Concrete prepared from biofuel byproduct

Dr. Neil Canter, Contributing Editor | TLT Tech Beat July 2013

An alternative binder from a sustainable source has been developed to improve the performance of cement.

KEY CONCEPTS
• The principle binder used in the manufacture of cement is derived from amorphous silica obtained from non-sustainable sources.
• A new binder source has been developed from the biomass left over from the processing of agricultural crops.
• Pretreatment of the biomass removed impurities, increasing the amorphous silica concentration and leading to improved performance strength and sustainability at a 20 percent replacement rate in Portland cement.

THE INCREASING USE OF BIOFUELS is leading to the need to find applications for byproducts also generated during their manufacture. One incentive to working with byproducts is that it will increase sustainability and reduce the carbon footprint.

Research has been underway to improve the performance of concrete, while also finding a process that is more environmentally friendly. Concrete is produced by mixing Portland cement, water and gravel or sand.

In a previous TLT article, work was described that showed fly ash, which is a byproduct from coal-burning, in electric power plants can be used as a replacement for cement in concrete (1). Introduction of up to 75 percent of the fly ash in place of cement produced concrete with comparable compressive strength with values between 4,000 and 5,000 pounds per square inch obtained during material testing.

Kyle Riding, assistant professor of civil engineering at Kansas State University in Manhattan, Kan., says, “Use of cement dates back to the Romans who prepared it from calcium hydroxide and amorphous silica obtained from the volcanic ash produced from nearby volcanoes. The resulting product is calcium silicate hydrate, which acts as a binder.”

In modern cement, the principle binder is tricalcium silicate, which, when mixed with water, also forms calcium silicate hydrate. Riding says, “This material is essentially the glue that holds cement together. The Romans used volcanic ash, which had a high concentration of silica. Supplemental materials are used that contain amorphous silica. The most common source is fly ash produced as the byproduct of coal combustion. Because of availability concerns and environmental regulations for coal combustion, there is need for alternative materials.”

Riding explains that environmental legislation that limits sulfur and NO_x emissions is also reducing the quality and availability of fly ash. Two other options that have been looked at as supplementary material are silica fume and slag cement.

Riding says, “Silica fume is a byproduct obtained from the manufacturing of silicon metal or ferrosilicon alloys. It is isolated from the gas stream emanating from the electric furnaces used in the production of silica fumes. Unfortunately, only 60,000 tons per year is available in the U.S. Slag cement is a byproduct of steel manufacture, but is also only available in limited quantities.”

There is a need for an alternative binder for use in the production of concrete. A candidate has been found that has the added benefit of being from a sustainable source.

AGRICULTURAL RESIDUE ASH
Riding, in collaboration with his graduate student, Feraidon Ataie, have determined that biomass left over from the processing of agricultural crops can be used if pretreated properly. Once pretreated, the biomass is then burned in a combustion process to produce a material known as agricultural residue ash (ARA) that has been found to improve the performance of cement.

Riding says, “ARA is suitable for this application because it contains a large concentration of silica. Plants consume minerals that contain silica as they grow. The silica becomes incorporated in the structure of the plant and varies in concentration, depending upon the specific crop. For example, 20 percent of the biomass in rice is silica. With wheat, the percentage of silica is 4 to 5 percent.”


Kyle Riding and his graduate student, Feraidon Ataie, have determined that pretreatment of a biomass known as agricultural residue ash leads to the manufacture of cement with a maximum increase of 25 percent in compressive strength. (Courtesy of Kansas State University)

Two sources of biomass that appear to be attractive to study are wheat straw and rice straw. The byproducts from cellulosic ethanol production made from straw feedstock material contain high levels of lignin and are not directly linked to the food supply.

One concern faced with incorporating ARA into cement is the presence of undesirable minerals such as potassium, sodium, calcium and magnesium. Riding says, “These elements can reduce the performance of the ARA produced after combustion of the biomass by limiting its reactivity. Other negative characteristics include a reduction in the temperature at which undesirable byproducts are formed during combustion, making it difficult to produce quality ARA.”

For this reason, the researchers decided to pretreat the biomass with either distilled water or dilute hydrochloric acid (0.1 N) at temperatures ranging from 23 C to 80 C in an effort to remove impurities. Riding says, “Dilute acid is widely used in biofuel production to remove some hemicelluloses present, as well as changing the structure of lignin and reducing the polymerization of the cellulose in the biomass.”

The researchers then burned the pretreated biomass at temperatures ranging from 500 C to 800 C. This was followed by measurement of such critical parameters as loss on ignition, internal surface area and amorphous silica content in the resulting ARA.

Pretreatments were very effective in removing impurities and resulting in an increase in the amorphous silica content. There is a direct correlation between removal of calcium, potassium and magnesium and an increase in the concentration of amorphous silica.

Riding says, “Pretreatment is very effective in producing a biomass that is much less sensitive to burning. The only concern during the combustion is not to have it occur at too high a temperature, which results in the formation of quartz.”

The researchers then used ARA at a 20 percent replacement rate in Portland cement. The result was the preparation of a material with improved performance strength. Pretreatment of both wheat straw and rice straw at 80 C for 24 hours led to ARA that exhibited the best result, which was a 25 percent increase in compressive strength.

This work shows that a new source of silica can be used to improve the sustainability profile of concrete. Future work will involve doing enzymatic hydrolysis of the biomass to determine if a more effective ARA can be produced.

Additional information can be found in a recent article (2) or by contacting Riding at riding@k-state.edu.

REFERENCES
1. Canter, N. (2011), “Greater Use of Fly Ash in Concrete,” TLT, 67 (7), pp. 14-15.
2. Ataie, F. and Riding, K. (2012), “Thermochemical Pretreatments for Agricultural Residue Ash Production for Concrete,” Journal of Materials in Civil Engineering, 10.1061/(ASCE)MT.1943-5533.0000721.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.