Compressive Strength Test of Cement: IS Code, Procedure, Formula & Calculation

Introduction

The compressive strength test of cement is one of the most important physical tests used to assess the quality and strength development of hydraulic cement.

Cement itself is not tested as a neat solid cube. Instead, its compressive strength is determined using standard cement mortar cubes prepared with cement and standard sand in specified proportions.

In India, the test is carried out according to IS 4031 (Part 6):1988 – Methods of Physical Tests for Hydraulic Cement: Determination of Compressive Strength of Hydraulic Cement Other Than Masonry Cement.

The test helps determine whether cement develops the minimum compressive strength required for its type and grade.

What Is Compressive Strength of Cement?

Compressive strength of cement is the maximum compressive stress that a standard cement mortar specimen can withstand before failure.

It is calculated as:

Compressive Strength = Maximum Load at Failure ÷ Loaded Area

The result is normally expressed in:

N/mm², which is numerically equal to MPa.

For example:

40 N/mm² = 40 MPa

Why Is Cement Tested Using Mortar Cubes?

Cement powder cannot be tested directly in compression.

If neat cement paste were used, the result would be highly influenced by:

• shrinkage;
• cracking;
• water content;
• compaction;
• specimen preparation; and
• paste characteristics.

Therefore, cement is tested using a standardized mortar made with:

1 part cement : 3 parts standard sand by mass

This provides a reproducible method for comparing cement strength.

IS Code for Compressive Strength of Cement

The main Indian Standard is:

IS 4031 (Part 6):1988

This standard covers determination of the compressive strength of hydraulic cement other than masonry cement using mortar cubes compacted by a standard vibration machine.

Other related standards include:

IS 650 – Standard sand for testing cement.

IS 10080 – Moulds and associated equipment used for cement and concrete tests.

IS 4031 (Part 4) – Determination of standard consistency of cement paste.

For Ordinary Portland Cement, the product specification is:

IS 269:2015 – Ordinary Portland Cement – Specification.

Size of Cube Used for Cement Compressive Strength Test

The nominal cube size is:

70.6 mm × 70.6 mm × 70.6 mm

IS 4031 Part 6 specifies a 70.6 mm cube mould conforming to the applicable apparatus standard.

The nominal loaded area is approximately:

70.6 × 70.6 = 4,984 mm²

which is approximately:

5,000 mm²

However, for accurate calculation, the standard requires the cross-sectional area to be determined from the mean dimensions of the specimen.

Materials Required

The test requires:

1. Cement sample
2. Standard sand conforming to the applicable standard
3. Potable or distilled water

The standard sand helps ensure that differences in strength are mainly due to cement properties rather than variations in natural sand.

Apparatus Required

The main apparatus includes:

• 70.6 mm cube mould
• vibration machine
• poking rod
• gauging trowel
• weighing balance
• graduated measuring cylinder
• non-porous mixing plate
• moist room or moist cabinet
• water-curing tank
• compression testing machine

The testing equipment should be properly maintained and calibrated.

Laboratory Temperature and Humidity

During specimen preparation:

Temperature of materials, water and moulding room = 27 ± 2°C

The laboratory relative humidity should be:

65 ± 5%

The moist room or moist cabinet should be maintained at:

27 ± 2°C

with relative humidity:

not less than 90%.

Mix Proportion for Cement Compressive Strength Test

For each cube, IS 4031 Part 6 specifies:

Cement = 200 g

Standard sand = 600 g

Therefore:

Cement : Standard sand = 1 : 3 by mass

The material for each cube is mixed separately.

Water Required for the Test

One of the most important points is that the water quantity is not simply a fixed water-cement ratio such as 0.40.

The water is calculated using the standard-consistency value.

The formula is:

Water = (P/4 + 3.0)% of the combined mass of cement and standard sand

Where:

P = percentage of water required to produce cement paste of standard consistency

For one cube:

Cement = 200 g

Standard sand = 600 g

Combined mass:

200 + 600 = 800 g

Therefore:

Water = (P/4 + 3.0)% × 800 g

This is the water requirement specified in IS 4031 Part 6.

Example Calculation of Water

Assume the standard consistency of cement is:

P = 30%

Then:

Water percentage = (30/4 + 3)

= 7.5 + 3

= 10.5%

Combined cement and sand mass:

= 800 g

Therefore:

Water = 10.5/100 × 800

Water = 84 g

So, for this example:

• Cement = 200 g
• Standard sand = 600 g
• Water = 84 g

Important

Do not use a fixed water-cement ratio unless it is specifically required by the applicable test method.

The correct IS method relates the water quantity to the standard consistency P.

