RHA furnance

General

Materials with a high content of amorphous silica can be used as Supplementary Cementitious Materials (SCMs). With calcium hydroxide available, they form strength supporting calcium silicate hydrate phases (CSH). The use of Rice Husk Ash as SCM has economic and ecological benefits: Ecologically, with the use of RH it is possible to reduce the amount of needed cement. Current cement production causes 5-7% of total global carbon emissions due to the chemical effect during the chemical transformation from calcium carbonate to calcium hydroxide: Carbon dioxide is chemically separated and emitted. Thus for the global decrease of greenhouse gas emissions, the building industry has a big impact.  Lowering the cement production causes a much lower emission of greenhouse gases.

At the same time, the per-capita-consumption of concrete grows globally each year. Subsequently, cement replacement and sustainable building solutions have to be found. Currently, SCMs like fly ash and blast ground furnace slag are already able to replace cement. But the most common used SCMs are no longer available in huge amounts.

 

Moreover, in many countries cement is a very expensive building material, so mostly less cement than needed or just rural building materials are used. Even in countries with much cement production the price of a ton cement will increase within the next years due to the emissions trading laws: Producers of cement will have to pay more fees because of their huge contribution of greenhouse gas emissions into the atmosphere: There are some economical predictions that the price of a ton cement will increase up to 500% within the next years. Altogether, research is ongoing to find suitable replacement materials for cement.

Low-Cost development of Rice Husk Ash and research plan

The production of RHA with the required chemical composition already is possible in industrial processes for generating electricity out of the husks (Thailand) as well as in laboratory muffle ovens for research use. Both solutions are not suitable for low cost development in building in rural areas. For this reason, an easy and cheap furnace has to be invented which runs without much exterior power to reduce the costs. Still, with easy burning process the RH has to be burnt properly to get a high amount of amorphous silica which has a high pozzolanic activity. The invention of a furnace for easy but proper production of Rice Husk Ash will be implemented in the building project. The building concept for this project was planned sustainably taking the cultural and current building situation into account.

Rice Husk as Replacement for Cement

Rice is one of the four most important global agriculture crops. From the paddy the husks remain as waste. The husks cannot be used as pet food due to its composition. Often, the husks are just burnt or left in the nature. Burning the husks properly generates a white ash with a high amount of amorphous silica which is an excellent building material for cement replacement.

The quantity and quality of reactive silica depends on the used Rice Husk, the treatment before and after burning and the time and temperature of burning (2). There are many research results from experimental programs which are presented very shortly:

The combustion periodand temperature has the main impact on the quality of Rice Husk as building material. Properly burnt, the ash contains up to 90% of amorphous silica and even up to 5% of alumina phases [8], which makes the ash a highly pozzolanic material with a good impact on strength and durability properties of concrete. The best time and combustion temperature varies between the different research grups. In general, it can be said that amorphous silica is formed under temperatures of 700°C [1-9]. XRD analysis show that over 800°C, mostly criztalline silica is formed which has no big impact on strength development of concrete [1]. Generally, temperatures between 500°C and 700°C are recommended.

In laboratory environment, controlled burning in muffle ovens is possible. On the field, often there is no possibility for controlled burning. Thus, some researches compare controlled to uncontrolled burning [xx, 8]. Uncontrolled burning often does not reach the required temperatures for the creation of amorphous silica and thus does not produce RHA as good building material. Nevertheless, it has to be taken in account that the laboratory production of RHA in muffle ovens does not solve the problem of using RH as building material in rural areas.

Zain et al. [4] and Mboya et al. [6] investigated a rudimentary two-step-combustion of RH where the dry husks first are incinerated by burning e.g. paper or cloth. The incineration time takes not more than one or two hours. The second step is the calcination of the husks at temperatures between 500 °C and 700°C. During the calcination, the husks transform to white ash which contains amorphous silica. Thus, the way of calcination is crucial for the chemical composition of the ash.

The cooling process influences especially the specific surface area of the ash and thus the porosity (4). Following, a suitable cooling process for little porosity should be chosen.

The last step is thegrinding process. The grinding process and thus the fineness of the ash influences the reactivity and as well the surface properties of the ash particles. The Los-Angeles-Method which is grinding with metal balls turned out to be suitable for a good quality and reactivity of RHA (4).

All in all, the current research activities on RHA have a good agreement on the proper treatment of the husks. Nevertheless, most of the research is practiced in the laboratory under controlled conditions and with little amounts of Rice Husks. What mostly is missing is a solution for a cheap and easy treatment of husk wastes which produces good or at least suitable ash for the use as building material.

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