Recover returned concrete within minutes

10 June 2014

Returned concrete is a reality and a liability in the readymix concrete industry. Traditional methods of handling this problem have significant operating and capital costs and are increasingly in conflict with municipal environmental regulations. A completely new approach to returned concrete recovery is Re-con Zero, an innovative additive technology supplied by Mapei South Africa. This remarkable product provides a solution to returned concrete within minutes, and involves zero waste and zero impact on the environment.

Traditional ways of dealing with returned concrete vary from environmentally undesirable disposal on landfill sites, or crushing the cured concrete for recycling as concrete elements such as masonry blocks, to a capital investment in a treatment plant to recover the aggregates and dispose of the cementitious sludge. A new Mapei technology does away with all of these options.

For the past five years, the Mapei South Africa technical service team has been introducing the local construction industry to the benefits of the advanced building technology additives, sealants, adhesives and concrete performance chemicals that are available from the international Mapei Group. Offering market-leading performance and handling advantages, Mapei’s 1400 products are recognised throughout Europe, America and the Far East, for their adherence to sustainability principles as ‘green’, environmentally-friendly formulations. Mapei South Africa is a member of the Green Building Council of South Africa in support of the drive to ensure that all local buildings are designed, built and operated in an environmentally sustainable way.
Re-con Zero additives are used to treat concrete in the returning truck mixer where, in a few minutes, it is transformed into granular material. This is discharged onto a paved area and allowed to cure, after which it can be used as aggregate for a new concrete mix. No waste is produced during this process and the minimal washing the mixing drum requires produces a waste water that is completely recyclable as mixing water.

Treatment with Re-con Zero is a two step process: first a polymer compound is added to the mixer drum at the rate of 0,5kg per cubic metre of returned concrete. The polymers absorb water, swell and begin to dissolve. After 4 minutes of mixing, the returned concrete is transformed into a granular material. The second step involves adding a set accelerator to the drum at a dosage of 6kg per cubic metre of concrete. This contains ettringite-forming compounds that, after 3 minutes, provide enough strength to enable the granular material to be discharged from the drum and the truck mixer to return to service.

“Our products are the outcome of extensive research and testing in international markets and Mapei’s 75-years of experience in the building industry,” says Mapei South Africa’s Antony Offenberg, Manager Admixtures and Grinding Aids. “This means that when Mapei gives the assurance that the Re-con Zero treatment takes minutes, it means exactly that. When we say there is zero waste, it also means exactly that: no liquid or solid waste – even the product’s bags are water soluble!”

Since the discharged granules are a fresh cementitious mix, they must be allowed to cure. To keep the granules free-flowing, the heap must be moved within the first 24 hours to break the weak bonds of hydrated cement paste between the particles. Once this has been done there is no further risk of caking (sticking together) and the material can be stored and used in the same way as normal aggregates. Re-con Zero works with all types of concrete and research has shown that the characteristics of new concrete produced with returned concrete aggregates are essentially the same as that of concrete made with virgin aggregates. One difference is an improvement in compressive strength, which seems to be related to an improvement in the interface transition zone (ITZ) between granules.

Re-con Zero reflects Mapei’s rigorous approach to sustainable development by responding to the three core elements of true sustainability:
• Economic advantages for the customer:
– One cubic metre of returned concrete is transformed into 2,4 tons of quality
– Saves the operating cost of traditional disposal measures
– Avoids costly investment in treatment plant and equipment
• Environmental advantages:
– Returned concrete is completely recycled
– Reduces the use of non-renewable virgin aggregates
– Correspondingly reduces the transportation of virgin aggregates since the returned concrete is processed at the readymix plant
– Assists with achieving targets related to recycled content in concrete
– Helps obtain or renew municipal operating permits
• Social advantages:
– The absence of hazardous or toxic substances in Re-con Zero and its ease of use contribute to a healthier and safer working environment

“We are pleased to be able to offer our readymix customers a cost-effective, operationally attractive solution to the ever-present problem of returned concrete,” comments Offenberg. “Mapei’s market-leading products are uniquely ‘green’, while introducing new performance benchmarks for speciality chemicals in the South African construction industry.”

Image caption: Using Re-con Zero, the returned concrete can be discharged as granules within minutes.

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