Frequently Asked Questions
Q: What is the difference between cement and concrete?
A: Many people, even construction professionals, use these two terms interchangeably. But cement and concrete are two very different materials. Simply, cement is the white or gray powder that is the essential ingredient in concrete. Concrete is the solid mass, produced by mixing cement, water and aggregates (sand and rock) together in specified ratios. When water mixes with the cement, special reactions occur that make an incredibly strong and durable solid. The aggregates are trapped within the cement and water mixture as it sets and hardens. Other materials are sometimes added to achieve desired results such as color, resistance to chemical attack or workability. Portland cement denotes a cement that is made to exacting standards, set by ASTM.
One hundred years ago, people all over North America began to experience the benefits of portland cement and the concrete made with it. Ads for cement sidewalks, cement houses and factories, and cement bridges and streets, peppered newspapers, magazines and technical journals. It is little wonder that people everywhere started calling this new wonder material cement. But technically, those structures are concrete sidewalks, concrete houses, concrete factories, concrete bridges and concrete streets.
Q: Why does concrete crack? Why are joints necessary?
A: There are two types of concrete: concrete that is cracked and concrete that has the potential to crack. When exposed to wetting and drying conditions concrete will expand and contract (similar to a sponge). If the stress associated with these volume changes exceeds the tensile capacity of the concrete, a crack will form. This specific type of crack is referred to as a shrinkage crack. Cracks are expected to form in concrete and act as a "pressure release valve". By intentionally jointing concrete, you decrease the thickness of the slab in that location. This allows a crack to form along a straight line within the joint since the thin section provides a path with less resistance than a thicker section.
Q: How is cement made?
A: Cement manufacturing starts at a quarry where limestone and shale are mined, crushed and blended with sand, mill scale and clay. These raw materials are introduced into a rotary kiln where temperatures can exceed 2700 oF. Under these conditions the raw materials experience chemical and physical changes to form clinker. Clinker and gypsum are introduced to a finish mill where they are ground until a desired fineness is achieved. Changes can be made to the raw materials, burning temperatures and finish mills to create different types of cement such as Type I, Type II, Type III, Type V, and Type N and S Masonry Cements.
Q: What is gypsum, and what role does it play in cement production?
A: Gypsum is a mineral (specifically, CaSO4…2H2O - hydrated calcium sulfate). Many Lehigh plants use gypsum that is quarried from a major reserve in Nova Scotia. Some plants are able to recycle industrial by-products with a similar chemical composition as natural gypsum. During the cement making process, after the clinker has been cooled, a small amount of gypsum--up to 5%--is added during the final grinding process. The clinker and gypsum are then interground, producing the super-fine powder called portland cement. Gypsum is a critical ingredient that helps to control the setting time of the cement when it is mixed into concrete. Without gypsum, the cement would begin to set immediately.
Q: Why are there different cement types? What's the difference?
A: For general construction, a Type I cement is used. Typical applications include paving, floor slabs, and building columns. A Type II cement is a modified Type I cement which has a lower heat of hydration than a Type I and greater resistance to sulfate attack. Type II cements can be used for dam construction and in concrete exposed to heavy concentrations of deicing salts (bridge decks). Type II cement is also recommended for use when there will be exposure to marine environments and sulfate attack (below-grade applications and waste water treatment facilities). Type III cement is a high-early strength cement. Type III is chemically similar to a Type I but is ground finer. This provides more reaction surfaces for hydration to occur. Type III can be used for the manufacture of concrete block, precast, prestressed structures as well as in ready mix where high-early strengths are necessary such as in the winter months. Type V cement is specifically designed for concrete exposed to severe sulfate environments. The cement is chemically modified to increase concrete's durability in concrete sewer pipe and sea water applications. Type IV cement was left out of the order above because it is not readily available. Type IV cement is a low heat of hydration product which was used in massive pours such as in dams and other large projects to minimize thermal stresses and cracking. Mineral admixtures or blended cements are used today for these applications.
Masonry cements are specifically manufactured for the masonry wall industry (brick and block). Types N, S and M are readily available and have unique applications. Each of these cements vary in compressive strength and it is important to keep in mind that stronger may not be better! Choose a masonry cement that will provide you with a compressive strength less than the building unit you are using. As a wall moves due to changes in temperature and moisture conditions, the mortar will accommodate these stresses by allowing a hairline crack to form between the unit and mortar (step cracking). A crack in a mortar joint is better than a cracked building unit! Refer to ASTM C270 on choosing the right mortar type for your application.
Q: What is hydration?
A: Hydration is the term used to describe the chemical reaction that takes place between cement and water. For 100 pounds of cement, only 25 pounds of water is needed to fully hydrate the cement (0.25 water/cement ratio). This reaction will generate three primary products: CSH paste (Calcium Silicate Hydrate), calcium hydroxide and heat. The paste is the glue or binder that combines the sand and stone into a rock hard mass. (In actuality, a w/c of 0.25 does not usually hydrate all the cement due to evaporation and the sealing off of spaces around each particle of cement. There is usually a small percentage of unhydrated cement particles in concrete of various w/c ratios and ages.)
Q: Why is fly ash (slag, XPM, silica fume...) used in concrete?
A: Supplemental cementitious materials such as fly ash are commonly referred to as pozzolans. Pozzolans have limited cementitious properties as individual materials but when introduced to a concrete mix, will combine with calcium hydroxide to form additional CSH paste. The materials can provide a cost savings to the concrete producer, extend additional durability features to the owner, be designed for high strength concrete and low heat of hydration applications. They also aid the flat work finisher with better workability and finishing.
Q: What is White Cement?
A: Sometimes we have had this question posed to us - Is white cement made by bleaching gray cement? Well, the answer is no. So, what exactly is white cement? White cement is a manufactured dry powder which when mixed with aggregates and water makes concrete, concrete products and mortars. White cement is a true portland cement meeting the American Society of Testing and Materials (ASTM) standards (ASTM C-150) and has the physical characteristics equal to or better than gray portland cement.
White cement is made from carefully selected raw materials that are extremely low in gray coloring elements. The raw materials used are limestone, clays, and gypsum very low in iron and manganese. Specialized manufacturing techniques are used during kiln burning, raw and finish grinding.
