Building lime has been a fundamental binding material in construction for centuries, used in mortars, plasters, and stabilisation works across a wide range of structures. Unlike cement, the strength properties of lime vary significantly depending on its source, calcination process, and hydration method. Evaluating the compressive strength of lime mortar is essential for ensuring that structural elements meet design requirements over their service life. The standard IS:6932-Part 8-1973 provides a detailed method for determining the compressive strength of building lime under controlled laboratory conditions. For a broader understanding of how strength testing applies to other materials, refer to our article on compressive strength of concrete and what causes low strength breaks in concrete cylinders, which discusses similar principles in concrete testing.
Understanding Building Lime and Testing Standards
Building lime is produced by calcining limestone or chalk at controlled temperatures to drive off carbon dioxide. The resulting quicklime is then slaked with water to produce hydrated lime or putty lime suitable for construction. The Bureau of Indian Standards publishes a comprehensive series of test methods under IS:6932 covering various physical and chemical properties of building lime. Part 8 specifically addresses compressive strength determination, while Part 7 covers both compressive and transverse strengths in a broader context.
The objective of this standard is to prescribe a uniform procedure for preparing and testing lime mortar specimens so that results from different laboratories are comparable. Without such a standard, variability in sample preparation, curing conditions, and loading rates would make it impossible to specify reliable strength values for design purposes. Understanding the full range of laboratory tests for building lime is essential for quality assurance in any project that uses lime as a binder.
Lime mortars offer better workability, self-healing properties, and vapour permeability than cement mortars. However, their strength development is slower and more dependent on proper curing conditions. The standard recognises this by specifying a 28-day testing regime that accounts for the slower carbonation and pozzolanic reactions that give lime mortar its ultimate strength.
Equipment and Apparatus Required
The equipment needed for compressive strength testing per IS:6932-Part 8-1973 is modest, making this test accessible to most field and laboratory settings. Two main items are required:
- Oven (300°C capacity) — Used for drying materials and maintaining controlled temperature conditions during sample preparation. The oven must be capable of reaching at least 300 degrees Celsius for any required drying or conditioning of lime and sand before testing begins.
- Compression testing machine — This machine applies a gradually increasing compressive load to the test specimens until failure occurs. It must deliver load at a controlled rate of 15 kg per minute as specified in the procedure and have sufficient capacity for lime mortar cubes, which typically achieve strengths in the range of 10 to 50 kg per square centimetre depending on the lime grade.
Metallic moulds are required to cast 5 cm cube specimens. These must be constructed from non-corroding material and coated with a thin layer of petroleum jelly before each use to prevent the mortar from adhering. The business of building a building business part 2 provides useful context on how material quality decisions fit into broader construction practice.
A mixing bowl, spatula, and graduated measuring cylinder are also necessary. The standard sand must conform to IS:650 or an equivalent specification, ensuring uniform particle size distribution for reproducible results.
Sample Preparation for Test Cubes
Sample preparation is the most critical step in the entire procedure, as inconsistencies at this stage produce unreliable results regardless of how carefully the testing is performed. The preparation involves two distinct stages:
Stage One: Lime Putty Preparation
- Take 500 grams of the lime sample to be tested.
- Add water in the proportion of 60 to 65 percent of the mass of lime, which amounts to 300 to 325 millilitres.
- Mix thoroughly for 5 minutes to obtain a uniform putty with no dry lumps remaining.
- Pass the resulting putty through a mixer twice to ensure complete homogeneity.
Stage Two: Mortar Mixing
- Take 350 grams of the prepared lime putty.
- Add standard sand with a mass equal to three times the dry lime content in the putty. Since the putty contains approximately 60 to 65 percent moisture, the equivalent dry lime mass is about 212 to 219 grams, so the sand required is approximately 636 to 656 grams.
- Mix the lime putty and sand thoroughly for 10 minutes continuously to produce a uniform mortar.
Water content at the mixing stage is critical. Too little water produces a stiff, unworkable mortar that cannot be properly compacted, while too much water leads to segregation and reduced strength. The prepared mortar must be used immediately after mixing to prevent setting or drying before casting. For more detail on assessing lime quality in the field, read about field tests on building lime for construction works.
