Brick Making Cement Ratio: How Block Plants Set the Mix

There is no single brick making cement ratio. India’s IS 2185 (Part 1) caps concrete blocks at one part cement to six parts combined aggregate by volume, and ASTM and EN standards set no ratio at all, only the strength and absorption the block must reach. Plants set the ratio by trial batches on their own aggregates, because grading, mix water and compaction change strength as much as cement does.

This guide is for investors and plant managers setting up a concrete block or cement brick plant. It explains what the standards say about mix proportions, why the same ratio gives different strengths in different plants, how to convert a volume ratio into batch weights, and how to run the trials that fix your mix. It covers the mix for non-fired concrete blocks and cement bricks made on a vibration-and-press machine. It is not about the mortar ratio for laying bricks, and it does not cover fired clay or autoclaved bricks.

Brick Making Cement Ratio: What the Standards Say

Most block standards specify performance, not proportions. The cement ratio is the producer’s decision, within a few limits.

Note what the IS 2185 limit means. It is a ceiling on richness, written as “not richer than” 1:6, so a 1:8 mix is permitted and a 1:5 mix is not. Clause 7.1.1 stops a producer from buying strength with cement alone. The blocks must still pass the strength grade, which runs on gross area from 3.5 N/mm² up to 15 N/mm² for Grade A hollow blocks. ASTM C90 sets a minimum net area compressive strength of 2,000 psi (13.8 MPa) (Concrete Masonry and Hardscapes Association, CMU-TEC-001). Check the current edition of each standard before designing to it.

Why the Cement Ratio Alone Does Not Fix Strength

Block mixes are stiff and earth-moist, not fluid like poured concrete. Their strength depends on how densely the machine packs them, and that depends on water and grading as well as cement.

A study of plant practice by the Kansas Geological Survey (Hardy, 1954) shows the effect. It made 8 × 8 × 16 in. lightweight blocks on two production machines with the same aggregate:

• At 14% water, 28-day strength on one machine was 752 psi with 14% cement and 720 psi with 16% cement, so more cement gave slightly lower strength. On the other machine, a mix with 10% cement reached 1,301 psi against 1,391 psi at 14%. The author notes that strengths “increased to a maximum and then decreased” as the cement-aggregate balance changed.
• Water was “by far the most important of the three variables”. Raising water from 14% to 15% markedly lowered block quality; above 15%, blocks slumped on one machine, and the other could not form the mix at all.
• The same mix gave very different strengths on the two machines, and the paper links vibration intensity to differences in yield, block weight and compressive strength.

The study is old and used one lightweight aggregate, so its numbers do not transfer to your plant. What it shows is the direction of the effects in a stiff, machine-compacted mix, and the 2014 study of a vibro press described below points the same way: mix moisture has to be set on the machine itself. A cement ratio copied from another plant, a textbook or a brochure is a starting point at best. Mix water, aggregate and machine decide where your strength peak sits.

Cement still matters, of course. A 1997 study of soilcrete (soil-cement) blocks in Building Research and Information found that “cured strength is more dependent on cement content than it is on compaction pressure for a well-cured block”. The boundary is important: that study compared compaction pressures on soil-cement blocks, while Hardy varied cement at fixed water in vibro-compacted aggregate blocks. The two do not contradict each other. Pressure and vibration bring the mix to its density, and cement, cured properly, sets the strength that density can reach, up to the point where the mix water no longer suits the machine.

Brick Making Mix Ratio and Strength: What Published Tests Show

Published tests give a sense of how strength moves with the ratio, but each applies only to the materials tested.

A 2023 study in the Nigerian Journal of Engineering (Auyo, Ejeh, Lawan and Aliyu) made 9-inch hollow sandcrete blocks at cement-to-sand ratios from 1:6 to 1:10 and reported these 28-day strengths:

The blocks in this table were made at a water-cement ratio of 0.45, which the authors found to be the optimum among the six ratios they tested (0.35 to 0.60). They report that 1:6, 1:7 and 1:8 met the 2.5 N/mm² minimum of the Nigerian Industrial Standard (2004) for sandcrete, and recommend 1:8 or richer, with 28-day curing, for commercial production.

