Properties of Cement
Cement is a fundamental building material widely used in construction, and its performance depends on a range of physical and chemical properties. For this discussion, we focus specifically on Ordinary Portland Cement (OPC), one of the most commonly used types of cement.
Understanding these properties is essential for ensuring the durability and strength of structures. Proper evaluation of cement quality allows for comparison between different sources, helping to make informed decisions for construction projects. Regular testing, whether directly on the cement, on hardened cement paste, or continuously on concrete made from the cement, is crucial to maintaining quality within specified limits.
The key physical and chemical properties of cement (with a focus on OPC) are discussed below:
i. Physical Properties of Cement
1. Fineness
Fineness refers to the particle size of cement and is expressed as the specific surface area. It is a key factor influencing the rate of strength gain and the uniformity of the material. Typically, finer particles provide a larger surface area, which enhances the hydration process and results in faster strength development.
As the fineness of cement increases, the rate of hydration accelerates due to the greater surface area available for chemical reactions. This, in turn, leads to the earlier attainment of strength. The relationship between cement fineness and compressive strength is illustrated in chart below , which shows that finer cement achieves higher strength both at 28 days and after one year of curing.
For consistent performance, it is essential to control the fineness of cement. According to standard specifications, the residue retained on a 90-micron sieve should not exceed 10%. This ensures uniformity and optimal performance in construction applications.
Want to practice MCQs on Cement?
From Portland to pozzolana, see how well you know the world of cement with our specialized MCQs.
2. Setting Time
When water is added to cement and mixed, it forms a paste that gradually loses its plasticity and eventually hardens into a firm material. The duration of this process is referred to as the setting time, which is further divided into two stages: initial setting time and final setting time.
- The initial setting time is the period during which the cement paste begins to lose its plasticity, making it less workable.
- The final setting time refers to the point at which the paste completely loses its plasticity and gains enough firmness to withstand specific pressure.
For Ordinary Portland Cement (OPC), the initial setting time should not be less than 30 minutes, while the final setting time should not exceed 600 minutes.
A sufficiently long initial setting time is critical to allow for operations such as mixing, transporting, placing, and finishing concrete. However, the setting time is influenced more by factors such as the water content in the mix and atmospheric temperature rather than the cement itself.
3. Soundness
Soundness refers to the ability of cement to resist excessive expansion after setting, ensuring stability and durability. Unsound cement can lead to undesirable expansion, causing severe cracking and disintegration of the hardened material.
The primary cause of unsoundness in cement is the presence of free lime (CaO) and magnesia (MgO), which can react and expand after the cement has set. To prevent unsoundness, the following measures are typically employed:
- Limiting MgO content: Ensuring that the magnesium oxide content is below 0.5%.
- Fine grinding: Grinding the cement particles finely to improve uniformity and reduce expansion risks.
- Aeration: Allowing the cement to aerate for several days to stabilize its composition.
- Thorough mixing: Ensuring proper mixing to achieve a homogeneous blend of ingredients.
By controlling these factors, the soundness of cement can be maintained, preventing long-term structural issues.
4. Compressive Strength
Compressive strength is one of the most critical properties of cement, reflecting its ability to withstand compressive forces. It is typically measured through compression tests conducted on cement-mortar cubes prepared in a 1:3 ratio (cement to standard sand) with a total area of 5,000 mm².
The sand used for mortar cube preparation must meet the requirements specified in IS: 650–1991 to ensure consistency and accuracy in testing.
For Ordinary Portland Cement (OPC), the minimum compressive strength requirements are as follows:
- 16 MPa at 3 days
- 22 MPa at 7 days
These values ensure the cement meets performance standards for structural applications, providing sufficient strength for early and long-term load-bearing capacities.
5. Heat of Hydration
Hydration is the chemical reaction between the silicates and aluminates in cement and water, forming a binding medium that hardens over time. This process generates heat, referred to as the heat of hydration. It is defined as the amount of heat, measured in calories per gram of hydrated cement, released during complete hydration at a specific temperature.
The amount of heat released varies based on the type of cement. For Ordinary Portland Cement (OPC), the heat of hydration should not exceed:
- 66 cal/g at 7 days
- 75 cal/g at 28 days
The heat of hydration also increases with temperature. For OPC, it ranges from 37 cal/g at 5°C to 80 cal/g at 40°C.
Understanding the heat of hydration is particularly important in large-scale concreting works, such as dams, where excessive heat generation can lead to cracking. Controlling this heat ensures durability and prevents structural damage.
6. Specific Gravity
The specific gravity of Ordinary Portland Cement (OPC) is typically around 3.15. However, this value can vary slightly depending on the raw materials used in cement manufacturing, such as components other than limestone and clay.
It is important to note that specific gravity is not a direct indicator of cement quality. Instead, it serves as a reference property used in calculations, such as determining the volume of cement required in a mix.
7. Consistency of Cement
Consistency refers to the ability of cement paste to flow and is a crucial factor in determining its usability and workability. It ensures that the cement paste is neither too thick nor too thin for proper application.
The consistency of cement is measured using the Vicat Apparatus through the Vicat Test:
- Preparation of Cement Paste: A paste of normal consistency is prepared by mixing cement with water in a specific proportion.
- Testing with the Vicat Apparatus:
- The paste is placed in the Vicat Apparatus mold.
- The plunger is brought down to touch the top surface of the cement paste.
- The plunger is allowed to penetrate the cement under its own weight.
