China Clay Sand and Stent

Definition

The by-products from the extraction of china clay from decomposed granite, consisting largely of two distinct materials: 'Stent', which is waste rock, and 'Tip Sand'. China clay tip sand is defined as the washed material produced as a by-product during the extraction of China Clay (kaolin) from kaolinitic granite, predominantly comprising quartz with some mica, and is relatively consistent from within each source production [HD 35/04].

Use

Based upon the Specification for Highway Works [MCHW Volume 1] and the Design Manual for Roads and Bridges [HD 35/04], china clay sand can be recycled into:

• Bitumen bound materials – china clay sand asphalt base is specified in Clause 901 of the Specification for Highway Works and may be used in a variety of base course and binder course mixtures.
• Concrete – may contain china clay sand as the fine aggregate.
• Pipe bedding – may use china clay sand.
• Hydraulically bound mixtures (HBM) for sub-base and base – china clay sand may be used as part of the fine aggregate within HBMs.
• Unbound mixtures for sub-base - may use china clay sand where the grading permits.
• Capping – may contain china clay sand or stent
• Embankments and Fill – china clay sand and stent are generally suitable for construction of embankments and as bulk fill.

Recycled China clay waste has been extensively incorporated into various concrete products as a replacement for virgin materials including in concrete mixtures and in precast concrete products.  Projects where these materials have been used include:

• One Coleman Street is a nine storey composite steel and concrete framed office block designed by Arup where it was originally decided to use china clay stent coarse aggregates in approximately 6000m³ of concrete.  With the use of stent coarse aggregate and increased proportion of fly ash the value of recycled and secondary aggregate content increased three fold.  For every cubic metre of concrete placed, one less tonne of primary aggregate had been quarried and one less tonne of china clay waste tipped in Cornwall [Marsh, 2006].
• The Olympic Delivery Authority has established a concrete batching plant on site where secondary and recycled materials, including china clay waste, replace primary aggregates.  Foundations of the Aquatic Centre, Handball Arena and the Stadium have used concrete containing more than 30 per cent recycled materials [ODA, 2009].

Relevant material and location

Approximately 3 Mt of English china clay was produced in 1999 in the South West of England, of which 87% was exported. English china clay extraction operations are concentrated around the granitic source rock of the South West of England. The largest producer operates 17 open pits in the UK; 14 of which are situated in Cornwall (12 are concentrated in the St Austell area). The remaining three pits are located in Devon and the South West side of Dartmoor [English China Clays International Ltd, 2002].

Tonnages in the UK (excluding Northern Ireland)

Arisings: 19.60 Mt (china clay waste) of which approximately 8.71 Mt is china clay sand and 5.44 Mt is Stent*
Relevant portion suitable for aggregate use: 13.70 Mt (china clay waste)
Aggregate use: 2.60 Mt (19% of the relevant proportion of china clay waste)
Non-aggregate use: 0%
Existing usable stockpiles: 150 Mt (china clay sand and stent)

Explanatory comments

The tonnage data are sourced from the DCLG survey [2007] and refer to china clay aggregate (china clay sand and stent combined).

*China clay sand and stent estimate is based on approximately 9 tonnes of waste being generated for every tonne of good china clay. This waste typically comprises (dependant on the quality and age of deposit) 4 tonnes of coarse china clay sand, 2.5 tonnes of coarse rock material (stent), 1.5 tonnes of overburden, and 1 tonne of fine residue [DCLG, 2007].

Arisings

Located in South West England. China clay is removed by directing high-pressure jets of water at a weathered granite cliff face. The water runs into the bottom of the pit and is pumped up to a treatment plant that settles out the china clay. The grain size and quality of the china clay sand arising may vary between sources, but generally it is only likely to require basic grading and washing processes, as would be applied to other natural aggregates, prior to use. Stent would require crushing and screening for use as an aggregate [Coventry et al, 1999].

There has been historic perceptions of china clay waste having weathered, low quality material.  This is characteristic of sources only in the area of South Dartmoor and there is a long history of satisfactory use in ready mix concretes over much of Cornwall and Devonshire [Marsh, 2006].  Quality control and specifications should be agreed prior to accepting consignments.

Stockpiles

Located in the South West of England, total stockpile quantities of up to 600 Mt, but much of this is not available. Recent work on potentially usable stockpiles indicates a broad estimate figure of 150 Mt [DCLG, 2007].

Trends in aggregate use

China clay sand and stent have been predominately used in the South West of England, which has a finite requirement for such aggregates.  Aggregate use has been steady at its current level [DCLG, 2007].

Future potential

There is commercial and sustainability potential for using china clay waste as secondary aggregate as it is exempt from the Aggregates Levy, the main constraint and hindrance on its use was the geographic location of its source. Improvements in rail transport, for example deliveries to the London Olympic site are by rail directly to the batching plant, and investment in the Port of Par has resulted in china clay waste becoming more competitive.

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