Words like cement and concrete often are used interchangeably but they are different.
In its most basic form, concrete is made from cement, water, and aggregates or rocks. Most concrete today is also comprised of supplementary cementing materials and chemical admixtures.
Fundamentals of Quality Concrete
Supplementary Cementing Materials (SCMs)
SCMs contribute to the properties of hardened concrete through hydraulic or pozzolanic activity. In addition to increased strength, they are also used to make mixtures more economical and to reduce permeability.
SCM’s are added to concrete in addition to or as a partial replacement of Portland cement or blended cements and are usually considered as a part of the total cementing system. The use of these materials in concrete has grown considerably over the past 30 years in that they are typically byproducts of industrial processes and their use can contribute to environmental and energy conservation practices.
A number of chemical admixtures can be added to a concrete mixture to enhance the characteristics of concrete in both its plastic, before it sets, and hardened state. These chemicals contribute to the ease of transport, handling, placement and finishing of concrete and to the ultimate strength, durability, and lifespan of the finished product.
There are five distinct classes of chemical admixtures:
Air-entraining - used to purposely place microscopic air bubbles into the concrete.
Water-reducing - used to reduce the required water content for a concrete mixture by about 5 to 10 percent.
Retarding - Slows the setting rate of concrete, are used to counteract the accelerating effect of hot weather on concrete setting.
Accelerating - used to increase the rate of early strength development and are especially useful for modifying the properties of concrete in cold weather.
Plasticizers - used to reduce water content by 12 to 30 percent and can be added to concrete with a low-to-normal slump and water-cement ratio to make high-slump flowing concrete.