Carbonates: Calcite and dolomite are the dominant minerals of carbonate rocks. Depending on their proportion, the rock is a limestone, with dominant calcite, or a dolostone, with dominant dolomite. If the amount of clay or fine silica is minimal, both these rock types are durable. Argillaceous carbonates (those with dispersed clay particles) present a durability problem.
An intermediate rock, a dolomitic limestone, is sometime problematic. The discrete crystals of dolomite within a calcite matrix may be subject to a de-dolomitization process under influence of strong alkalis. The de-dolomitization process results in volume increase and expansion of the rock, known as the destructive alkali-carbonate reaction in concrete. Fine silica or silicate dispersed in the carbonate can result in alkali-silica reaction.
Chert, a form of amorphous silica, is sometimes found interbedded with or as nodules within limestone formations. Crushed rock containing chert may also be subject to alkali-silica reaction. However, not all cherts are reactive. Leached and weathered forms are more reactive then solid, unaltered chert.
Sandstones: Sandstones consist mainly of quartz (sometime some feldspar, as in arkose), with various types of cements - argillaceous (clay), carbonate, iron oxide, and silica. The latter produces quartzites, some of the hardest aggregates know. The properties of sandstones depend more on the cement than on the quartz grains; although in some cases, the quartz grains may have been derived from tectonically affected rocks and are strained. ASR reactions are thus possible. The degree and type of cementation determines the strength, abrasion resistance, and impact resistance of sandstone aggregate.
Siltstones: Some well indurated siltstones are sometime used, although the fine-grained character of the rock and its relative weakness make the rock a less desirable aggregate. Shales, the very fine grained detrital rocks, are not durable, and are in fact undesirable, if mixed in with above aggregates. Shale may be used as fill material.