Preparation of Cement Mortar

Step 1: Weigh the Materials

For one cube, weigh:

200 g cement

and

600 g standard sand

Calculate the required water using:

(P/4 + 3.0)% of combined cement and sand mass

Step 2: Dry Mixing

Place the cement and standard sand on a clean, non-porous plate.

Mix them dry using a trowel for approximately:

1 minute

until the mixture appears uniform.

Step 3: Add Water

Add the calculated quantity of water.

Continue mixing until a uniform mortar is obtained.

The total mixing time should not be less than:

3 minutes

If a uniform colour is not obtained within:

4 minutes

the mix should be rejected and a fresh batch prepared.

Preparation of Cube Mould

Before casting:

1. Clean the mould thoroughly.
2. Apply a thin film of petroleum jelly to the joints between mould halves.
3. Apply petroleum jelly between the mould and base plate to prevent water leakage.
4. Apply a thin coating of mould oil to the internal faces.
5. Assemble and clamp the mould securely.

Correct mould preparation helps prevent leakage and dimensional defects.

Casting Procedure

Step 1: Position the Mould

Place the assembled mould on the vibration-machine table.

Clamp it firmly.

Attach the hopper to the top of the mould.

Step 2: Place First Portion of Mortar

Immediately after mixing, place part of the mortar into the mould.

Prod the mortar:

20 times in approximately 8 seconds

to help remove entrained air and avoid honeycombing.

Step 3: Add Remaining Mortar

Add the remaining mortar.

Again prod it:

20 times

in the prescribed manner.

Step 4: Vibrate the Cube

Compact the mortar using the standard vibration machine.

The vibration period is:

2 minutes

at:

12,000 ± 400 vibrations per minute.

Step 5: Finish the Surface

After vibration:

• remove the mould from the vibration machine;
• smooth the top surface using a trowel; and
• identify the specimen properly.

Initial Curing of Cement Mortar Cubes

Keep the filled moulds in a moist cabinet or moist room.

The amended requirement is:

24 ± 1 hours

after completion of vibration.

After this period:

1. remove the cubes carefully from the moulds;
2. immediately place them in clean fresh water.

Water Curing

After demoulding, the cubes are completely submerged in clean fresh water.

Water temperature:

27 ± 2°C

The curing water should be renewed:

every 7 days.

The cubes should remain immersed until immediately before testing.

After removal from water, they must not be allowed to dry before testing.

Number of Cubes Required

For each required test age:

Three cubes are tested

under IS 4031 Part 6.

For example, if cement is to be tested at:

• 3 days
• 7 days
• 28 days

then normally:

3 cubes × 3 ages = 9 cubes

are required.

The actual test ages depend on the applicable cement specification.

Testing of Cement Mortar Cubes

Step 1: Remove Cube from Water

Take the cube from the curing tank immediately before testing.

Do not allow it to become dry.

Step 2: Position the Cube

Place the cube in the compression testing machine.

The cube is tested:

on its sides

without packing between the specimen and steel platens.

Step 3: Apply Load

Apply load continuously and uniformly from zero.

The loading rate should be:

35 N/mm²/min.

Continue loading until the specimen fails.

Step 4: Record Maximum Load

Record the maximum load carried by the cube.

Formula for Compressive Strength of Cement

Compressive Strength = P / A

Where:

P = maximum load at failure, N

A = loaded cross-sectional area of specimen, mm²

Result:

N/mm² or MPa

Example Calculation

Suppose:

Maximum load:

P = 200 kN

Convert to newtons:

200 kN = 200,000 N

Assume measured loaded area:

A = 5,000 mm²

Then:

Compressive Strength = 200,000 / 5,000

= 40 N/mm²

Therefore:

Compressive Strength = 40 MPa

Observation Table

Average strength:

(40 + 41 + 39) / 3

= 40 MPa

Therefore:

Average compressive strength = 40 MPa

Reporting of Test Result

IS 4031 Part 6 requires the measured compressive strength to be calculated from:

maximum applied load ÷ actual cross-sectional area

and expressed to the nearest:

0.5 N/mm².

The three specimen results should be examined before calculating the final result.

Acceptance of Individual Cube Results

Do not consider:

• visibly defective specimens; or
• specimens whose strength differs by more than 10% from the average strength of all test specimens.

If, after discarding unsuitable results, fewer than two valid strength results remain, the test should be repeated.

This is an important quality-control requirement and should not be omitted from laboratory reporting.

Compressive Strength Requirements of OPC

For normal OPC grades under IS 269:2015, the commonly specified minimum compressive strengths are:

The grade designation corresponds to the specified minimum 28-day compressive strength of standard cement mortar.

