How Mapei’s solution transforms returned concrete and washing slurry into resource materials, while significantly reducing costs

Over the past few years, the rising price of fuel and virgin raw materials has caused an increase in the total cost of ready-mix concrete production. This situation has been exacerbated by the Covid-19 pandemic due to disturbances in global logistics and raw material supply chains. In combination with the growing awareness in society about the need for more circular production models, the management of waste streams in concrete production has come into the spotlight as a major cost and environmental issue.

Returned concrete and slurry from washing of trucks, which previously could be disposed at low cost, is now becoming more and more expensive to handle. As a result, Mapei has developed an innovative solution that transforms returned concrete and washing slurry into resource materials.

Sven-Henrik Norman, corporate product manager of Re-Con Line at Mapei Group, shares more about this solution and illustrates how it can be applied to minimise the use of water and raw materials like sand, aggregates and cement, while also reducing water pollution.

Circular process: more sustainable, lower cost

The Re-Con Line from Mapei helps concrete producers become more sustainable by offering a solution that transforms, reduces and recycles waste streams.

“A 50,000 cu m yearly production normally has a rate of returned concrete of 5%, amounting to 2,500 cu m or approximately 5,800 t. It also has a waste stream of washing slurry. If there is no possibility to reuse the returned concrete, it will have to go to landfill as waste,” explained Mr Norman.

“The slurry must in any case be deposited in landfill or in an approved deposit as it contains heavy metal contamination like hexavalent chrome. The cost of this handling can be tens of thousands of Euros for the production in this example.”

However, if the Re-Con Line solution is applied in concrete production, substantial savings can be achieved, said Mr Norman. “Not only are handling costs eliminated with the Re-Con Line solution, the transformation of the waste materials into recyclable aggregates also leads to savings by reducing the need for virgin raw materials.”

Mr Norman described the process using a flow diagram (see image 1). There are two scenarios for the trucks that return from a delivery at a jobsite to the batching plant: either it contains some returned concrete (A), or it is empty of concrete but has about 200 kg of cementitious residue that covers the inside of the mixer drum in a thin layer (B). 

In scenario A, the returned concrete is treated with the Re-Con Zero Evo process, which transforms the fresh returned concrete into a granular material that is emptied on the ground and left to dry and harden for eight to 24 hours. After that, the truck can have a light wash and get ready for the next job.

For the concrete truck operator, the process has taken about 15 minutes and instead of a flow of concrete that needs to be handled and processed, the batching plant now comprises an available volume of aggregates. This material is called Re-Con Zero aggregates, and it can either be sold as a base course or backfill material, reused in the production of new concrete or as a dry washing aggregate (as in scenario B).

In scenario B, when a truck returns empty but dirty, it goes directly to the Re-Con Dry Washing process. Approximately 1 cu m of aggregates, or 2,500 kg is fed into the empty truck and then rotated back and forth inside the drum for about four minutes. During this rotation, the aggregates clean the inside of the drum and absorbs around 70% of the cementitious residue onto the surface aggregates, forming a new layer that will dry and harden in 12-24 hours. 

The truck empties the aggregates in a material storage unit, and then proceed to a light wash where the remaining residue in the form of fine sand with a minor element of cement is collected for sedimentation and dewatering. The need to dry wash an empty truck varies from case to case. Normally, it is only required after the last load of the day; but in a hot climate or special cases, concrete trucks also have to be cleaned during the day in between deliveries.

The dry washing aggregates can be reused for about 10-15 times before its slurry absorbing abilities are no longer efficient enough. It is then used in process (C) as replacement for virgin raw materials in new concrete, or in (D) where it is sold externally as aggregates. 

In scenario C, the use of Re-Con Agg admixtures can save a lot of water and thereby cement, by keeping the concrete made with recycled aggregates flowable for a longer time after mixing.

Re-Con Zero Evo

Since concrete generally needs to be used within one to four hours from the mixing, over-ordering and quality issues can result in a daily flow of tens of cubic metres of concrete back to a single mixing plant,  generating a waste stream that requires significant resources, time and costs to handle and in many cases transport it to landfills.

With Re-Con Zero Evo, the returned concrete can be transformed into a recyclable aggregate material. The process uses a two-part system including a superabsorbent polymer and a hardener. "The water in the concrete is bound by the superabsorbent, while the hardener accelerates and stabilises the process,” explained Mr Norman.

“The returned concrete can be transformed in the truck mixer, or in a separate process if the truck is needed quickly for a new job. Once finished, the recycled aggregate can be used in new concrete or sold as backfill material, leading to substantial savings in handling costs.” 

Mr Norman added that the aggregates can also be used in an intermediate process of absorbing truck washing slurry in the Re-Con Dry Washing process.

Re-Con Dry Washing

The Re-Con Dry Washing method greatly reduces washing slurry waste and washing water consumption from concrete trucks by using the absorbing properties of the Re-Con Zero Evo aggregates made from transformed returned concrete.

“This method makes it possible to recycle washing slurry into a recycled aggregate, instead of having to process, transport and deposit it at landfills,” said Mr Norman. “Much less water is needed to complete the cleaning of the truck mixing drum after it has been dry washed. 

“The residual washing water has a lower solids content, lower pH and a lower risk of containing heavy metals like hexavalent chrome. All this is due to the unique absorption of cementitious residue onto the dry washing aggregates.” 

Mr Norman further revealed that Mapei has analysed and tested the Re-Con Dry Washing aggregates together with the renowned research institute SINTEF in Norway, and discovered that the Re-Con Dry Washing particles absorb carbon dioxide through the process of carbonation. “Thus, the Re-Con Line offers unique possibilities to produce carbon negative aggregate materials and Mapei Research continues in this field.”

Re-Con Agg admixture

There is also a growing shift to use more recycled materials and to utilise manufactured sands and lower quality sands in higher ratios in concrete production. Mr Norman noted that recycled aggregates generally have a higher water absorption due to their porous surface structure.  

“Manufactured sands with high fines content and low-quality sands containing clay minerals can also cause a higher water consumption in concrete mixing. With a high water demand, a higher usage of cement automatically follows, since the water/cement ratio is dictated through industry standards.” 

Mapei’s Re-Con Agg admixtures allow increased ratios of water-demanding sand and aggregates to be applied without increasing the need for cement. The Re-Con Agg polymers block the porous surfaces and crystalline structures in demanding aggregates and sand from absorbing the mixing water from the concrete in its fresh state. 

“Instead of getting soaked up into the porosities or laminar mineral layers of the sand or aggregates, the mixing water stays available in the fresh concrete, keeping it flowable for much longer than it would have been without these admixtures,” said Mr Norman. 

“The alternative would have been to add more water and more cement to the mix, leading to higher costs and a bigger carbon footprint.”

The article courtesy of Realtà Mapei International 95.