How Bricks are Made

How Bricks are Made

20th January 2016

Last week we took a brief look at the history of bricks, how they were made in pre modern China and the subsequent developments in the production of bricks that have enabled them to become one of the most common building materials on the planet.  Today we’re going to take a look at the three different ways in which bricks are produced – mudbrick, fired brick and chemically set brick.

Mudbrick

Mudbricks are made from a mixture of loam, mud, sand and water mixed with a binding agents like straw or rice husks and then air dried until they are ready to use.  This is the way the earliest bricks were made and the oldest that have been discovered so far date to before 7,500 BC in the Upper Tigris region of the Middle East.  Mudbricks were actually in use in the Pre-Pottery Neolithic period and were used by the Mesopotamians to construct their cities. 

The earliest bricks were typically flat on the bottom and curved on the top and are called plano-convex mudbricks.  Some bricks were formed in square moulds and rounded so that the middle was thicker than the ends.  In Ancient Egypt, mud was collected from the Nile River and then poured into a pit.  The mud was then trampled on by workers (or slaves) and straw was added to solidify the mixture. 

In the warmer regions of the world, the mudbricks were dried in the sun which is probably why the use of bricks first developed in the Middle Eastern countries with hot climates. 

Fired Brick

In other, cooler regions, mudbricks would have to be fired in a kiln – a much more expensive prospect as it involved the use of timber to fuel the kilns.  This made brickmaking more labour intensive as well as more costly.  The oldest kiln-fired bricks discovered are from the 2nd millennium BCE site in China.  Bricks were being produced in pre-modern China on a surprisingly large scale about 3,000 years ago. 

Nowadays, fired bricks are formed by one of three processes – soft mud, dry press or extruded.  The soft mud method is the most common because it’s the most economical method.  Raw clay is mixed with 30% sand to reduce shrinkage and then pressed into steel moulds with a hydraulic press.  The shaped clay is then fired at 900 – 1000 ° to harden and strengthen it for building purposes. 

Chemically Set Bricks

These bricks are not fired but are produced using an alternative curing process that’s speeded up by the application of heat and pressure in an autoclave.  This produces two types of bricks – calcium silicate bricks and concrete bricks.  Calcium silicate bricks (also known as sand lime or flint lime bricks) are not made from clay but are produced by binding a silicate material with lime.  The raw materials include lime mixed with sand, quartz, crushed siliceous rock or crushed flint which are mixed until the lime is completely hydrated.  The mixture is then pressed into moulds and cured in an autoclave to speed up the chemical hardening process. 

Concrete bricks are typically pale grey and are made from a dry, small aggregate concrete formed in steel moulds by vibration and compaction.  The finished blocks are then cured using low pressure steam.