Classification of Cements
Cements are divided into two main categories based on the way they set and harden:
- Hydraulic – set under water through a chemical reaction.
- Non‑hydraulic – require air to harden.
Hydraulic cements, such as Portland cement, can develop strength even when immersed, while non‑hydraulic cements rely on drying.
Raw Materials for Ordinary Portland Cement (OPC)
Modern OPC is produced by blending two types of raw materials:
- Calcareous materials – rich in calcium oxide (CaO), providing the lime component.
- Argillaceous materials – rich in silica (SiO₂), alumina (Al₂O₃), and iron oxide (Fe₂O₃), supplying the silicate and alumina phases.
These components are finely ground, mixed, and heated in a kiln to form clinker, which is then ground with a small amount of gypsum.
Key Oxides and Compounds in Portland Cement
The essential oxides present in clinker are CaO, SiO₂, Al₂O₃, Fe₂O₃, MgO, and gypsum. Among the crystalline phases, two are most important for early strength development:
- Tricalcium silicate (C₃S) – hydrates rapidly, providing the initial set and early compressive strength.
- Tricalcium aluminate (C₃A) – influences heat evolution and sulphate resistance; its rapid reaction must be moderated.
Gypsum is added to clinker specifically to retard the reaction of C₃A, preventing a flash set and allowing a workable paste.
Types of Portland Cement
Different types of OPC are formulated for specific performance criteria:
- Type II – offers moderate sulphate resistance and a lower heat of hydration, suitable for environments with sulphates.
- Type IV – designed for massive concrete structures where a very low heat of hydration is required to limit temperature rise.
Blended Cements
Blended cements combine OPC with supplementary cementitious materials (SCMs). Two common families are:
- Type 1S – OPC blended with blast‑furnace slag.
- Type 1P – OPC blended with pozzolanic material.
While blended cements can improve durability and long‑term strength, they do not always increase early strength; the effect depends on the specific SCM used.
Hydration Reactions and Products
When water contacts cement, a series of hydration reactions occur, producing the primary binding phases:
- Calcium silicate hydrate (C‑S‑H) – the gel that imparts most of the concrete’s strength.
- Calcium hydroxide (Ca(OH)₂) – a by‑product that can affect durability.
The hydration of C₃S can be represented by the balanced equation:
2C₃S + 6H → C₃S₂H₃ + 3Ca(OH)₂
Here, “H” denotes water, and the resulting C‑S‑H gel (C₃S₂H₃) forms the solid matrix that binds aggregates together.
Setting versus Hardening
“Setting” refers to the loss of workability as the paste stiffens, marking the transition from a fluid to a rigid state. “Hardening” follows setting and involves the development of mechanical strength as hydration products accumulate.
Influence of Fineness and Temperature
Fineness, measured by the Blaine method, is a critical quality parameter. Typical Portland cement must have a minimum surface area of 280 m²/kg. Higher fineness increases the available surface area, accelerating hydration and enhancing early strength.
The kiln temperature for clinker formation ranges from 1400 °C to 1600 °C. This temperature window ensures sufficient sintering while allowing the formation of the desired mineral phases.
Heat of Hydration and Expansion Issues
The heat released during hydration (HOH) is expressed in joules per kilogram (J/kg). Excessive heat can lead to thermal cracking in large pours, which is why low‑heat cements (e.g., Type IV) are selected for massive structures.
Unsound cement may expand after setting due to the presence of free lime, magnesia, or excess gypsum. These components hydrate slowly, causing volumetric expansion and potential cracking.
False Set and Flash Set
A “false set” occurs when the paste loses workability with little heat evolution, often caused by dehydrated gypsum. In contrast, a “flash set” is a rapid, exothermic stiffening resulting from the absence of gypsum, which allows C₃A to react violently with water.
Quality Indicators
Loss on ignition (LOI) measures the amount of carbonisation, free lime, and magnesium lost upon heating. High LOI values indicate poor storage conditions and can compromise durability.
Insoluble residue above 7 % by mass negatively impacts concrete strength, reducing early compressive strength by up to 11.5 %.
Specialty Cements
Aluminous (high‑alumina) cements are not primarily decorative; they are used for refractory applications and rapid hardening due to their high Al₂O₃ content.
C₄AF, another clinker phase, acts as a flux, lowering the temperature required for clinkering but contributing little to the final strength of the cement.
Summary
This lesson covered the fundamental classification of cements, the raw materials and key compounds that define OPC, the hydration chemistry that creates strength‑bearing phases, and practical considerations such as fineness, heat of hydration, and quality control. Understanding these concepts equips practitioners to select appropriate cement types, control setting behavior, and ensure durable concrete structures.
