Concrete study evaluates fly ash, slag and silica fume

Materials World magazine
,
1 Jul 2017

Supplementary cementitious materials could help to reduce the damaging impact of de-icing salt on roads. Kathryn Allen reports.

A new method for producing concrete using by-products of coal furnaces, the metal industry and silicon and ferrosilicon alloys – fly ash, slag and silica fume – has proved more resistant to salt deterioration than current concrete mixes used for road and pavement surfaces. 

Calcium chloride salt, used to de-ice roads, reacts with calcium hydroxide in concrete, forming calcium oxychloride. This can expand when formed, creating internal pressure and producing cracks in the concrete. Occurring at room temperature, this reaction can happen before ice appears. 

Developed by Dr Yaghoob Farnam, Assistant Professor at Drexel University’s College of Engineering, USA, the new concrete mix contains less calcium hydroxide, therefore producing less calcium oxychloride than Portland cement, while also using recycled materials. The paper, Evaluating the use of supplementary cementitious materials to mitigate damage in cementitious materials exposed to calcium chloride deicing salt, is published in Cement and Concrete Composites. 

Farnam said, ‘We do not replace calcium hydroxide. We replace a portion of cement using supplementary cementitious materials (SCMs) and use the blended cement to make concrete. This is a very simple process and some batching plants have the necessary equipment to do this.’ The SCMs convert calcium hydroxide to calcium silicate hydrate, calcium aluminate hydrates or calcium silicate aluminate hydrate. 

Using cement mixtures with varying amounts of SCMs, Farnam found that, while after eight days of exposure to salt the Portland cement showed signs of deterioration, the concrete samples with higher amounts of fly ash, silica fume and slag did not show damage. The optimum amounts of SCMs are still being calculated, but according to Farnam, as a percentage of the mixture replacing cement, 20-25% of fly ash was found to be very effective, while it was 30% for slag and more than 10% for silica fume. 

Farnam found that using higher concentrations of calcium chloride resulted in more calcium oxychloride being produced, so, in theory, using lower concentrations would reduce road damage, but the de-icing effect would also be reduced. 

Farnam said, ‘The SCMs have been used in the concrete industry for many years to increase concrete durability and corrosion resistivity. Our research provokes engineers and decision makers in pavement industry to encourage using SCMs as they mitigate chemical deterioration caused by calcium chloride de-icing salt.’ It is hoped that the new mix will allow the lifespan of concrete roads and pavements to reach the designed target of 20-40 years. Farnam’s team is also working on developing a protective layer for concrete using bacteria that prevents calcium oxychloride formation. 

To read the paper in full, visit bit.ly/2rL3VV6