In a previous blog, we attempted to answer the question: is concrete a sustainable material? Many of the issues with concrete stem from the use of Portland cement. As the demand for concrete has increased, the need for a more environmentally friendly alternative is needed.

In addition to the environmental concerns, traditional cement is beyond the reach of many in rural areas in terms of access and cost. As we have seen a rise in the cost of building materials lately, it’s expected for this lack of access to get worse.

A Note on Pozzolans

When creating any kind of Portland cement substitute, producing a reactive pozzolan is important. A pozzolan is a silicate-based material that reacts with calcium hydroxide to form extra cementitious materials. Standard pozzolans used are silica fume, fly ash, and slag. These can replace Portland cement pound for pound.

Rice Husk Ash

Rice husk is a waste product produced through the cultivation of rice. It is readily available and not a potential pollutant on its own. The production of rice husk ash (RHA) through burning has been studied as a potential additive to concrete to reduce the amount of Portland cement needed.

This isn’t necessarily new. In fact, RHA in concrete was first patented in 1924. But this patent was based on uncontrolled combustion, which creates a subpar product. A study done in 2005 and another done in September 2020 show that RHA is a valid supplement.

The study done in 2005 in particular looked at controlled combustion. The researchers studied three types of field ovens – an annular (ring-shaped) oven, pit burning, and a brick oven. While the researchers also used a lab oven to compare results, they acknowledge that sophisticated oven and techniques are not feasible in rural areas.

The results found that the annular enclosure oven produced the best results. The duration and type of incineration was significant. The presence of lots of oxygen during burning is necessary to reduce the carbon content.

Lower strength values were found in concrete made with RHA from pit burning. This is possibly from the longer burning time and slow cooling rate. A shallower pit could result in better ash; however, this was not studied.

Coal Bottom Ash

Coal is most commonly used as fuel for energy and steam production. Burning coal leaves behind various ashes – fly ash, coal bottom ash, and many others. Coal bottom ash (CBA) is heavier and coarser than the rest and tends to settle at the bottom. It is also porous, light, and glassy with a similar color to Portland cement.

While coal is not the cleanest energy source, using the ash is a good way to recycle the waste. Otherwise, the residue is usually dumped near the power plants.

CBA needs to be ground to a finer grain size, but researchers found that found that ground CBA resembles fly ash physically and chemically. Fly ash is already used to replace Portland cement needed in a concrete mixture.

The addition of ground CBA seems to initially reduce the compressive strength, but over time the strength recovers. Researchers find it to be a viable solution to reducing the environmental impact of coal burning waste, as well as reducing the “environmental impacts and reduction in overall production cost of concrete”