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Concrete made from ash

Waste from burning sugar cane bagasse may substitute part of the sand used in civil construction

EDUARDO CESARA black mountain, comprising 3.8 million tons of ash and the burned remains of sugar cane bagasse; this is the waste produced during a year by the incineration of bagasse in Brazilian sugar and alcohol mills. For some time, industries in the sector have burned bagasse and cane straw to generate electricity for their own consumption, and if there is any excess production, it is sold to third parties. The ashes resulting from the burning are discarded in landfill sites or thrown onto sugar cane plantations as fertilizer, although there are doubts about its true effectiveness. For each ton of bagasse incinerated around 25 kg of ashes are generated. This material was studied by the team, coordinated by civil engineer, Almir Sales, a professor at the Federal University of São Carlos (UFSCar), and the results show that this waste may have an environmentally suitable destination and be transformed into an important input in the manufacture of mortar and concrete for use in civil construction. The results were presented in an article published in February in the on-line version of the journal, Waste Management.

The proposal of the researcher from UFSCar is to substitute part of the sand used in preparing mortar and concrete with ash from sugar cane bagasse. The research, which started three years ago and is carried out with financial support from FAPESP, showed that substitution of 30% to 50% in mass of natural sand with ashes not only preserves the physical and mechanical characteristics of good quality concrete, but also brings benefits. “In this substitution range, concrete made from ash may gain up to 20% more resistance than conventional concrete,” says Sales, who was helped by PhD student, Sofia Araújo Lima, and a further five scientific research undergraduate students. Furthermore, this type of concrete will reduce the need to find areas for disposing of the waste and at the same time, less sand will be used, thus reducing the environmental impact on the riverbeds from which it is removed.

“The extraction of natural sand and gravel demands a lot from nature. Most of the sand ports and quarries cause environmental damage to watercourses. We’re beginning to have difficulties in finding sand and natural stone for use in civil construction,” says the researcher from UFSCar. Recently, he says, there has been an increase of 500% in the price of sand in São Luís, in Maranhão, resulting from the cancellation of environmental licenses for extracting sand in the city.

Concrete made from bagasse ashes can, in principle, be used in most civil construction applications. The initial proposal is that it be employed in the manufacture of curbs, gutters and storm drains. “Several city administrations are already working with using waste for producing concrete artifacts. We believe that one of them may be interested in producing our concrete on a pilot basis,” Sales says. For some special applications, like structural and high performance concrete, more studies will be necessary.

It looks like sand
To reach the conclusion that ash from bagasse is a substitute for sand, the researcher carried out a series of trials. Microscopic physical characterization showed that it has a profile that is very close to that of natural sand, with a part that is crystalline and a high concentration of silica. The studies carried out with samples collected from four sugar mills in São Paulo also revealed a lack of any elements in the waste suitable for use as fertilizer. “This is a largely inert material that is weak for use as fertilizer. We found no significant concentrations of potassium that would justify the use of ash as an element for correcting the acidity of the soil, as has been happening,” Sales points out.

Another surprising fact revealed by the trials on the ash analyzed was the presence of a large quantity of heavy metals, among which were lead and cadmium. As a result, its use for fertilizing sugar plantations may present the risk of contaminating the soil and the water table. Cautiously, Sales emphasizes that more detailed studies need to be done in the soil area, by taking a larger sample.”We’re working with ash produced by a limited number of mills. Even so, this is an indication that needs to be checked,” he says. These chemical elements were probably incorporated into the sugar cane in the growing phase, when insecticides, herbicides and maturing products are used to increase productivity and bring forward the cutting date. After the bagasse was burned, the heavy metals passed into the ash.

According to Sales, the treatment that enables the bagasse ashes to be used as a substitute for sand is simple and cheap. In cases in which the ash has fragments of poorly burned bagasse, it needs to be sieved to remove them. However, in mills that have newer furnaces, already prepared for the efficient production of electricity, the bagasse is fully burned, thus dispensing with the need for sieving. The second phase of the treatment is limited to milling to control the size of the ash grains – in technical terms the granulometric size is adjusted. “With milling it’s possible to get ash that is very like sand, except it’s black.,” says the researcher, clarifying that the investment to acquire the mills used to process the ash is relatively low and can be diluted in the furnace costs.

Durability on trial
Once the ideal substitution dosage of sand for ash in the cement has been defined (between 30% and 50%), the next step in the research will be to test the durability of the concrete. In these trials, planned to be carried out over the next 12 months, a check will be made as to whether concrete made with ash has characteristics that are suitable for protecting frameworks; in other words, if in addition to being durable, it can protect the steel used in concrete construction from corrosion. During these tests, the test body specimens will be exposed to the environment, thus simulating a real situation. “Early tests are encouraging and the tendency is that its durability can be confirmed,” says Sales.

According to the researcher from UFSCar, other countries are already looking for alternatives to sand and gravel for making mortar and concrete. Such is the case in the Netherlands, Denmark and Belgium, where these inputs have been replaced by construction and demolition waste (CDW). “The addition of CDW for recomposing sand and gravel is widespread in many developed countries, reaching levels above 70%,” he says. In Brazil, several cities already have recycling plants in which CDW is separated, crushed and transformed into sand and gravel for producing concrete artifacts. “This is the way to go. We must learn to transform the waste generated in various sectors, including agribusiness, into concrete and other components for civil construction, without always having to extract from the environment.”

Concrete made from what is left over after burning sugar cane bagasse is not the researcher’s first piece of work that has focused on taking advantage of waste and transforming it into a product with added value. Five years ago, he coordinated a project that resulted in a patent, the aim of which was to incorporate the sludge generated in water treatment stations into concrete and mortar. Instead of sending the sludge produced in stations to landfill sites or incinerators, it can be used for manufacturing concrete artifacts.

The project
Use of sugar cane bagasse in the production of artifacts for urban infrastructure: characterization of waste and the assessment of mortar and concrete (nº 08/06486-4); Type Regular Research Awards; Coordinator Almir Sales – UFSCar; Investment R$ 124,592.61 (FAPESP)

Scientific article
SALES, A.; LIMA, S.A. Use of Brazilian sugarcane bagasse ash in concrete as sand replacementWaste Management. 2010. On-line version. doi:10.1016/j.wasman.2010.01.026

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