Always verify the latest edition, amendments and applicable cement specification before material acceptance.

What Does OPC 53 Mean?

OPC 53 does not mean that concrete made using the cement will automatically achieve 53 MPa.

It means that the cement must satisfy the prescribed compressive-strength requirement when tested using the standardized cement-mortar test method.

Concrete strength depends on many additional factors, including:

• water-cement ratio;
• aggregate quality;
• mix proportions;
• admixtures;
• mixing;
• compaction;
• curing; and
• concrete age.

Therefore:

Cement grade ≠ Concrete grade

Cement Strength vs Concrete Cube Strength

This is a very important distinction.

Cement Compressive Strength Test

Uses:

70.6 mm cement mortar cubes

Mix:

1 cement : 3 standard sand

Test standard:

IS 4031 Part 6

Purpose:

Assess strength of cement.

Concrete Compressive Strength Test

Commonly uses:

150 mm concrete cubes

Materials:

cement + fine aggregate + coarse aggregate + water + admixtures, where applicable.

Purpose:

Assess compressive strength of concrete.

Therefore, a cement mortar cube test and concrete cube test are different tests.

Why 70.6 mm Cube Is Used

The nominal 70.6 mm cube provides a loaded area close to:

5,000 mm²

This specimen size is suitable for standard cement mortar and allows convenient testing while limiting material requirements.

It also provides a standardized specimen geometry for comparing different cement samples.

Why Standard Sand Is Used

Natural sand varies in:

• grading;
• particle shape;
• mineral composition;
• surface texture;
• moisture; and
• impurities.

Such variations could significantly affect mortar strength.

Using standardized sand reduces this variability and makes cement test results more comparable.

Why Three Cubes Are Tested

Testing three specimens provides a more reliable estimate of the strength than using a single cube.

It also helps identify:

• casting defects;
• compaction problems;
• abnormal specimen results; and
• testing variation.

Factors Affecting Cement Compressive Strength

Several factors can affect the measured result.

Cement Composition

Different clinker minerals influence early and later strength development.

Cement Fineness

Finer cement generally hydrates more rapidly and may develop early strength more quickly.

Water Content

Incorrect water quantity can significantly affect mortar strength.

Standard Sand

Use of non-standard or improperly graded sand can invalidate the test.

Mixing Time

Insufficient or excessive mixing can affect uniformity.

Compaction

Inadequate compaction increases voids and lowers measured strength.

Curing Temperature

Incorrect curing temperature can change hydration and strength development.

Specimen Moisture

Allowing cubes to dry before testing can affect results.

Loading Rate

Applying load too quickly or too slowly can alter measured compressive strength.

Common Errors in the Test

Using a Fixed Water-Cement Ratio

The IS test does not simply use a fixed w/c ratio such as 0.40.

The required water depends on:

P = standard consistency

and is calculated using:

(P/4 + 3.0)% of the combined cement and sand mass

Using Ordinary River Sand

Use the specified standard sand, not uncontrolled natural sand.

Casting All Cubes from One Uncontrolled Batch

IS 4031 Part 6 states that the material for each cube is mixed separately.

Using 50 mm or 150 mm Cubes

The standard cement mortar specimen is a:

70.6 mm cube

Using Concrete Cube Loading Procedure Without Verification

Cement mortar cubes should be loaded according to the requirements of IS 4031 Part 6.

Allowing Cubes to Dry

Specimens should remain wet until immediately before testing.

Precautions

1. Use a representative cement sample.
2. Use standard sand conforming to the applicable standard.
3. Determine standard consistency correctly.
4. Calculate water accurately.
5. Maintain the test-room temperature at 27 ± 2°C.
6. Maintain proper laboratory humidity.
7. Use a clean, non-porous mixing surface.
8. Mix cement and sand uniformly.
9. Follow the specified mixing time.
10. Prevent loss of mortar during transfer.
11. Oil mould surfaces correctly.
12. Prevent leakage through mould joints.
13. Compact each specimen properly.
14. Maintain vibration at the specified frequency and duration.
15. Keep moulds in the moist environment for 24 ± 1 h.
16. Cure cubes at 27 ± 2°C.
17. Renew curing water periodically.
18. Do not allow specimens to dry before testing.
19. Align cubes properly in the CTM.
20. Apply load uniformly at the specified rate.
21. Record individual cube results.
22. Check for abnormal results before calculating the final average.

Practical Importance for QA/QC Engineers

The cement compressive-strength test is useful for:

• cement acceptance;
• verification of manufacturer test certificates;
• laboratory quality control;
• investigation of cement-related strength problems;
• comparison between cement batches; and
• confirming compliance with the applicable cement specification.

