Concrete Ingredients

Concrete Ingredients

The basic ingredients in concrete are cement, aggregates, and water. The type, quality, and proportioning of these ingredients affect the curing rate, compressive strength, and durability of the concrete. Chemical admixtures can be used to enhance one or more properties of the concrete or to improve its handling and placing characteristics.

Cement

Cement is not the same thing as concrete. Many people mistakenly refer to “cement” sidewalks or “cement” driveways and the like, but cement is only one of the ingredients in concrete. It is also an ingredient in masonry mortar, stucco, and other materials.

• Cement + water = cement paste
• Cement + water + sand =cement mortar
• Cement + water + sand + lime = masonry mortar
• Cement + water + sand + coarse aggregate = concrete

Aggregates

The aggregates most commonly used in concrete are sand, gravel, crushed stone, crushed slag, and pumice. Cement and water are mixed with aggregates to produce concrete. Concrete contains both fine and coarse aggregates. When cement is mixed only with fine aggregate, it is called cement mortar, which is used typically for patching and small repairs, or for coating a concrete surface to provide a smooth, even finish. Masonry mortar is different from a simple cement mortar because it contains other ingredients as well.

Water

As a rule of thumb, water used for mixing concrete should be drinkable. Any water that is drinkable is generally free of harmful impurities. In urban areas where municipal water supplies are available, contaminated water is usually not a problem.

Admixtures

Admixtures are substances other than cement, water, or aggregates which are added to concrete mixes for the purpose of altering properties of the fresh or hardened concrete. Admixtures are not generally required to produce high quality, low cost concrete, but they may sometimes be necessary or desirable to alter specific properties of the concrete for specific conditions or circumstances.

Chemical Admixtures

Set accelerators speed up the setting time and early strength development of concrete. This can be helpful in winter weather to reduce the length of time required for curing and protection and to compensate for the effects of low temperatures on strength development.

Concrete Additives

Concrete Additives

Concrete additives are fine materials which are generally added to concrete in significant proportions (around 5–20%). They are used to improve or obtain specific fresh and/or hardened concrete properties.

Types of inorganic concrete additive:

Type I

Virtually inactive materials such as lime fillers, quartz dust and color pigments.

• Pigments

Pigmented metal oxides (mainly iron oxides) are used to color concrete. They are added at levels of 0.5–5% of the cement weight; they must remain color-fast and stable in the alkaline cement environment. With some types of pigment the water requirement of the mix can increase.

• Rock flours (quartz dust, powdered limestone)

Low fines mixes can be improved by adding rock flours. These inert materials are used to improve the grading curve. The water requirement is higher, particularly with powdered limestone.

Type II

Pozzolanic or latent hydraulic materials such as natural pozzolans (trass), fly ash and silica dust.

Fly ash is a fine ash from coalfired power stations which is used as an additive for both cement and concrete. Its composition depends mainly on the type of coal and its origin and the burning conditions.

Silica dust (Silicafume) consists of mainly spherical particles of amorphous silicon dioxide from the production of silicon and silicon alloys. It has a specific surface of 18–25 m² per gram and is a highly reactive pozzolan.

Standard dosages of silica dust are 5% to 10% max. of the cement weight.