Two points stand out. First, strength fell by only about 15% from 1:6 to 1:10 (our calculation from the table), which suggests that in this study the sand and compaction did more than the cement to set strength. Second, these sandcrete blocks were made for the Nigerian sandcrete standard; their strengths sit below the lowest IS 2185 Grade A class (3.5 N/mm²) and well below the ASTM C90 minimum, which is stated on net area. A plant that sells to those standards as well needs to design the mix for them from the start, usually with graded aggregate rather than sand alone.

Aggregate Grading: The Hidden Half of the Mix Ratio

Grading decides how much cement paste is needed to fill voids and bind the particles. A well-graded aggregate packs densely and needs less cement for the same strength.

A 2017 study at the British Columbia Institute of Technology (Bown et al., CSCE conference) made weak concrete mixes intended to represent low-strength masonry blocks, using several sands and coarse aggregates. Strength was measured mainly on cast cylinders tested to CSA A23.2, consolidated by compaction and cured at ambient conditions, at water-cement ratios from 0.65 to 0.85. They are not machine block mixes, but the aggregate effects carry over. The study found that:

• Strength tracked the fineness modulus of the fine aggregate: the lower the fineness modulus, the lower the strength. The authors report that strength range “was directly correlated to the shape of the gradation curve for the fine aggregate”.
• Unwashed river sand carried excess silt and clay fines, which the authors call detrimental because they disrupt the bond between paste and aggregate.
• Sand-only mixes needed more binder than sand-plus-coarse mixes; the study used binder-to-aggregate ratios of 1:3 and 1:4 respectively, by its own lab method.
• Coarse aggregate type mattered little in weak mixes, where failure ran through the paste, and moderately in stronger mixes.

IS 2185 (Part 1) asks that aggregates be clean, free from deleterious matter and conform to IS 383, with grading “as near as possible” to IS 383. Before you set any cement ratio, test your sand and crushed stone for grading, silt and clay content. If the local sand is fine and dirty, washing or blending it in coarser crushed fines may save more cement than any change in the ratio.

A 2014 paper in Key Engineering Materials describes a method that starts from the aggregate: combine aggregate fractions for the lowest volume of voids, then find the moisture content by compaction tests on small specimens. The compacted specimens correlated at 95.3% with the compressive strength of blocks made in a vibro press.

Water: Water-Cement Ratio vs Optimum Moisture

In poured concrete, the water-cement ratio is the main lever on strength. In a block mix, water is set by what the machine can compact.

• Too dry. The mix does not compact; blocks have an open texture, weak edges, low density and low strength.
• Too wet. The blocks slump or stick when the mold lifts, and the machine cannot form them (Hardy, 1954).
• The working range is narrow. In the BCIT cylinder mixes, those with unwashed river sand at a water-cement ratio of 0.65 were too dry, and those with laboratory sand at 0.85 were too wet; within the workable range, the lower the water-cement ratio, the higher the strength (Bown et al., 2017). These were cast cylinders, not machine block mixes, but the narrow window is the same lesson.
• Lab moisture is not machine moisture. The 2014 Key Engineering Materials study found the moisture for press production was 10.71% lower than the value set by the compaction test, because a block press applies more compaction energy.

For this reason, plants control block mixes by moisture content, often with a moisture probe in the mixer, rather than by a fixed water-cement ratio. IS 2185 (Part 1) says mix water should be electronically measured and controlled in the mixing drum. Aggregate moisture changes with weather and stockpile position, so a fixed water addition without measurement shifts the mix from one batch to the next.

From Volume Ratio to Batch Weights

A ratio such as 1:6 by volume is easy to say and hard to batch accurately. Weigh batching turns it into repeatable batch weights.

Step 1: Convert volume parts to mass.

Mass of each material per batch = volume parts × loose bulk density of that material

Measure the loose bulk density of your own cement, sand and crushed stone in a container of known volume; bulk density of sand changes with moisture, so measure it dry and correct for the moisture you find on the day.