Step-by-Step Testing Procedure
Twelve cubes with 5 cm sides are cast from the prepared mortar using metallic moulds. The procedure follows a carefully timed sequence of casting, curing, and testing that spans 28 days. Each step is designed to simulate the conditions lime mortar would experience in actual construction while maintaining strict control over environmental variables.
| Step | Day | Action | Conditions |
|---|---|---|---|
| 1 | Day 0 | Cast 12 cubes from lime-sand mortar | Metallic moulds, hand compaction |
| 2 | Days 0 to 3 | Store undisturbed in container | 90%+ RH, 27±2°C, 72 hours |
| 3 | Days 3 to 7 | Demould and air-dry in laboratory | Ambient lab air, 4 days |
| 4 | Days 7 to 14 | Immerse in clean water | Continuous water storage |
| 5 | Day 14 | Test 6 cubes in compression | 15 kg/min load rate |
| 6 | Days 14 to 28 | Remaining 6 cubes stay in water | Continuous water storage |
| 7 | Day 28 | Test remaining 6 cubes | 15 kg/min load rate |
During casting, moulds are filled by hand with mortar pressed in using the thumb, lightly tamped to remove air voids, and smoothed off with two or three strokes of a trowel. This manual method differs from mechanical vibration used for concrete cubes, reflecting the different workability and sensitivity of lime mortar. Understanding the broader techniques for strength calculation of building materials helps put this specific method into the wider context of structural material evaluation.
The curing environment during the first 72 hours is particularly important. The container must maintain at least 90 percent relative humidity at a temperature of 27 plus or minus 2 degrees Celsius. This high-humidity environment prevents the mortar from drying out too quickly, which would disrupt the hydration and carbonation processes. After demoulding, cubes air-dry in the laboratory for four days before being immersed in clean water. At 14 days from casting, six cubes are removed, wiped surface-dry, and tested at a load rate of 15 kg per minute. The remaining six cubes are tested at 28 days, providing a direct comparison between early and mature strength development.
Result Calculation and Reporting
The compressive strength of each individual cube is calculated by dividing the maximum load applied at failure by the cross-sectional area determined from the mean dimensions of the specimen. The result is expressed in kilograms per square centimetre (kg/cm²), consistent with traditional Indian engineering practice. At each testing age, the average strength of the six specimens is taken as the compressive strength of the mortar.
| Lime Class | Description | Typical 28-Day Strength (kg/cm²) |
|---|---|---|
| Class A | Eminently hydraulic lime | 15 to 50 |
| Class B | Semi-hydraulic lime | 8 to 15 |
| Class C | Fat lime (non-hydraulic) | 3 to 8 |
| Class D | Magnesian or dolomitic lime | 5 to 15 |
Individual results that deviate significantly from the mean should be investigated, as they may indicate flaws in sample preparation, casting, or testing. The test report should include the grade or source of the lime, the water content used in putty preparation, the batch number of standard sand, curing temperature and humidity records, individual cube strengths, and average strengths at both 14 and 28 days. This comprehensive reporting allows engineers to make informed decisions about the suitability of a particular lime for specific applications. For additional context on how lime functions as a construction material, see our detailed article on lime building material construction.
Safety Precautions During Testing
Testing laboratories must adhere to strict safety protocols to protect personnel and maintain equipment integrity. The following precautions are essential when carrying out compressive strength tests on lime mortar:
- Wear hand gloves when removing containers from the oven after heating to prevent burns.
- Use safety shoes and a laboratory apron during the entire testing period to protect against falling specimens and lime dust.
- Inspect the compression testing machine before each use to ensure proper calibration and mechanical function.
- Turn off the electrical supply to all equipment after testing to prevent electrical hazards.
- Clean all equipment thoroughly before and after each test to prevent cross-contamination between samples.
- Apply petroleum jelly to mould surfaces to prevent mortar adhesion and ensure clean specimen release.
In addition to these lab-specific precautions, personnel should be trained in the safe handling of lime, which can cause skin irritation and eye damage if handled without proper protection. First aid supplies should be readily available in the testing area, and all staff should be familiar with emergency procedures for chemical exposure and crush injuries from heavy equipment.
Conclusion
The compressive strength test for building lime specified in IS:6932-Part 8-1973 provides a standardised, reliable method for evaluating the quality of lime intended for construction use. By following the prescribed procedures for sample preparation, casting, curing, and testing, engineers can obtain consistent and comparable results that inform material selection and structural design decisions. The test captures both 14-day and 28-day strength values, providing insight into the rate of strength development, which is a key consideration when lime mortar is specified for projects with tight construction schedules.
Lime remains a valuable building material, particularly for restoration work, sustainable construction, and applications requiring vapour-permeable mortars. Understanding its mechanical properties through proper testing is essential for using it effectively. When combined with other quality control measures such as field testing, visual inspection, and chemical analysis, compressive strength testing forms a complete picture of lime quality. For further reading on quality assurance in modern construction, our article on building wrap selection installation and performance of weather resistive barriers for modern building envelopes covers another important aspect of construction quality assurance.