Introduction

Compressive strength is one of the most essential mechanical properties of cement and represents the maximum compressive load that a hardened cementitious material can withstand before failure. Since mortar and concrete are primarily subjected to compressive stresses in structural applications, the compressive strength of cement serves as a direct indicator of its quality, performance, and suitability for construction.

This property is of great importance because it ascertains the quality of cement which is quite useful in building structures. With the increase in compressive strength of cement, properties such as flexural strength, abrasion resistance, durability, and impermeability also increase. Therefore, by performing the compressive strength test of cement we can determine different characteristics of cement.

Hydration Chemistry and Strength Development

When cement is mixed with water, a complex series of hydration reactions begins. These reactions gradually convert the cement compounds into hardened crystalline and gel-like products. The most significant hydration reactions contributing to strength development are:

1. Hydration of Tricalcium Silicate (C3S):

$$ 2C_{3}S + 6H_{2}O \longrightarrow C!-!S!-!H + 3Ca(OH)_{2} $$

2. Hydration of Dicalcium Silicate (C2S):

$$ 2C_{2}S + 4H_{2}O \longrightarrow C!-!S!-!H + Ca(OH)_{2} $$

Where:

• $C_{3}S$: Tricalcium silicate
• $C_{2}S$: Dicalcium silicate
• $C!-!S!-!H$: Calcium Silicate Hydrate (C‑S‑H gel)
• $Ca(OH)_{2}$: Calcium hydroxide
• $H_{2}O$: Water

The C‑S‑H gel formed during hydration is the primary strength‑giving product. It fills voids, binds particles together, and contributes to the progressive increase in strength.

Strength development timeline:

• C₃S is responsible for early strength (3–7 days)
• C₂S contributes to long‑term strength gain (after 28 days)

Why Mortar Cubes Instead of Pure Cement?

The compressive strength test of cement mortar is not carried out on plain cement because the high heat of hydration of cement can cause thermal cracks during setting and hardening. These cracks would give inaccurate results.

The test is performed on mortar cubes (cement + sand, typically 1:3 by mass). Sand:

• reduces heat generation,
• acts as inert aggregate to prevent thermal cracking,
• provides a standardized medium for testing.

Test Procedure

The standard mortar cube size is $70.6\ \text{mm} \times 70.6\ \text{mm} \times 70.6\ \text{mm}$. Cubes are prepared, compacted, cured, and tested at:

• 3 days (early strength),
• 7 days (intermediate),
• 28 days (standard strength).

Calculation of Compressive Strength

During testing, each cube is loaded until failure. Compressive strength is:

$$ \sigma_{c} = \frac{P_{\max}}{A} $$

Where:

• $\sigma_{c}$ = compressive strength (N/mm² = MPa)
• $P_{\max}$ = maximum load at failure (N)
• $A$ = loaded area (mm²)

For the standard cube:

$$ A = 70.6\ \text{mm} \times 70.6\ \text{mm} = 70.6^{2}\ \text{mm}^{2} \approx 4984\ \text{mm}^{2} \approx 5000\ \text{mm}^{2} $$

So, approximately:

$$ \sigma_c ;\approx; \dfrac{P_{\max}}{5000}\ \text{(N/mm}^2\text{ or MPa)} $$

Significance of Results

The results indicate:

• cement's ability to resist compressive loads,
• rate of strength gain,
• overall performance in structures.

Higher compressive strength generally means improved durability, abrasion resistance, and lower permeability.