Chert

Chert, also known as flint, is a silica-rich sedimentary rock. Chert is very hard and its conchoidal fractures can leave sharp edges that do not dull easily. For this reason, it was one of the earliest materials used for tools and weapons.

In some areas, chert is ubiquitous as stream gravel and fieldstone and is currently used as construction material and road surfacing. Part of chert’s popularity in road surfacing or driveway construction is that rain tends to firm and compact chert while other fill often gets muddy when wet.

Cherts are subject to problems when used as concrete aggregates. Deeply weathered chert develops surface pop-outs when used in concrete that undergoes freezing and thawing because of the high porosity of weathered chert. The other concern is that certain cherts undergo an alkali-silica reaction with high-alkali cement. This reaction leads to cracking and expansion of concrete and ultimately to failure of the material.