch together with the cement analysis, and the plant’s laboratory keeps the two in step.

6. The SO&sub3; Determination and the Gypsum Content

The sulfur trioxide content of the cement is the basis of the gypsum account, and the standard determination is the gravimetric barium sulfate method: the cement sample is dissolved in hydrochloric acid, the sulfate is precipitated as barium sulfate with barium chloride, the precipitate is filtered, ignited and weighed, and the SO&sub3; percentage follows from the precipitate mass and the sample mass with the stoichiometric factor of barium sulfate (SO&sub3;/BaSO&sub4; = 80.06/233.39 = 0.343). The workbook records the test as a weighing cycle (the tare and the final weighing of the precipitation filter or crucible) and converts the result into the SO&sub3; percentage of the cement.

The gypsum content follows from the SO&sub3; with the accounting structure of the plant: the clinker itself carries a small SO&sub3; content (typically 0.3–0.8 percent, depending on the fuel and raw material sulfur), so the sulfate added as gypsum is the difference between the cement SO&sub3; and the clinker SO&sub3;, converted by the SO&sub3; content of the gypsum used (dihydrate gypsum, CaSO&sub4;·2H&sub2;O, contains 46.5 percent SO&sub3;):

Gypsum (%) = (SO&sub3; cement − SO&sub3; clinker) / (SO&sub3; of gypsum) × 100

The workbook applies this structure with its own constants (the 0.3 percent clinker SO&sub3; and the gypsum conversion), and the resulting gypsum percentage is compared with the theoretical recipe of the type. The gypsum account is the control of the set regulation: too little gypsum gives flash set in the concrete, too much gives false set and strength loss, so the gypsum dosing is one of the tightest proportioning controls of the mill, and the account is its verification instrument.

7. The Fly Ash Account and the Loss on Ignition Correction

The fly ash content of the cement is accounted both by the proportioning balance (the fly ash is dosed by the mill feeders) and by the analytical signature of the ash: the carbon content measured by the fly ash loss on ignition. The workbook uses a fly ash loss on ignition of 0.3 percent as its accounting constant — the value of the ash actually used at the plant — and the correction logic is the same in both directions: the measured cement loss on ignition at 550 °C contains the contribution of the fly ash carbon, so the carbonate carbon (the basis of the limestone calculation) is corrected by subtracting the fly ash contribution, and the fly ash percentage is balanced against its own loss on ignition in the final composition. The accounting sequence of the workbook therefore interleaves the three additive determinations: the limestone from the CO&sub2; (corrected for the ash carbon), the gypsum from the SO&sub3; (corrected for the clinker sulfate), and the fly ash from the proportioning and the ash loss on ignition.

The total additive amount is the sum of the limestone and the fly ash (the two mineral additions, as opposed to the gypsum, which is the set regulator), and the workbook reports it as its own line of the account. The clinker percentage closes the balance:

Clinker (%) = 100 − (gypsum + limestone + fly ash) (%)

and the account is complete when the four constituents sum to 100 percent and each one lies in its expected range for the declared type. The closing check of the account is the reconciliation against the theoretical recipe of the selected cement type: the measured gypsum, limestone and fly ash are compared with the theoretical values of the type matrix, the differences are reported, and the plant uses them as the correction signals for the mill proportioning system.

8. Balancing the Cement: The Reconciliation with Production

The laboratory account is one side of the balance; the production silo is the other, and the two must agree. The reconciliation procedure of the plant runs at three levels. At the batch level, the account of each composite sample is compared with the proportioning setpoints of the mill at the time of the sample: the feeders dose the clinker, gypsum, limestone and fly ash at measured rates, and the accounted percentages should match the setpoints within the feeder accuracy (typically ±0.5–1 percent of the composition). A mismatch at this level points to a feeder calibration error, a blocked or flooded feeder, or a homogeneity problem of the feed materials. At the silo level, the accounts of the batches filled into a cement silo are averaged and compared with the total masses drawn from the raw material silos and the cement shipped from the product silo: the mass balance of the silo must close within the inventory tolerance, and a systematic difference is the classic signature of a scale or a silo level measurement problem. At the monthly level, the averaged clinker factor from the accounts is compared with the plant’s monthly production statistics (clinker produced, additions consumed, cement shipped), and the agreement of the three levels is the plant’s evidence that its quality accounting is sound.

The reconciliation has a direct cost consequence: the clinker factor from the account determines the cement production cost and the carbon accounting of the plant. A plant whose accounted clinker factor is 83 percent on its CEM II/A line converts 830 kg of clinker per tonne of that cement, and the difference between the accounted factor and the theoretical recipe is the efficiency signal of the proportioning and blending system. The account is therefore not a laboratory curiosity: it is the quality and economics document that the plant manager, the production manager and the sales department all read, and its numbers appear in the monthly reports, the product certificates and the sustainability reporting.

9. The Complete Worked Example of the Account

The workbook’s example data illustrate the complete accounting cycle on a CEM I-type sample. The 550 °C cycle weighs a sample of about 0.99 g in a crucible of 20.79 g tare; the 950 °C cycle runs its own weighing sequence with the same sample size and tare; and the SO&sub3; cycle records its precipitation weighings. The loss on ignition results follow from the mass balances: the 550 °C loss is the difference of the first weighing cycle, the 950 °C loss its counterpart, and the CO&sub2; result is the difference of the two losses. The limestone percentage is then the CO&sub2; multiplied by 2.27 with the corrections; the gypsum percentage is the SO&sub3; balance converted through the clinker sulfate and the gypsum factor; the fly ash percentage is taken from the proportioning with its loss on ignition correction; and the clinker percentage closes the account to 100 percent. The final block compares every value with the theoretical recipe of the selected type and reports the total additive amount, exactly as the plant’s daily account is produced.

