Concrete Basics

• A basic concrete mix consists of a binding compound, sand and coarse aggregate, such as gravel. Cement consists of a manufactured mixture of natural compounds, including silica, lime and minerals. Although mixtures vary according to application, concrete might contain as much as five times more sand and gravel than cement. Mixed with water and allowed to cure, the cement hardens to bind the sand and gravel in a rock-hard form. However, even cured concrete retains water, and the composition of cured concrete includes both water and trapped air bubbles, in addition to cement and aggregates.
  
  

Portland Cement

• Portland cement is the binding compound or "glue" that hardens to hold together a concrete mix's gravel and sand. Portland cement consists of several mineral ingredients, including calcium, metals and silicon, combined with both gypsum and lime. Portland cement manufacturers grind and crush the ingredients, combine them in precise proportions and typically process them in massive ovens, called kilns. The resulting product and its many variations apply to all types of masonry products, including mortars and concrete mixes.
  
  

Aggregates

• The size of the coarse and fine aggregates applied to concrete mixtures varies according to the dimensions of the concrete project and the project's application. For example, whereas thin concrete slabs require gravel of small diameter, thick slabs might accommodate rocks up to 6 inches in diameter. Alternatively, masonry mortars consist of varying degrees of fine aggregate or sand. Regardless of size, concrete mix requires aggregate of consistent diameter.
  
  

Water Quality

• Although nearly any potable water is suitable for concrete mixtures, water quality affects the long-term performance of concrete. According to the National Ready Mixed Concrete Association, water of poor quality can cause several types of surface blemishes on cured concrete, including stains and efflorescence. Additionally, impure water might contain compounds that cause steel reinforcements, such as rebar or wire mesh, to prematurely corrode.