Why is concrete is a mixture and not a pure substance?

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Pure substances are substances composed of identical particles. The particles in a pure substance can be identical element particles or identical compound particles. For example, a sample of pure water is composed of identical `~H_2O ` compound particles. 

Mixtures are substances that are composed of two or more different types ...

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Pure substances are substances composed of identical particles. The particles in a pure substance can be identical element particles or identical compound particles. For example, a sample of pure water is composed of identical `~H_2O ` compound particles. 

Mixtures are substances that are composed of two or more different types of particles. The particles in a mixture are still composed of elements and compounds, but they are not all identical. For example, a mixure of salt and water is composed of dissolved salt compound particles and water compound particles.

Cement is a mixture composed of several different compound particles. These include: calcium oxide, silicon dioxide, aluminum oxide, iron oxide, and calcium sulfate dihydrate. Cement is a mixture because it is composed of different types of compound particles. Each of the components of concrete by themselves would be pure substances. For example, a sample of just calcium oxide would be a pure substance because the particles in the sample would all be identical calcium oxide compounds.

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