What is the difference between cement and concrete?

Posted by in Concrete | March 26, 2012

If you ever watched the old television show the Beverly Hillbillies, you probably remember Granny referring to the swimming pool as the “cement pond.”  This is not an uncommon term as we often hear someone referring to a cement driveway, cement patio, cement wall, etc.

However, engineering and construction textbooks refer to the material of these construction components as concrete.  What is the difference between cement and concrete?

Concrete is a commonly used construction material that is basically comprised of cement powder, sand, gravel (aggregate), water, and entrained air.  When mixed together the water and cement powder become a dark gray cohesive paste that bonds the sand and gravel together.  The sticky properties of the paste and the subsequent the hardened properties of the light gray colored material are due to a chemical reaction called hydration.  That is, the water molecules and the cement molecules chemically bond to form a new material.  The process of chemical hardening and evaporation (drying) of excessive water in the paste is called curing.  The chemical reaction is what causes the chemical burn when skin comes in contact with the wet paste.

The chemical reaction and the evaporation of excess water from the paste during curing results in a light gray hardened material that occupies less volume that the dark gray paste.  The decrease in volume results in the shrinkage of the material.  The shrinkage of the material is what causes the cracking that appears in concrete driveways, sidewalks, walls, and floor slabs when they are restrained from shrinking by the subgrade or adjacent building components.

In practice of engineering, the curing process is considered theoretically complete after 28 days.  That is, the expected strength and usually the expected water loss have occurred by the 28th day of curing.  However, in actuality, the curing process continues on for years.  That is one of the reasons why shrinkage cracking may not appear for years.

The sand and gravel components of concrete are basically inert filler materials that provide strength and reduce the effects of the shrinkage of the water and cement paste.  The entrained air generally improves the workability of the concrete while in the paste stage during construction and the durability of the concrete to repeated freezing and thawing in the hardened stage.

Cements that react chemically with water are called hydraulic cement.  The most commonly used hydraulic cement is called Portland Cement.  Portland Cements are commonly found in concrete, mortar for masonry, stucco, and in some base coats for plaster walls.

Non-hydraulic cements, such as gypsum (commonly found in drywall board and joint compounds used for patching drywall) and lime harden as the water evaporates.  The re-introduction of water causes the gypsum or the lime to be re-tempered (re-soften).  Interestingly, lime added to a mortar mix can provide a type of healing for narrow width cracks by softening when wet, flowing into a narrow crack, and then hardening when the lime dries.  This self- healing is a bonus that aids in the water shedding ability of masonry walls.

On occasion, we have seen a driveway, a floor slab, a wall, or even a pond that has been cast with little or no sand or gravel.  These are identified by the lack of sand and gravel aggregate on an exposed fracture surface.  These would more appropriately be described with the adjective “cement” rather than concrete.

Thus, there is a difference between cement and concrete.  Cement is a component of concrete; however, a hardened, gray cementitious material with little or no sand or gravel may be considered to be cement.

But what distinguishes cement and concrete from the mortar used in a brick or block wall?  That will be discussed in a future article.