Here’s a thing – there is no such thing as a “cement mixer” – it’s a concrete mixer! To the lay-person, the terms cement or concrete may well mean the same thing but, being the pedants we are, we discuss the difference between the cement and concrete.
“Portland cement” (with a small “p”) is the generic term for the type of cement used in most concrete. Portland cement is made from mined materials including limestone, shale, iron ore and clay, which is crushed and put in a cement kiln. The raw material is subjected to extreme temperatures (up to 3,000 degrees Fahrenheit) in a rotating kiln to remove water and carbon dioxide. The residue forms a small marble-sized pellets of “clinker”. Each ton of raw material creates three quarters of a ton of clinker, which is ground to a fine powder to form portland cement. Gypsum and/or limestone is sometimes added during the grinding process. Around 85% of cement is formed from lime and silica.
There are several types of portland cement, each manufactured with different chemical or physical requirements for various applications. Cement is an ingredient of concrete, which is a combination of aggregates (sand or gravel), portland cement and liquid. Cement usually comprises 10-15% of the concrete mix by volume.
The cement and water harden to bind the aggregates togetherConcrete is formed through a process called hydration, whereby. Once the ingredients are mixed the process begins rapidly, but it take many years to complete and concrete only reaches its maximum hardness after several years.
The strength of concrete is controlled by the amount of cement used, but the predominant influence of concrete strength is actually the ratio of water used to bind the aggregates and cement together. Using too much water will produce a weaker concrete. The water to cement ratio should be as low as possible. This will improve the durability of the concrete’s surface resulting in less flaking and spalling.
When concrete is laid, any finishes should not be carried out until the surface water sheen has evaporated. This will avoid any excess water being worked into the concrete which will cause the surface to be weaker and less durable.
Large expanses of concrete pavement and flooring are usually laid with expansion joints to allow any cracks that form due to shrinkage during the drying process to form in a straight line.
In places where the concrete might be subject to freezing and thawing, the concrete should be air-entrained. Air entrainment is the intentional creation of tiny air bubbles in the mix. It is a surface-active substance (surfactant) that reduces the tension of the surface between the water and solids, and allows bubbles to form which will resist flaking of the surface.
As a hydraulic cement, portland cement sets and hardens due to a chemical reaction, which means it will still harden under water.