WRAP (The Waste & Resources Action Programme)

Use of recycled plasterboard in unfired clay-gypsum blocks

Executive summary

Over one million tonnes of waste plasterboard are estimated to be produced each year in the UK from construction and demolition activities. Most of this waste is currently sent to landfill, even though it can easily be recycled. WRAP is working to divert plasterboard waste from landfill by seeking to overcome the barriers to plasterboard recycling. One area of its work is to develop markets for the materials from plasterboard recycling (recycled gypsum and reclaimed paper). This project evaluated the use of waste plasterboard as the source of the gypsum in novel unfired gypsum–clay construction blocks.

The collaborative project was led by Akristos Limited and also involved two brick companies (Baggeridge Brick and Hanson Red Bank), two plasterboard recyclers (Roy Hatfield Limited and Atritor Limited) and a number of consultants including the BRE Centre in Innovative Construction at the University of Bath and Natural Building Technologies.
Trial design and results

A series of laboratory-scale and production-scale trials were carried out during the project at the Measham works belonging to Hanson Red Bank. Production constraints during the project timescale unfortunately limited the work that could be undertaken at Baggeridge Brick’s Sedgley works.

Target performance characteristics for the gypsum–clay block product were developed from two benchmark products – a clay–fibre block (Karphosit) and a gypsum block (Promonta) – imported into the UK. Die designs for extrusion trials at Hanson Red Bank featuring a block design of 440 × 220 × 120 mm were also developed. Some processing was required before waste plasterboard could be used as a feedstock in the manufacture of the gypsum–clay blocks. The process at Roy Hatfield incorporates a series of shredding, crushing and air-separation processes to produce a 10–15 mm material. This was the material used in the laboratory- and production-scale trials. The Atritor process based on a turbo separator also produced comparable material (<15 mm), but the plant did not have the capacity to produce sufficient material for production-scale trials.

Preliminary development work using Keuper Marl/Grog clay blend (from the Measham works of Hanson Red Bank) and Etruria Marl clay blend (from the Sedgley works of Baggeridge Brick) suggested that 50% addition of recycled plasterboard (by volume) would be most appropriate.

All larger scale trials used a blend of processed plasterboard supplied by Roy Hatfield Ltd on a 50 : 50 volume basis with Keuper Marl clay. Use of a front-end loader and a bucket blending technique was shown to more suitable than use of a bulk bag discharge unit for the preparation of the amount of plasterboard–clay mix required for the production-scale trials. Before being used in the trials, the blend was dry ground in a hammer mill to give a free-flowing mix containing particles <2 mm in size.

The project team examined the compressed earth block machine available from Dutch company Oskam V/F in order to evaluate the potential of pressing (the traditional method for manufacturing concrete blocks) for production of the novel blocks. However, it was felt that pressing a fine plasterboard–clay mix could be problematical and extrusion was felt to a more suitable technology.

Initial trials using the horizontal extruder and a standard die at Hanson Red Bank’s Measham works were successful in producing a block product containing 100% clay. Successful production of a small number of blocks with the 50 : 50 recycled plasterboard–clay blend on the more labour-intensive vertical extruder at Measham gave confidence to proceed to a production-scale trial using the more efficient horizontal extruder together with its purpose-built die. The gypsum–clay blocks made on the horizontal extruder were used for performance testing (compressive strength, flexural strength, abrasion resistance, etc.) and to build a number of demonstration walls.

A traditional cement : sand mortar proved ineffective but a mortar system based on clay, silica sand and sodium silicate produced excellent results. A thin joint mortar system was applied successfully using a cartridge gun or trowel.

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