• Les Ellaby

Carbon Neutral Concrete Blocks without OPC

Updated: Aug 28, 2020

Cement production generates large quantities of fine grained a (<250µm grain size) waste material called cement kiln dust (CKD).

Abeln et al. (1993) define CKD as a “fine particulate matter that consists of entrained particles of clinker, raw materials, and partially calcined raw materials”.

The production of CKD means there is a financial loss to the cement manufacturing company.

Valuable raw materials are consumed and processed. There is a high energy usage involved and there is a collection, disposal and storage problem of the CKD.

Improvements have been made in cement production to reduce the generation of CKD, but there are still large volumes of the CKD held in stockpiles and land-filled every year.

There are strict environmental regulations associated with the storage and containment of CKD, therefore finding a use for it would be beneficial to the cement industry.

The washing of CKD to remove chlorides is a costly and energy consuming process, which is not always favoured by the cement producer.

CKD land-filled close to water courses or agricultural areas increases the risk of damage to human health and aquatic life.

It has been suggested long term exposure to CKD can lead to colon, liver and respiratory cancer.

As a partial binder in construction products CKD causes problems with poor reactivity and low strength achievement with OPC, resulting in severe exotherms, delamination of formed materials produced, and retarded hydration.

PowerCem were faced with the problem of incorporating a high percentage of CKD into a construction product, whilst immobilizing sulphates and chlorides.

Having trialled various blends of OPC/CKD and Sand, GGBS was introduced into some of the blends in the hope that this would help contain the sulphates and chlorides.

Progressively, GGBS was used as the co-binder with the CKD and Sand.

This caused an expulsion of salts that could be welcomed for some applications, but it was deemed to be of an erratic nature and unpredictable, so we introduced our own product ImmoCem into the various blends.

GGBS/ImmoCem/Sand/CKD did show very much reduced expulsion of salts at the 72 hour mark and beyond.

Having succeeded in containing the sulphates and most of the salts in the matrix, it was decided to introduce the plentiful low lime type F Fly Ash and Incinerator Bottom Ash into the mix as replacements for GGBS.

This replacement was decided on to both significantly reduce both cost and also embodied CO2 in the end product.

The mixing was carried out in early December 2013 with specimens tested for compressive strength and flexural strength carried out at 13 days just before the Christmas holiday period.

Compressive strength was recorded at 9MPa and flexural strength was an excellent 3.2MPa.

NEN 7375:2004 was chosen to assess the mix against the Dutch Soil Quality Decree for formed/monolithic products.

The test is normally carried out over a 64 day period at 0.25, 1, 2.25, 4, 8, 16, 32 and 64 day intervals. It is though acceptable to use the first 4 stages only and divide the limiting values for Chloride and Sulfate leachate by 4. In this case the limiting values were determined to be 27500mg/m2 for chlorides and 41250mg/m2 for sulphates.

The ImmoCem modified CKD mix achieved leachate values for Chlorides and Sulphates at levels below the Dutch Soil Quality Decree threshold. The results were 20480mg/m2 for Chlorides and 518mg/m2 for Sulphates.

The material as mixed has a density of 1.7t/m3, so qualifies as a medium light weight material for building blocks etc, which would be very low carbon and sustainable product for the UK construction industry or any organisation seeking closed loop zero carbon solutions.

ImmoCem is a Nano technology cement modifier, which improves the performance of cement and other binders. Eliminating cracking and maximising leach free solidification of organic and inorganic contaminants.

It is supplied to the UK market by PowerCem Technologies (UK) Ltd, based in Derby.

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