Concrete is used in varying amounts in a wide variety of buildings, from a simple beetle statue to a Megalopolis.
Concrete is made in a Clinker Vat from a mixture of cement, gravel, and water. Cement, in turn, is mixed from bauxite, gypsum, clinker, and water, also using a Clinker Vat.
To build a Clinker Vat, the following materials are required:
Clinker Vats run on petroleum, charcoal, or coal -- you choose the fuel type when you build the vat. In practice, nearly all Clinker Vats in existence are of the petroleum-burning variety.
Making cement in a Clinker Vat is a labor-intensive process that lasts one hour. During this time, the vat must be stirred continuously to prevent the cement from hardening. As the hour passes, more frequent stirring is necessary. Stirring cement is an Endurance-based activity, and thus the process will require multiple stirrers and/or a supply of Endurance food.
Cement is made in batches of 100. Each batch of cement requires 10 bauxite, 10 gypsum, 800 clinker, and 500 water. Additionally, 40 petroleum is needed to start the Clinker Vat (assuming it is a petroleum-burning vat).
The following hour is divided into thirty 2-minute time blocks. During each block, the Clinker Vat must be stirred a certain number of times, otherwise the cement batch is ruined. The first two 2-minute blocks require 1 stir apiece, the next two blocks require 2 stirs apiece, and so on, until the final two 2-minute blocks which require 15 stirs apiece.
You must rest after stirring the Clinker Vat, the rest period determined by your Endurance. If a 2-minute period elapses without the required number of stirs, the cement batch is lost (and the hardened cement must be removed with a sledgehammer before the vat can be used again).
Once the hour is up, the 100 cement may be taken from the vat.
Concrete is made in batches of 250. Each batch of concrete requires 250 gravel, 15 cement, and 100 water. (Thus, each cement run produces enough cement for 1500 concrete, with 10 cement left over.)
Unlike the cement-mixing process, making concrete is instantaneous. It does not require Endurance, nor does the Clinker Vat need to be fueled.