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Section 6: Hydraulic Cement

Hydraulic cements are defined as cements that not only harden by reacting with water but also form a water-resistant product. Hydraulic cements set and harden by reacting chemically with water. During this reaction, called hydration, cement combines with water to form calcium-silica-hydrates, which are the materials providing cementing actions.

The primary constituents of hydraulic cements are calcium and silica. Small amounts of iron, alumina, and sulfates also exist. Sources of raw materials for the manufacture of cements include limestone (for calcium), clay (for silica, alumina, and iron) and gypsum (sulfate). These raw materials are crushed, milled, and proportioned in such a way that the resulting mixture has the desired chemical composition. They are fed into a kiln where temperatures of 2,600 to 3,000°F change the raw material chemically into cement clinker, grayish-black pellets about the size of 1/2-in. diameter marbles. The clinker is cooled and then pulverized, resulting in hydraulic cement. In the pulverization process, small amount of gypsum is added to control the hydration of aluminates.

The four principal compounds of hydraulic cement are:

  • tricalcium silicate, 3CaO SiO2 (alite: C3S)
  • dicalcium silicate 2CaO SiO2 (belite: C2S)
  • tricalcium aluminate, 3CaO Al2O3 (C3A), and
  • tetracalcium aluminoferrite, 4CaO Al2O3 Fe2O3 (C4AF).

Tricalcium silicate (alite: C3S) hydrates and hardens rapidly and is largely responsible for initial set and early strength. In general, the early strength of hydraulic cement concrete is higher with increased percentages of tricalcium silicate.

Dicalcium silicate (belite: C2S) hydrates and hardens slowly and contributes largely to strength increase at ages beyond one week.

Tricalcium aluminate (C3A) liberates a large amount of heat during the first few days of hydration and hardening. Gypsum, which is added to cement during final grinding, slows down the hydration to control the heat of hydration. Without gypsum, cement sets rapidly, called flash set. Large amounts of C3A make cement vulnerable to external sulfate attack and, for sulfate resistant cement, its amount is limited to a maximum of 8% for Type II and 15% for Type III.

Tetracalcium aluminoferrite (C4AF) reduces the temperature required to change the raw material chemically into cement clinker, thereby making the cement manufacturing process more energy efficient. It hydrates rather rapidly but contributes very little to strength. Most color effects in concrete are due to tetracalcium aluminoferrite and its hydrates.

ASTM C 150, Standard Specifications for Portland Cement, provides for eight types of hydraulic cements as follows:

Type I

normal

Type IA

normal, air-entraining Type I cement

Type II

moderate sulfate resistance

Type IIA

moderate sulfate resistance, air-entraining

Type III

high early strength

Type IIIA

high early strength, air-entraining

Type IV

low heat of hydration

Type V

high sulfate resistance


  • Type I: A general-purpose cement suitable for all uses where the special properties of other types are not required. This is the most widely used cement type for pavement concrete.
  • Type IA: A type I cement with small quantities of air-entraining material interground with the clinker during cement manufacture.
  • Type II: When moderate sulfate resistance or moderate heat of hydration is desired.
  • Type IIA: A type II cement with small quantities of air-entraining material.
  • Type III: For use when high early strength is desired. For TxDOT paving concrete, this cement type is allowed only for Class HES concrete.
  • Type IIIA: Air-entraining Type III cement, where air entraining is desired.
  • Type IV: For use when a low heat of hydration is desired.
  • Type V: For use when high sulfate resistance is desired.

There is a Type I/II cement that is widely used in TxDOT projects. This cement meets the requirements for both Type I and Type II cements. Therefore, this cement can be used for concrete where either Type I or Type II cement is required.

In addition, there are blended hydraulic cements available. Blended hydraulic cements are produced by intimately and uniformly blending hydraulic cement with other types of fine materials. The primary blending materials are fly ash, ground granulated blast-furnace slag, and other pozzolans. In Item 421, two classes of blended cements are used:

  1. Type IP consists essentially of an intimate and uniform blend of Portland cement and Class F fly ash.
  2. Type IS consists essentially of an intimate and uniform blend of Portland cement and ground granulated blast furnace slag.
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