Raising a glass to river sand substitutes

Raising a glass to river sand substitutes

Danny Chung

Windows and curtain walls spring readily to mind when glass is mentioned as a building material but as one of the constituent materials in mortar or low strength concrete products?

Not so strange as it seems according a research report compiled recently by the Construction Industry Council on finding suitable replacements for river sand in concrete and in cement-sand mortar.

The 55-page report said that due to limits placed on dredging for river sand due to adverse effect on the environment and stability of river banks, it was necessary to look for alternatives.

The report identified manufactured sand (crushed rock fine that has been processed), crushed waste glass and recycled aggregate as possible alternatives to be used in concrete and mortar.

Other alternatives such as furnace bottom ash and granulated blast furnace slag were ruled out.

“Regarding crushed waste glass, about 120,000 tonnes of waste glass is produced every year in Hong Kong but only a small amount is recycled. In theory, it should be possible to crush waste glass into sand size for use as aggregate in mortar,” the report said.

It suggested that trial production and application be carried out to look at the feasibility of using waste glass.

According to the report, only about four percent of waste glass is crushed and used as aggregate in the production of precast concrete paving blocks.

“Most of the waste glass is just dumped to landfills as waste,” the report said.

Due its high brittleness, glass was unsuitable for use in concrete in structural applications due to risk of alkali-silica reaction, which could affect the durability of concrete.

However crushed waste glass could be used as fine and course aggregates in materials such as concrete paving blocks in non-structural walls, concrete blocks for pervious pavement and in-situ concrete in minor works.

Mortar for plastering, rendering screeding and masonry could also use crushed waste glass as the aggregate.

Researchers said recycling rates for glass could jump to 50 percent or more if glass was used in mortar.

However the report noted the high cost of collection and transport which added to the cost of producing crushed waste glass.

“More support to the recycling industry (such as helping to do research, helping to collect more waste glass for recycling, and promoting the use of crushed waste glass for sustainable development) needs to be provided or otherwise, the production of crushed waste glass to be used as aggregate in mortar will have no market,” the report said.

Eco blocks, invented by Hong Kong Polytechnic  University, use glass in its manufacture. (Danny Chung)

Eco blocks, invented by Hong Kong Polytechnic University, use glass in its manufacture. (Danny Chung)

One initiative getting serious about recycling glass is Green Glass Green.

According to its website, the initiative was started by The Hong Kong Dumper Truck Drivers Association and funded by the government’s Environment and Conservation Fund.

“Witnessing first hand the amount of the waste glass dumped at the fills, the Association launched this project as a step towards a greener and more sustainable Hong Kong,” the website said.

It has been collecting glass since June 2010 for use in Eco block, pavement block invented by Hong Kong Polytechnic University and manufactured by a Laputa Eco-Construction Material Company under licence.

As regards river sand substitutes in concrete, the report noted that concrete producers started using crushed rock fine in place of river sand a few years ago.

Even so the high fines content can cause problems in concrete and the report proposed that crushed rock fine be processed to reduce and control the fines content, as well as improve particle shape to suit different applications.

This “manufactured sand” would be a better alternative to both river sand and unprocessed crushed rock fine as aggregates for concrete and mortar.

“However, manufactured sand for mortar and manufactured sand for concrete are not the same and research studies are needed to determine the separate requirements for them,” the report said.

The current CIC report is only the first phase and researchers said in the second phase, drafting a standard for aggregates in mortar as well as further research on using glass and the other alternatives should be conducted.


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