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New materials

High quality cement with rice husk

Unprecedented process extracts silica from agricultural waste for use in concrete

In preparing an unprecedented process for obtaining highly pure silica from rice husks, research at the University of São Paulo (USP) at São Carlos opens up the possibility of an ample supply of this material for high quality cement production. Using the new technique, 99% pure silica with high chemical reactivity is obtained, making it very attractive for industrial use, particularly in the construction industry.

The technology was consolidated  and improved in the project High Performance Concrete Using Rice Silica, coordinated by professor Jefferson Libório, of the Civil Engineering Laboratory of the Department of Architecture and Town Planning of the São Carlos Engineering School, and financed by FAPESP.

Libório works along with another group of researchers at São Carlos, coordinated by professor Milton de Souza, of the Physics and Materials Science Institute. They systematized the system for the new method of extracting silica. As a result, professor Libório made possible the use of the additive for producing structural concrete with variable properties, intended for use in various sectors of the construction industry.

According to professor Souza, the results already obtained by Libório in laboratory scale trials, miniature samples of concrete containing the silica obtained on a laboratory scale show the good quality of the product and the excellent resistance of the cement.

It led to a patent
“Our process is original and no other is as good”, promises Souza with the authority of one who has combed through patent databases in the main countries that conduct experiments in the field. The method created also led to one of the first patents to be awarded by the Technology Patenting and Licensing Nucleus (Nuplitec), which FAPESP created recently.

Extracting silica from rice husks, as well as making use of agricultural waste that is normally disregarded (and polluting), is a renewable resource and unprecedented in the construction industry, where it serves as an alternative to silica obtained from waste products of the production of iron-silicon or metal-silicon alloys. With these residues of the steel industry, at least 85% pure silica is obtained meeting the specifications of the Brazilian Technical Standards Association (ABNT in the Portuguese acronym).

Sufficient leftovers
The advantage of the new method is that the agricultural source is plentiful. Of the ten million tons of rice the country grows each year, two million tons of husk are left over – which would yield 400,000 tons of silica, sufficient to supply the structural concrete market and others. Depending on the amount of cement used, the proportion of silica in concrete varies from 5% to 10% and it is particularly important for high performance concrete. Processing husks to extract silica can, according to Souza, have an influence on various stages of economic activity. In the first place, rice is a cheap source for a high quality input to the construction industry, a segment that accounts for 14% of Gross Domestic Product (GDP). Considering that a kilogram of the highest purity silica costs US$ 36, jokes Souza: “It’s a better business to grow rice for the husk and have the rice itself as the by-product”.

Protein from the soil
Souza also predicts improvements in rice growing: “What transports this material from the soil to the plant and from the plant to the husk is a protein, which can be manipulated so that we can have strains that will facilitate the processing”.

A former director of the Physics Institute, Souza’s first contact with the subject took place about ten years ago, when he was given a packet of black powder resulting from burning the rice husks, still a common practice in rice growing. At the time, the study disciplines and the characterization of materials at São Carlos were already researching the use of ash of rice husks in producing concrete, but with not very encouraging results.

“When the studies began on 1991, we couldn’t do very much. To separate the silica from the black powder we needed to use temperatures of more than 1,000 ºC, which damaged the silica obtained”, tells Souza.

Five years later, now with the help of postgraduate chemistry students – among whom Souza makes special mention of the co-author of the patent Paulo dos Santos Batista – there was a unification of projects in the materials science field. So the direction of the work changed: “We discovered a way of treating the husk before extracting the silica”. At the time, Souza and Libório maintained the initial contact and discussed the possibility of using silica together with Portland cement.

Souza knew the importance of the nanoparticles in processing ceramics: “Because the silica is dispersed in the cellulose of the husk, it could be extracted in form of aggregates of nanoparticles. It was from this angle that studies resumed in 1997. The nanoparticles, around a millionth of millimeter in size, have a high degree of chemical reactivity and sinterizing ability, which lowers the burning temperature and improves the quality of the ceramic products”. Libório adds: “These silica particles are lighter that cigarette smoke”.

