2/2015 – Recycling cut-off glass in concrete blocks

Reusable elements in concrete design, a vital component of sustainability

In a recent article in Concrete Beton, Bryan Perrie, MD of the Concrete Institute, pointed out that more attention should be paid to re-use and recycling of building materials at the design stage to more actively promote sustainability in the construction industry.

Says Perri: “There is growing concern worldwide about this wastage of materials such as concrete, wood, gypsum, metals, bricks, glass, plastics, and salvaged building components. Yet there is tremendous potential to recycle so many elements of the construction cycle.”

Recycled glass aggregate concrete mix

Recycled glass aggregate for concrete mix

Samples of the crushed glass were sieved through a series of screens

Samples were sieved through a series of screens

The Terraforce concrete retaining wall and erosion control system can already be considered a sustainable product and has been since its inception 35 years ago – long before it became fashionable. It is a hollow core and closed faced system allowing for maximum water absorption and plant growth, once installed. It is also an easy to use, cost-effective mortar less system that requires less material and equipment staging areas and fewer requirements of large pieces of equipment for manufacture and installation than cast in-situ solid concrete solutions.

Blocks underwent extensive crushing tests

Blocks underwent extensive crushing tests

Inspecting the results of the crushing tests

Inspecting the results of the crushing tests

In the Western Cape, Klapmuts Concrete and Cape Retaining Systems, Terraforce concrete retaining block manufacturers and contractors are also committed reducing their carbon footprint as much as possible. Like most Terraforce manufactures locally and abroad, Klapmuts previously acquired their own crusher to re-use all broken and reject products. Says Johan van Wyk, CEO of Klapmuts Concrete:

“We recycle all reject blocks as well as blocks that get damaged during the “split” operation of the rock face blocks. We accept “clean” building rubble (no timber, plastic, metal, etc.) from builders as well as all the rejects from another building block manufacturer in our area. To ensure that the rubble can be used in our products it without comprising quality it was graded by a SANAS accredited laboratory.”

The blocks produced a steady result of 11MPA

Crush test:  a steady result of 11MPA

The glass is pulled into the mould machine

The glass is pulled into the mould machine

Wanting to take this principle a step further, when approached in 2013 by Renzo and Fabio Schincariol of Nelson Glass, a major processor of plate glass in Cape Town, van Wyk immediately took interest in their proposal to use crushed, recycled glass in their concrete mix for block production.

Says Keith Clarke, Manager of Re – Use Glass, Nelson Glass’s brand new department focused solely on recycling glass debris and cut-offs: “The idea of recycling glass was conceived in 2012, as a result of mounting concerns over waste and cost of dumping glass cut-offs at landfill site across the Western Cape by glass companies and their subsidiaries. Subsequently I was commissioned by Renzo and Fabio Schincariol to conduct a feasibility study over a period of nine months to establish the viability of such a plant.”

After two years of planning and red tape Nelson Glass invested in a crushing plant. Says van Wyk: “As Nelson Glass cannot use the crushed glass themselves, they were looking for possible buyers of the product, and this is where we saw an opportunity to get involved.”

At Nelson Glass, the raw material (off-cuts of glass or broken glass) is collected from most of the big processing companies. Says Clarke: “This is done with our own waste removal truck by placing waste skips at their premises which when full are collected and brought to the recycling facility.

The broken glass is recycled by running it through a crushing and screening process which converts it into a reusable chips or sand format that can be supplied to the building industry for the manufacturing of cement blocks or slabs, replacing sand and stone used in the industry.”

Klapmuts Concrete started experimenting with the first samples of crushed glass late in 2014. Says van Wyk: “Samples of the crushed glass were sieved through a series of screens to compare it with regular crushing dust, blocks were produced and crushing tests were done. Interestingly the crushed glass particles do not have sharp edges and can be handled exactly the same as normal aggregates.”

broken glass is recycled by running it through a crushing and screening process

Broken glass recycled by a crushing and screening process

The byproducts: 3-6mm glass aggregate and glass dust

The byproducts: 3-6mm glass aggregate and glass dust

From these results, as well as studies done in the USA, it was determined that glass can be used in masonry products, but there is a decrease in compressive strength of the product that increases as the percentage of glass increases.   Finally, the decision was made to use crushed glass in limited quantities in almost all our products. It was the perfect opportunity to use even more recycled product in our concrete mix, replacing even more virgin material in the manufacture of our products.”

Klapmuts currently uses two products – a course product that compares favourably with standard 6mm stone and a dust that is very similar to crusher dust. Says van Wyk: “The mix designs were adjusted to accommodate these products without jeopardizing the strength of the blocks. The finishes of the products are exactly the same as with normal aggregates except for the rock face blocks where there now is a slight twinkle on the rough face. “

Bottom line is that glass waste that would have ended up in a landfill as a hazardous and non-degradable by-product of the glass industry is transformed into a useful, eco-friendly substitute, reducing the carbon footprint (reduction mining of raw material and waste disposal) of both Klapmuts and Nelson Glass respectively.

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