- Normal Consistency Criterion:
- A cement paste is considered to have normal consistency when the plunger penetrates to a depth of 10 ± 1 mm from the bottom of the mold.
ii. Chemical Properties of Cement
The chemical properties of cement include important parameters such as Loss on Ignition (LOI) and Insoluble Residue.
1. Loss of Ignition
Loss on Ignition (LOI) measures the weight loss of cement when it is subjected to high temperatures (around 900–1000°C). This weight loss occurs due to the evaporation of moisture and carbon dioxide, which are present in combination with free lime (CaO) or magnesia (MgO).
LOI is a critical indicator of the freshness of cement. A higher LOI often suggests the presence of hydroxides or carbonates of magnesium and lime, which are inert substances lacking cementing properties. Such inert materials dilute the cement’s quality and reduce its effectiveness.
To maintain high-quality cement:
- LOI should be as low as possible.
- Typically, LOI is around 2%, but it should not exceed 4%.
By monitoring LOI, manufacturers and users can ensure the consistency and reliability of cement in construction applications.
2. Insoluble Residue
The insoluble residue in cement refers to its inactive components, which do not contribute to its cementing properties. It is an important chemical property that indicates the presence of impurities in cement.
The insoluble residue is determined through the following test procedure:
- One gram of cement is mixed with 40 ml of water and 10 ml of concentrated hydrochloric acid (HCl).
- The mixture is stirred and boiled at a constant temperature for 10 minutes. Any lumps present are broken during this process.
- The solution is filtered, and the residue on the filter is washed sequentially with sodium carbonate (Na2CO3) solution, water, hydrochloric acid, and finally water again.
- The filter paper with the residue is then dried, ignited, and weighed.
The percentage of insoluble residue is calculated based on the weight of the residue. A lower percentage indicates better quality cement.
- The maximum allowable value for insoluble residue in Ordinary Portland Cement (OPC) is 0.85%.
Controlling the insoluble residue ensures higher purity and better performance of cement in construction applications.
iii. Chemical Properties of Cement Clinkers
The various constituents of raw materials combine during the burning process to form cement clinker. These compounds, identified as Bogue compounds, possess the properties of setting and hardening in the presence of water. The proportions of these compounds influence the performance and properties of Portland cement. Below is a detailed breakdown of their individual behaviors:
1. Tricalcium Silicate (C₃S – Alite)
- Proportion: 25–50% (typically ~40%).
- Behavior and Properties:
- Considered the best cementing material.
- Hydrates rapidly, generating high heat (500 J/g).
- Contributes significantly to early strength (mainly 7-day strength) and hardness.
- Improves resistance to freezing and thawing.
- Excessive C₃S content increases the heat of hydration and solubility of cement in water.
- The rate of hydrolysis and the gel character are critical for developing early hardness.
2. Dicalcium Silicate (C₂S – Belite)
- Proportion: 25–40% (typically ~32%).
- Behavior and Properties:
- Hydrates and hardens slowly, contributing to long-term strength (effective after one year).
- Offers resistance to chemical attacks.
- Lower heat of hydration (260 J/g) compared to C₃S.
- Excessive C₂S content decreases early strength, reduces freezing/thawing resistance, and makes clinker harder to grind.
- Has limited impact on strength during early ages (less than a month).
3. Tricalcium Aluminate (C₃A – Celite)
- Proportion: 5–11% (typically ~10.5%).
- Behavior and Properties:
- Hydrates rapidly, responsible for flash setting of finely ground cement.
- Regulated by adding gypsum (2–3%) during grinding to control hydration speed.
- Contributes to initial setting and produces a high heat of hydration (865 J/g).
- Excessive C₃A content decreases sulfate resistance, reduces ultimate strength, and increases cracking due to volume changes.
4. Tetracalcium Alumino Ferrite (C₄AF – Felite)
- Proportion: 8–14% (typically ~9%).
- Behavior and Properties:
- Generates the least heat during hydration (420 J/g).
- Contributes minimally to the cementing value.
- Excessive C₄AF content slightly reduces overall strength.
- Assists in reducing the melting temperature of raw materials during clinker production, improving energy efficiency in the kiln.
Frequently Asked Questions
The physical properties of cement include:
- Fineness: Refers to the particle size and surface area of cement, influencing hydration rate and strength.
- Setting Time: Includes initial and final setting times, ensuring sufficient workability.
- Soundness: Determines the stability of cement against expansion after setting.
- Compressive Strength: Indicates the cement’s ability to withstand compressive loads.
- Heat of Hydration: The heat released during the chemical reaction of cement with water.
- Specific Gravity: A measure of cement density, typically ~3.15 for OPC.
- Consistency: Refers to the flowability of cement paste, tested using the Vicat Apparatus.
The chemical properties include:
- Loss on Ignition (LOI): Weight loss due to moisture and carbonates; should be <4%.
- Insoluble Residue: Indicates non-reactive impurities; should not exceed 0.85%.
Cement grades (e.g., 33, 43, and 53) represent the compressive strength (in MPa) achieved after 28 days of curing. Higher grades exhibit:
- Faster strength development.
- Better performance in load-bearing structures.
- Higher heat of hydration, requiring careful control in mass concreting.
Cement plaster strength depends on:
- Fineness: Improves bonding and hydration.
- Water-cement ratio: Excessive water reduces strength.
- Setting time: Determines workability and stability.
Indian Standards (IS) codes for testing cement properties include:
- IS 4031: Methods of physical tests on cement.
- IS 650: Specifications for standard sand used in tests.
- IS 269: Specifications for Ordinary Portland Cement.