Step 2: Scale to the mixer. As an illustration with assumed bulk densities, not recommended values: if cement is taken as 1,400 kg/m³ and combined aggregate as 1,600 kg/m³, a 1:6 mix by volume becomes 1,400 : 9,600 by mass, or about 1 : 6.9 by mass (our calculation). For a batch with 100 kg of cement, that gives about 686 kg of aggregate.

Step 3: Add water by moisture, not by ratio. Set the target moisture content of the mix from your trials, measure the free moisture already in the aggregates, and add the difference.

Step 4: Estimate cement per block. Cement per block = cement content of the compacted mix (kg/m³) × solid volume of the block. As an illustration: a 390 × 190 × 190 mm hollow block has a gross volume of about 0.0141 m³; at 55% solid material, its concrete volume is about 0.0077 m³ (our calculation). Multiply by the cement content measured on your trial blocks to get cement per block. Then:

Bags per day = blocks per day × cement per block (kg) ÷ bag mass (kg)

Use your own trial cement content and your planned daily output; we do not give a default cement content, because it is the number the trials are meant to find.

IS 2185 (Part 1) allows by-products in the mix: fly ash to IS 3812 (Part 1) may replace up to 25% of the cement where 33, 43 or 53 grade ordinary Portland cement is used, and fly ash to IS 3812 (Part 2) may replace up to 20% of the fine aggregate (clauses 6.1.1 and 6.2.2). For pressed bricks with much higher fly ash content, see the fly ash brick manufacturing process.

Setting the Mix by Trial Batches

The mix is fixed by a short, structured trial on your materials and your machine, not by a table.

1. Fix the target. Name the product, standard and strength class: for example, a hollow block to IS 2185 (Part 1) Grade A(5.5), or a loadbearing unit to ASTM C90. The target must cover the minimum average and the minimum individual value, with margin for plant variation.
2. Test the aggregates. Grading, fineness modulus, silt and clay content, and bulk density for each source.
3. Blend for packing. Combine sand and crushed fines or chips to approach a dense grading; the lowest-void blend usually needs the least cement.
4. Run three cement levels. For example, for an IS 2185 market, start at the richest mix the standard allows (1:6 by volume) and add two leaner steps such as 1:7 and 1:8; in the 2023 Nigerian sandcrete study, 1:8 was the leanest ratio that met the national 2.5 N/mm² minimum. These are starting points for a trial, not recommended mixes; keep the target moisture the same across the three.
5. Find the moisture window on the machine. Run each mix at two or three moisture levels and watch for slumping, sticking and open texture.
6. Cure trial blocks as production will. Strength depends on curing, so blocks cured in a laboratory tank can test stronger than the same mix left in a dry yard.
7. Test to the product standard. Compressive strength, density and absorption at the specified age, using the method in our guide to concrete block testing.
8. Choose the leanest mix that passes with margin, then re-check it whenever the sand source, cement type or mold changes.

The same mix may need adjusting between products. A paver needs a dense, wear-resistant mix; a hollow block needs a mix that holds thin face shells as the mold lifts. Our articles on the hollow block machine and the interlocking brick machine explain how the machine and product shape the mix.

Curing: where the cement ratio pays off

Cement only gives strength if it hydrates, and hydration needs moisture in the first days. A rich mix that dries out on the yard can test weaker than a lean mix that was cured well.

IS 2185 (Part 1) calls for curing to IS 456 clause 13.5 or mist curing, with steam curing as an alternative; the drying period before use is covered in our hollow block machine article. On the block lines described on our brick making machinery page, blocks cure in curing chambers without a kiln, autoclave or steam boiler. Whatever the method, cure trial blocks exactly as production blocks will be cured, or the trial will mislead you.

FAQ

What is the cement ratio for making bricks?

There is no universal ratio. For concrete blocks, IS 2185 (Part 1) sets a ceiling of 1 part cement to 6 parts combined aggregate by volume, and ASTM C90 and EN 771-3 set no ratio, only performance. Plants choose the leanest mix that meets their strength class by trial batches with their own aggregates and machine.

