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, calcium hydroxide, and a few other compounds.

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 and is then 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, a small amount of gypsum is added to control the hydration of aluminates.

The four principal compounds of hydraulic cement are:

Tricalcium silicate (alite: C

3

S)

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: C

2

S)

hydrates and hardens slowly and contributes largely to strength increase at ages beyond one week.

Tricalcium aluminate (C

3

A)

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

C

3

A

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 (C

4

AF)

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 C150, Standard Specification for Portland Cement, provides for eight types of hydraulic cements; the most commonly used cements are listed below.

Most cement producers in Texas produce a Type I/II cement. This type of cement is widely used in TxDOT projects. This cement meets the requirements for both Type I and Type II cements, and can be used in concrete where either Type I or Type II cement is required.