Heat and pressure
The first step in obtaining the silica was to remove the components that reacted with it, such as salts. “In the beginning, we washed the husks with acid, but it was a very long  process”, explains Souza. “Then we decided to put the husks in an autoclave (a chamber withstanding high temperature and pressure), where they are washed with acid at a controlled temperature. In going through the autoclave, the rice husks lose the salts.After other simple procedures, the material is calcined at more than 400ºC.” This part of the final stage of obtaining the purest, whitest form of silica is followed by procedures that professor Souza carefully keeps to himself.

Silica’s contribution in improving cement has been known for more than two decades. Libório recalls that, in Brazil, around 1985, the engineer Epaminondas do Amaral Filho “donated to the Structures Laboratory of the São Carlos Engineering School imported silica, of ferrosilicon and metal silicon alloys”. This enabled Libório to begin research into this material with a view to building a structural element (a pillar), “Since then, we have carried out various studies on improving cement, with reports made to Brazilian and international congresses. Thus, 60 MPa (Mega Pascal) concrete was produced, the equivalent of 600 kilograms per square centimeter of resistance to simple compression”. He analyzes the trial results, samples of little more than 7 centimeters in diameter, based on this parameter (1 MPa = 10 kg/cm2), which show how much compression a piece of concrete can withstand.

It has always been taught that concrete is extremely durable. Libório makes the reservation: “Concrete is an excellent building material, but its durability has limits. Deterioration generally takes place because of improper use and because of the interaction of poor quality concrete with substances present in the air or in water, such  water and drainage networks (all with concrete structures) and corrosive environment waste, such as carbon dioxide”.

Avoidable disaster
For Souza, a cheap raw material like rice silica will help solve problems already detected and prevent new works deteriorating. In the first case, it could be used to manufacture special lining plates: “Bridges and viaducts cannot simply be put down on the ground and rebuilt”.

Other sectors could benefit and Souza points out one of them: “Nowadays, irrigation channels in the Northeast need to be reinforced with thick layers of rubber, because of the concrete’s permeability. A high performance concrete that did away with the need for reinforcement could represent a huge saving, making irrigation a reality in that region”.

Active rice husk silica will be an alternative as efficient as other sources of silica when the processing difficulties have been solved satisfactorily. Under present conditions, rice husk is still burnt and left in the field, where it causes considerable damage: “Nothing grows in the soil where the burnt material is thrown. In the form of dust this ash can cause silicosis in people, a serious lung disease”, warns Souza.

Processing the husks has been the object of many studies since the beginning of the 90s. The great challenge was to achieve a high degree of purity in the isolated silica, economically feasible at a production cost and on a production scale. “That is precisely the innovation we now have”, says Souza.

Maximum compression
The consequences are very practical. Libório explains: “Much construction work still uses conventional concrete, resistant to compression of 25 MPa, measured at the age of 28 days, although uses of 60 Mpa have been recorded, even reaching values of up 80 MPa”.

In the project, Libório goes on, these numbers rise dramatically: “We have already achieved compression resistance of 120 MPa at 28 days of age and 50 Mpa at one day, and we expect to exceed 200 MPa. At this level the concrete is no longer of interest to the construction industry; it serves other purposes such as, for example, manufacturing molds for parts in the mechanical industry”. Other studies, led by Libório, are underway, with the addition of rice husk silica to products such as paints, polymers, decorative plaster, sealants, counterweights, and special floorings.

The addition of high quality silica to cement is only one stage. Besides improving the processes of collecting rice husks and extracting the silica, we now need to show all users of structural concrete technology, such as civil engineers and architects, the benefits of this new material and this new technology for the construction industry. That is what the researchers intend to do.

Jefferson Libório graduated in civil engineering at USP’s Engineering School at  (EESC), where he did his master’s degree and doctorate. He is the director of the Brazilian Concrete Institute.
Milton Ferreira de Souza graduated in Chemistry at the University of Brazil (RJ), did his doctor’s degree in Physics at USP’s EESC and complemented his studies at the universities of Illinois and Utah, in the United States.

High performance concrete and mortar coating with active sílica from rice husks (nº 98/08112-0); Type Thematic Grants; Investment R$ 230,993.27 and US$ 166,979.51