Eco-rate - Buying Guide

Concrete Buying Guide
`A quick guide to strategic product selection`

Background

Concrete is the most common building material in the world and is second only to water as the most consumed substance in the world. Twice as much concrete is used in construction than all other building materials combined, including wood, steel, plastic and aluminum.

Concrete itself is a relatively environmentally sound building material due to its durability, color and recyclability. Roads and structures built from concrete last longer and require less maintenance than other materials. When used in freeways, concrete can result in less vehicle fuel consumption. And because concrete reflects light, it helps reduce heat island effect and lowers the energy demand required to illuminate roadways. Also, concrete is recyclable, with 45 - 80% of crushed concrete usable as aggregate in new construction.

What its made of

Concrete is made out of aggregates (rock and sand), hydraulic cement and water. The aggregate constituent of concrete is typically mined but occasionally is made from recycled concrete or slag. The hydraulic cement is either straight portland cement (limestone, shale, plus gypsum) or a mixture of portland cement and fly ash (see below).

How sustainable it is

As stated above, concrete has attributes such as durability, relatively light color and recyclability. However, the process of manufacturing concrete requires and energy-intensive process, especially the process by which the portland cement constituent of the concrete mixture is made. Producing one ton of cement results in the emission of approximately one ton of CO2, created by fuel combustion and the calcination of raw materials.This accounts for approximately 8% of total global CO2 emissions.

Fly ash

Fly ash is a by-product of burning coal for electricity production, typically captured from the chimneys of coal-fired power plants. In the past, fly ash was released into the atmosphere, but recently mandated pollution control equipment now requires that it be captured prior to release. Once captured, the fly ash is stored at coal power plants, placed in landfills, or recycled.

There will be an estimated 600 million tons of fly ash produced worldwide by the end of this year, about 80% of which will end-up in landfills.

Fly ash works in tandem with cement in the production of concrete products. It has been shown to improve the strength of concrete as well as increase its workability because it reacts chemically with the lime that is given off by cement hydration, creating more of the glue-effect that holds concrete together. It is readily available and involves no added cost, or, in some instances, is cheaper (when fly ash is added to concrete, the amount of cement that is necessary can be reduced).

Using concrete that contains fly ash reduces the need for more cement production and makes use of an otherwise wasted and toxic product. In North America, fly ash is typically used to replace an average of 8% of the cement in concrete, while in many European countries, the replacement rate is greater than 25%.

The use of fly ash as a pozzolanic ingredient was recognized as early as 1914, although the earliest noteworthy study of its use was in 1937. Before its use was lost to the Dark Ages, Roman structures such as aqueducts or the Pantheon in Rome used volcanic ash (which possesses similar properties to fly ash) as pozzolan in their concrete. As pozzolan greatly improves the strength and durability of concrete, the use of ash is a key factor in their preservation.

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