The example demonstrates the accounting structure that the engineer must maintain in every batch: each determination is an independent measurement with its own weighing cycle, each conversion is stoichiometric and documented, each correction (fly ash carbon, clinker sulfate, limestone purity) is explicit, and the account closes to 100 percent before it is signed. A cement account that closes to 99 or 101 percent is a warning in itself: the missing percent is the analytical error, and the laboratory traces it (repeat determination, check the constants, verify the balance) before the account is accepted into the batch records. The same example structure is repeated for the CEM II/A and CEM II/B columns of the matrix by simply moving the type selection marker: the theoretical recipe changes (more limestone, less clinker), the same determinations run, and the accounted composition must move with the recipe — which is exactly how the workbook supports the plant producing several types in sequence through the same mill and the same silos.

10. Testing Frequency, Standards Compliance and Documentation

The cement account operates inside the plant’s quality system, and its testing frequency follows the requirements of the standards and the certification regime of the plant. The loss on ignition and the sulfur trioxide determinations are part of the routine chemical testing of finished cement, performed on composite samples drawn from the mill discharge at the defined sampling rate (in most plants, a composite sample per production shift or per defined tonnes, and daily chemical analysis); the account is then produced for each composite. In parallel, the physical testing program (fineness by Blaine, setting time, soundness, compressive strength at 2, 7 and 28 days) covers the strength class declaration, and the two programs together form the batch documentation. The conformity evaluation of the plant — internal quality control plus external surveillance testing by the certification body — reviews the accounts, the test records and the certificates, and the plant’s compliance depends on the traceability of every account line to its laboratory record.

The documentation discipline of the account is therefore as important as the chemistry: the batch number, the sample identification, the date and time of the sampling, the cement type declaration, the silo of origin, the laboratory results with the weighing records, the accounted composition and the signature of the quality manager form one record, and the record is retained for the period required by the quality system. When a customer complaint or a certification audit demands the proof of a delivered batch, the plant must be able to reproduce the complete account of that batch from the retained records, including the corrections that were applied. The workbook’s structure supports this discipline by keeping every determination and every conversion on its own line of the calculation, so that the account is a transparent, auditable document rather than a black-box result, and the plant’s laboratory procedures cite exactly this structure as the standard method of the cement account.

11. Frequently Asked Questions

Q1. What does the cement account calculate?

The finished cement composition: the percentages of clinker, gypsum, limestone and fly ash in every batch, from the laboratory determinations (loss on ignition at 550 and 950 °C, carbon dioxide, sulfur trioxide) and the known properties of the constituents (fly ash loss on ignition, clinker sulfate, gypsum SO&sub3;, limestone purity), checked against the theoretical recipe of the declared cement type.

Q2. How is the limestone content of cement determined?

From the carbon dioxide content of the cement: the CO&sub2; is obtained as the difference between the loss on ignition at 950 °C and the loss on ignition at 550 °C, and the limestone percentage follows from the CO&sub2; multiplied by the stoichiometric factor 2.27 (the CaCO&sub3;/CO&sub2; mass ratio), with corrections for the fly ash carbon and the carbonate purity.

Q3. How is the gypsum content of cement determined?

From the sulfur trioxide analysis (gravimetric barium sulfate method): the gypsum percentage is the cement SO&sub3; minus the clinker’s own SO&sub3;, divided by the SO&sub3; content of the gypsum used (46.5 percent for dihydrate), the classic accounting structure of the workbook with its 0.3 percent clinker sulfate constant.

Q4. What is the clinker factor and why does it matter?

It is the percentage of clinker in the cement, the closing balance of the account (100 minus gypsum, limestone and fly ash). It determines the production cost and the carbon footprint of the cement, feeds the monthly production reconciliation and the product certificates, and is the plant’s measure of its clinker efficiency.

Q5. Why must the loss on ignition of the fly ash be corrected?

Because fly ash contains residual carbon measured by its own loss on ignition, and that carbon also appears in the cement’s 550 °C loss. Without the correction, the carbonate carbon (and therefore the limestone percentage) would be overestimated; the workbook applies the fly ash loss on ignition (0.3 percent in the example) explicitly.

Q6. What does an account that does not close to 100 percent mean?

It means the analytical error of the batch: a weighing or conversion error, a wrong constant, or a sample problem. The account must close to 100 percent before it is signed into the batch records, and a non-closing account triggers the repeat determination and the check of the analytical constants.

12. Final Summary

The cement account is the complete quality and economics accounting of the finished cement: the type selection with its theoretical recipe, the gravimetric determinations (loss on ignition at 550 and 950 °C, carbon dioxide, sulfur trioxide), the stoichiometric conversions (limestone from the CO&sub2; by 2.27, gypsum from the SO&sub3; with the clinker sulfate correction, fly ash with its loss on ignition), and the final balance into clinker, gypsum, limestone and fly ash percentages that close to 100 percent. Every value is checked against the declared cement type and against the mill’s proportioning setpoints, and the account feeds the product certificates, the clinker factor economics and the plant’s carbon reporting. Produced per batch on the plant’s sampling schedule, reconciled with the silo and monthly production balances, and retained with the full laboratory records, the account is the auditable quality document that the certification body and the customer both rely on.

The complete Cement Account workbook is part of the 931-file Complete Cement Technical Package, together with the mill and kiln calculation tools, the dedusting design workbook and the full engineering library of books, manuals and courses covering the entire cement process from quarry to dispatch. Get the whole package with one PayPal payment — instant download, lifetime access — and keep the complete cement engineering reference on your desk.

This file is part of the Complete Cement Technical Package (931 files) available from cementequipment.org. Respective rights holders; library copy for the licensed single user.