However, cement compressive strength should not be used alone to assess overall cement quality.

Other tests may include:

• fineness;
• standard consistency;
• setting time;
• soundness;
• chemical tests; and
• other requirements depending on the cement specification.

Sample Laboratory Test Report

Project: __________

Cement manufacturer: __________

Cement type: __________

Cement grade: __________

Batch/Lot number: __________

Date received: __________

Date cast: __________

Test age: __________

Standard consistency, P: __________ %

Cement per cube: 200 g

Standard sand per cube: 600 g

Calculated water: __________ g

Cube size: 70.6 mm

Cube 1 strength: __________ MPa

Cube 2 strength: __________ MPa

Cube 3 strength: __________ MPa

Average strength: __________ MPa

Applicable requirement: __________ MPa

Result: Pass / Fail as applicable

Tested by: __________

Checked by: __________

Frequently Asked Questions

What is the compressive strength of cement?

It is the compressive stress carried by a standard cement mortar cube at failure when tested under specified conditions.

Which IS code is used for compressive strength of cement?

IS 4031 (Part 6):1988 covers determination of compressive strength of hydraulic cement other than masonry cement.

What cube size is used for cement compressive strength?

A nominal:

70.6 mm × 70.6 mm × 70.6 mm

mortar cube is used.

What is the cement-to-sand ratio?

The standard mortar uses:

1 : 3

cement to standard sand by mass.

How much cement and sand are used for one cube?

For each cube:

Cement = 200 g

Standard sand = 600 g

How much water is required?

Water is calculated as:

(P/4 + 3.0)% of the combined mass of cement and standard sand

where P is the standard-consistency water percentage.

How many cubes are tested?

Three cubes are tested for each specified curing age.

What is the loading rate?

The load is applied uniformly at:

35 N/mm²/min

under IS 4031 Part 6.

What is the formula for compressive strength?

Compressive Strength = Maximum Load / Loaded Area

Is cement strength the same as concrete strength?

No.

Cement strength is measured on standardized cement mortar cubes, whereas concrete strength is measured on concrete specimens containing cement, aggregates and water.

What does OPC 53 mean?

It means the cement is required to achieve the specified minimum 28-day compressive strength for OPC 53 when tested using the standard cement-mortar procedure.

It does not mean that every concrete made with OPC 53 will achieve 53 MPa.

Why is standard sand used?

Standard sand reduces variation due to natural sand properties and makes cement test results more reproducible.

Key Points to Remember

• Test standard: IS 4031 Part 6
• Cube size: 70.6 mm
• Cement : standard sand = 1 : 3
• Cement per cube = 200 g
• Standard sand per cube = 600 g
• Water = (P/4 + 3.0)% of combined mass
• Dry mixing ≈ 1 minute
• Total mixing = 3–4 minutes
• Prodding = 20 strokes per layer
• Vibration = 2 minutes
• Vibration frequency = 12,000 ± 400/min
• Initial moist curing = 24 ± 1 h
• Water curing temperature = 27 ± 2°C
• Three cubes per test age
• Loading rate = 35 N/mm²/min
• Result = Load / actual loaded area
• Report strength to the required precision
• Cement grade and concrete grade are different

Conclusion

The compressive strength test of cement is a standardized laboratory test used to determine the strength development of hydraulic cement.

According to IS 4031 (Part 6):1988, cement is tested in the form of 70.6 mm mortar cubes prepared using cement and standard sand in a 1:3 ratio by mass.

For each cube, 200 g of cement and 600 g of standard sand are mixed with water calculated from the cement’s standard consistency.

The cubes are compacted using a standard vibration machine, initially cured in a moist environment, then submerged in water at 27 ± 2°C, and tested at the ages required by the applicable cement specification.

The compressive strength is calculated by dividing the maximum failure load by the loaded cross-sectional area.

Engineers should always distinguish between cement mortar strength and concrete compressive strength, and the final cement acceptance decision should be based on the applicable cement specification, latest amendments and project requirements.

References

1. IS 4031 (Part 6):1988 – Methods of Physical Tests for Hydraulic Cement: Determination of Compressive Strength of Hydraulic Cement Other Than Masonry Cement.
2. IS 4031 (Part 4) – Determination of Consistency of Standard Cement Paste.
3. IS 650 – Standard Sand for Testing Cement.
4. IS 269:2015 – Ordinary Portland Cement – Specification.
5. Applicable project specifications and latest amendments to relevant standards.

Datasheet for compressive strength test of cement cubes