THEORY

Soundness of cement is defined as the property by virtue of which a cement paste maintains its volume stability and does not undergo any appreciable expansion after the setting and hardening process has been completed. It represents the ability of cement to resist volume changes that could lead to disruption of the hardened cement paste.

The soundness test is critically important because unsound cement can cause:

• Development of cracks and fissures in hardened concrete
• Distortion and disruption of the set and hardened mass
• Disintegration of concrete structures
• Entry points for water and atmospheric gases (CO₂, SO₃) which are injurious to embedded reinforcement
• Severe compromise of structural durability and serviceability
• Potential structural failures over time

CAUSES OF UNSOUNDNESS

Unsoundness in cement primarily results from:

a) Excess Free Lime (CaO)

When limestone is insufficiently burnt during clinker formation, unreacted free lime remains. This free lime hydrates very slowly after the cement has hardened, causing expansion.

Chemical reaction: CaO + H₂O → Ca(OH)₂ + Heat

This delayed hydration is accompanied by significant volume increase (approximately 98% expansion).

b) Excess Magnesia (MgO)

Similar to free lime, excess magnesia hydrates slowly after hardening:

MgO + H₂O → Mg(OH)₂

The permissible limit of MgO in cement is typically 5-6% by mass.

c) Excess Calcium Sulphate (Gypsum)

Added as a retarder during grinding, excess gypsum can react with tricalcium aluminate (C₃A) to form ettringite, causing expansion.

TESTING PRINCIPLE

Since the expansion due to unsoundness occurs very slowly under normal atmospheric conditions and may not be evident for several months or years, accelerated testing methods are employed to detect this tendency in a short time period. The test accelerates the hydration of free lime and magnesia by:

• Using elevated temperatures
• Providing adequate moisture
• Creating conditions that promote rapid expansion

TEST METHODS

Two standard methods are prescribed:

Le-Chatelier Method

This method involves:

• Preparing cement paste with reduced water content (0.78P, where P is water required for standard consistency)
• Moulding the paste in a specially designed split cylinder
• Initial curing at 27 ± 2°C for 24 hours
• Boiling the specimen for 3 hours
• Measuring the expansion of indicator arms

Principle: Boiling accelerates the hydration of free lime and magnesia, causing expansion that can be measured.

Acceptance Criteria: As per IS 8112:2013 for OPC 43 Grade, expansion shall not exceed 10 mm.

Autoclave Method

This method involves:

• Preparing cement paste bars of 25 × 25 × 282 mm with standard consistency
• Initial curing in moist room for 24 hours
• Subjecting specimens to high-pressure saturated steam (2.1 MPa, ~216°C) for 3 hours
• Measuring linear expansion as percentage of gauge length (250 mm)

Principle: High temperature and pressure create extreme conditions that rapidly accelerate any tendency for expansion.

Formula for Autoclave Expansion:

Expansion (%) = [(L₂ - L₁) / L] × 100

Where:

• L₁ = Initial length measured after 24 hours of moist curing (mm)
• L₂ = Final length measured after autoclaving and cooling (mm)
• L = Effective gauge length = 250 mm

Acceptance Criteria: As per IS 269:2015 for OPC 33 Grade and IS 8112:2013 for OPC 43 Grade, autoclave expansion shall not exceed 0.8%.