It is time for the industry to take a more hands on approach to dealing with cementitious wastes
As tunnelling on one of Europe’s largest construction projects gets into full swing, the full cost of handling the large volumes of the cementitious waste generated by such projects is only just becoming apparent. Although minuscule compared to the cost of Crossrail itself, it is not insignificant, and unless minimised can add millions to the overall project cost.
Crossrail will use a significant amount of cement, be that in the form of conventional concrete, shotcrete (for the sprayed concrete lined tunnels), grout from sealing the tunnel lining segments in place or compensation grouting. On completion of each activity any excess cementitious material must be discarded and equipment washed out before the next construction cycle.
The characteristics of this waste vary from water contaminated with small quantities of cement through to neat grout/concrete. While the volume of waste can be minimised by appropriate batch sizing there is always a small amount left in the mixer, pump and delivery line. This can be collected and
either reprocessed by a concrete reclaimer, allowed to harden prior to recycling as a secondary aggregate, or disposed of in accordance with waste management regulations.
“After each activity any excess cementitious material must be discarded and equipment washed out before the next construction cycle.”
However it is the flush water from rinsing equipment that is more challenging to handle. This typically has an elevated pH (with values as high as pH
13 not being uncommon) and has a variable solids content, which depending on the source can vary from simply cement to a mixture of cement, bentonite, additives and tunnel spoil. Consequently the characteristics of this material can vary from dilute water containing a small amount of suspended solids (which rapidly settle out of suspension) through to a viscous, non settling liquid with the consistency of “McDonald’s milkshake”.
In addition to the variable consistency, the amount of wash water generated varies throughout the day. Often the volume or its solids content is too great to allow it to be incorporated into the next mix, therefore the excess has to be disposed of. Off site disposal is expensive as the slurry cannot be discharged directly into sewer or landfilled (as the landfilling of liquid wastes is illegal) but has to tankered off site to a licenced waste management centre for further treatment. Typical tankering costs are in excess of £100/tonne, which for a larger site generating circa 30m3/day amounts to around £100k per month.
Treating these materials on site, rather than handing the problem over to a third party would help reduce costs and vehicle movements. Dewatering the waste so that it can be handled in a similar manner to any other construction waste, and either re-using the water or pH adjusting it prior to discharge off site would be a better approach. On smaller sites this can be achieved by a batch treatment system using a settlement and filter bag system; on the bigger projects it can be handled by filter pressing.
So with the cost of tankering cementitious waste off site ever increasing, surely the moment has come for the industry to adopt these methods and handle the problem itself.