What Is The Difference Between Concrete and Cement?
They may look similar, and they may have similar purposes, but concrete and cement are not the same thing. That doesn’t stop people from getting them mixed up, but among construction crews, the term concrete is not interchangeable with cement. In fact, that cement mixer isn’t carrying cement at all—it’s carrying concrete.
The primary difference between cement and concrete is what they’re made from. While cement can be incorporated into a project by itself, it’s more often used as a component of concrete. In other words, you can think of cement as a major ingredient in concrete, but not the only one.
What Materials Does Concrete Consist Of?
Concrete is a mixture of cement, water, and aggregates like crushed stone, sand or gravel. Together, the water and cement are mixed together to create a paste, and the aggregate material is added to create a type of rock-like mass. The water and cement paste facilitate hydration and the curing process, while the concrete mix provides strength.
What is Cement Made From?
The official term for cement is “Portland” cement, though that’s not a brand name. It comes from Joseph Aspdin, the creator of Portland cement, who gave the building material its name because it looks like stone from the Isle of Portland.
All cement is Portland cement, and even though there are many cement mixes available, they mostly contain the same substances. They include clay, iron ore and limestone, among other materials like gypsum or shale. These materials are screened and placed in a rotary kiln, where they’re heated to more than 2,600 degrees Fahrenheit. Following heating, the material forms into a clinker – many thousands of them, in fact. The clinkers are ground down into a powder that is ready for use as a concrete ingredient.
There Are Several Types of Cement - For Several Types of Applications
There are many kinds of cement, each with their own properties. Though there are many more, the American Society for Testing and Materials (ASTM) recognizes five types of cement for construction. They include:
- Type I – Type I cement is general purpose cement, appropriate for the vast majority of projects and settings. If additives aren’t required to guarantee concrete performance, Type I cement is sufficient.
- Type II – Type II is also a general-purpose cement, but it comes with moderate resistance to sulfates. Sulfates travel freely through water, so if the concrete will be exposed to water-rich environments, Type II cement is recommended. Sulfates are also present in some soils.
When sulfates are exposed to cement, they can cause the formation of ettringite, an expansive crystal-like substance. Too much ettringite will cause tensile stress inside the concrete, and eventually cracks will form in the material.
- Type III – Type III cement is also called high early strength cement because it’s used when rapid curing is required. Type III cement is expected to cure to structure-bearing strength within 24 hours of placement.
- Type IV – Type IV is low heat of hydration cement. During curing, some heat is generated as a by-product of water and cement reacting. Too much hydration-based heat can result in thermal cracking for large concrete pours. In these settings, Type IV cement allows for precise thermal control during curing.
- Type V – Type V cement is similar to Type II cement, only its sulfate resistance is much greater. Type V cement is used in applications where constant exposure to sulfates is expected, so you’ll find it in underwater and submerged structures. You’ll also find it along the coast and in roadways.
Why is Concrete Used Instead of Cement in Construction Projects?
Cement has a number of uses in construction, but concrete is the material of choice when building structures, sidewalks and parking lots. It comes down to one thing – durability.
Concrete is much more durable than cement. The presence of aggregate fortifies the concrete from cracking, which is something cement has trouble with. The silicas inside cement are tough, but they can’t handle the kind of compressive forces that concrete can. Cracks start small, but they eventually add up to total material failure.
Concrete can also be made tougher with steel – specifically rebar. Reinforced concrete provides excellent compressive and tension strength, ensuring it stands up even during major natural disasters.
Concrete Doesn't Dry - It Cures
As it turns out, there’s a lot of confusion regarding concrete-related terms. The mix-up between concrete and cement is one – another is confusing drying and curing.
Most people assume that concrete dries, but it doesn’t. Instead, it cures. What’s the difference?
- Drying – When a material dries, the free water content inside the material evaporates off. The interaction between the material and the water may be minimal prior to and during the drying process. Most materials dry – they don’t cure.
- Curing – Some materials, including concrete, do cure. During the curing process, water doesn’t evaporate, but reacts with the concrete instead. Specifically, hydrating the concrete allows calcium silicate hydrate bonds to form inside it, strengthening its internal structure. This process never completely stops, but it’s most active for the first few days after the concrete is hydrated.
Curing is a critical part of the concrete-setting process. It greatly improves the concrete’s durability, and the longer it’s facilitated, the stronger the concrete will be. In addition to time, it’s important to nail the ratio between cement and water. Too much water will compromise the concrete’s structure and could result in surface flaking. Too little water and bond formation will end before the concrete is of adequate strength.
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