Cement vs. Concrete:
Portland Cement is the most widely used type of cement in the world. It consists of Limestone and several other materials heated in a kiln resulting in a substance called "clinker". The clinker is then ground and a small amount of gypsum is added to create Ordinary Portland Cement.
Portland Cement is an ingredient in Concrete. Usually comprising approximately 10% of the mix by weight. The remaining ingredients are gravel, sand and water. Admixtures can be added to the mix to alter various properties.
All concrete will crack regardless of whether or not its compressive strength is high enough. In fact a mix with a high Portland cement content can actually crack more easily due to the increased amount of paste, hydration rate and from heat. As concrete hydrates (water combining with cement), the material undergoes shrinkage.
Standard concrete weighs approximately 150 lb/cuft which is 4,050 lb/cuyd and approximately 2,400 kg/cuM, depending on the aggregates and mix design.
A large factor in the ultimate strength of concrete is influenced by the water-cementitious ratio (w/cm). With all other factors being equal, concrete with a lower water-cement (cementitious) ratio makes a stronger concrete than that with a higher ratio. A ratio of 0.50 or less is required in some situations for high strength, faster curing or when the concrete is exposed to freezing and thawing. With a standard 4,050 lb/cuyd batch with a w/cm of 0.50 the mix would have approximately 405 lbs of cement and 202.5 lbs of water.
Some experimental concretes have been made with a compressive strength of up to 800 MPa (116,000 PSI). Most standard strength concrete used in industry has a compressive strength between 20 - 40 MPa (3000 - 6000 psi)