Many people are confused about the difference between cement and concrete. Lets talk about what cement and concrete are made from and how they are used. This is a quick review of what concrete and cement are so you can pick the right cement mix for your concrete project.
Many people use the words cement and concrete interchangeably but cement is actually an ingredient in making concrete. Cement makes up about 10% or so of volume in concrete mixtures. Portland cement is a powder made from limestone, clay and pulverized rock that is mixed with other materials like sand and water to form concrete.
When you visit your local hardware or home improvement store you’ll find that there are many choices in cement and concrete. Depending on the size of your project you may choose to either get a small premixed 50 lb bag of concrete or a larger bag of Portland cement, a bag of sand. and pea gravel and make the concrete mixture yourself.
Portland Cement Types and what they are used for. Types I, II, and III are used for most projects.
Type I – general purpose – for most uses
Type II – moderate sulfate resistance – for projects exposed to water or dirt with sulfates
Type III – high early strength – used for rapid construction projects and cold-weather pouring
Type IV – slow reacting – used for massive projects such as dams (rare)
Type V – high sulfate resistance – for projects with high levels of sulfates
Concrete hardens rapidly at first and then the process slows down. The actual curing time for concrete varies, dependent on weather conditions, temperature, mixture proportions, the size and shape of the concrete area, and the specified strength of the concrete.
For many projects concrete is moist-cured for a period of time. This ensures that the concrete maintains an adequate level of moisture while curing. The strength of fully cured concrete can be almost doubled when fully wet-cured as opposed to no moist-curing applied.
Many websites specify a curing time of 28 days, but this is not long enough for the concrete to reach full strength. The American Concrete Association Committee 301 indicates that ASTM C 150 Types I, II, III, IV, and V cement will reach 70 percent of strength within 14 days, with some types reaching this point earlier than others, but most concrete gets significantly stronger by about 90 days. If using moist-curing for at least the first week, concrete may see an additional strength increase of 50% or more by the 90 day point over the strength level at the 28 day point.
Cement hardens over a long period of time so picking the right temperature to pour concrete is important for strength and durability. Higher temperatures can improve the initial strength of the cement in the early days, but can also decrease the long-term strength of the cement by 25% or more. In general temperatures of 90 degrees or less should have an impact of less than 10% of the final strength.
Do not pour concrete if the temperature is below 40 degrees Fahrenheit.
Cement and Concrete Industry Resources
There are some great videos on www.quikrete.com. Check them out by visiting their site. The “pouring concrete in hot weather video” is helpful if you are pouring concrete in the summer time. They have many DIY videos on mixing concrete, concrete slabs, setting posts, building deck footings, resurfacing concrete slabs, and filling cracks in concrete. This is one of my favorite cement and concrete resource sites.
Another company that makes cement and concrete mix is www.sakrete.com. They also have concrete project videos. They have a calculator to figure out how much material you need for your project. Their videos about how to remove concrete from tools and repairing cracks in concrete are great resources to watch.
Visit the Portland Cement Association at www.cement.org for all kinds of history, in-depth technical info on how cement is made, aggregates, concrete and curing concrete. The article on air entrained concrete is very helpful for extreme freezing project areas.