2.2.3      Aggregate - The Filler in Concrete

     The focus of this monograph is the chemical reactions and microstructure of cement paste. We won't be talking nearly as much about aggregate, which mostly just sits around in the concrete taking up space. The most important properties of concrete are its strength (how much load it can support) and its durability (how long it will last in its environment). To a first approximation, these are both controlled by the cement paste rather than by the aggregate. In the case of strength, this is because the aggregate particles are normally much stronger than the cement paste, so the concrete fails (breaks) when the strength of the weaker cement paste matrix is exceeded. A similar situation occurs with durability. Cement paste is inherently more susceptible to environmental damage than the aggregate due to its pore system, which allows water and dissolved ions to enter and leave the paste.

     However, there are some ways that the properties of concrete are affected by the aggregate, and these will be discussed in this monograph. The workability (consistency) of the fresh concrete, which determines how easy it is to pour and place the concrete, depends on the shape and size distribution of the aggregate particles. The structure of the cement paste in a narrow region surrounding the aggregate particles is more porous than the bulk cement paste, which affects several properties. This region is called the interfacial transition zone (ITZ). Finally, in some cases the aggregate reacts with the cement paste, resulting in cracking, expansion, and deterioration.

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