What is the mix ratio for cement bricks and blocks?

It depends on the aggregate, the product and the target strength. A 2023 Nigerian study of sandcrete hollow blocks found that 1:6 to 1:8 met the national 2.5 N/mm² minimum for sandcrete. Blocks for ASTM C90 or IS 2185 Grade A must reach higher strengths, so their mix is usually designed around graded aggregate. Test your own materials before fixing a ratio.

Does more cement always make a stronger block?

Not in a stiff block mix. A 1954 Kansas Geological Survey study found block strength rose to a maximum and then fell as cement content changed at fixed water, and water content mattered more than cement. Grading and compaction set the density the cement has to work with, so extra cement can be wasted on a poorly graded or badly compacted mix.

How much water should a block mix have?

Enough to compact fully without slumping when the mold lifts. The working window is narrow: in a 1954 plant study, raising water from 14% to 15% markedly lowered block quality. Because aggregate moisture varies, plants control water by measuring mix moisture, as IS 2185 (Part 1) recommends, rather than by a fixed water-cement ratio.

Can fly ash replace cement in concrete blocks?

Partly. IS 2185 (Part 1) allows fly ash to IS 3812 (Part 1) to replace up to 25% of ordinary Portland cement, and fly ash to IS 3812 (Part 2) to replace up to 20% of the fine aggregate. Fly ash gains strength more slowly, so test the blend at the age your standard specifies and adjust curing if needed.

Planning a Concrete Block or Cement Brick Plant?

The mix you settle on is batched, mixed, compacted and cured on a concrete block and brick making machinery line, where the mix can also take fly ash and slag alongside cement, sand and crushed stone. Send us your local material information and we will check the mix.

Sources

• IS 2185 (Part 1):2005 Concrete Masonry Units – Specification, Part 1: Hollow and Solid Concrete Blocks – Bureau of Indian Standards, 2005: https://law.resource.org/pub/in/bis/S03/is.2185.1.2005.pdf
• Standard Specification for Dry-Cast Loadbearing Concrete Masonry Units, ASTM C90-24a – ASTM International, 2024: https://store.astm.org/c0090-24a.html
• CMU-TEC-001-23, Concrete Masonry Unit Specifications – Concrete Masonry and Hardscapes Association, 2023: https://www.cmha.org/resource/cmu-tec-001/
• EVS-EN 771-3:2011+A1:2015 Specification for masonry units – Part 3: Aggregate concrete masonry units (dense and lightweight aggregates) – Estonian Centre for Standardisation and Accreditation (national adoption of the CEN standard), 2015: https://evs.ee/en/evs-en-771-3-2011-a1-2015-consolidated
• Application of the Tri-Axial Type Diagram to the Study of Concrete Masonry Batch Mixes in Plant Practice, Bulletin 109, Part 2 – Kansas Geological Survey, 1954: https://www.kgs.ku.edu/Publications/Bulletins/109_2
• The Effects of Adjusting Aggregate Fineness and Mix Proportions to Produce Concrete Masonry Blocks of Various Strengths – British Columbia Institute of Technology, CSCE Annual Conference, 2017: https://legacy.csce.ca/elf/apps/CONFERENCEVIEWER/conferences/2017/pdfs/EMM/FinalPaper_604.pdf
• Effect of Mix Ratio and Curing on Compressive Strength of Sandcrete Hollow Blocks, Nigerian Journal of Engineering 30(3), 43–47 – Nigerian Journal of Engineering, 2023: https://doi.org/10.5455/nje.2023.30.03.07
• Use of Compactation Tests for Defining Process Mixtures Parameters to Concrete Blocks, Key Engineering Materials, vol. 600, 375–385 – Key Engineering Materials, 2014: https://www.scientific.net/KEM.600.375
• Soilcrete blocks: experimental work to determine whether cement or compaction pressure is more effective, Building Research and Information 25(4) – University of Warwick repository, 1997: https://wrap.warwick.ac.uk/17556