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The Highways Agency undertook research to look into this, and indeed produced an outline specification (not published). There was industry interest a few years ago about the use of recycled materials being incorporated in structural concrete (at the higher value end of recycling). The issues with the retaining wall blocks made from recycled materials is interesting (and worrying). There are also major concerns over their long term durability. This is compared to 18-35 N/mm2 compressive strength usually required for such blocks. The blocks, which are supposed to be used in high capacity load-bearing retaining walls reaching six or seven metres in height, have a compressive strength of 10.5 N/mm2. The potentially reactive aggregate content may have affected the compressive strength of the blocks. A wide range of materials were found to have been used as fine and coarse aggregates including varying sizes of CRT, plastics, copper wire (with its plastic insulation), and fibrous and friable materials. Tests were carried out on a sample and significant concerns over the applicability of the aggregates used in the blocks were found. They are manufactured by a European precast concrete manufacturing company and contain, amongst other material, Cathode Ray Tube (CRT) glass without proper crushing and treatment to remove lead and other contaminants. A UK firm has come across a new large precast concrete block range which seems to use a wide range of secondary recycled aggregates in its mix, including some potentially hazardous waste.