This is a unique emerging idea. Although it is not being practiced in USA commercially but there are future prospect to use such materials. While there are global warnings about greenhouse gas emissions, sustainable practices in industrial sectors are desperately required to minimize carbon di oxide emissions for environmental safety. The use of wheat straw and rice husk ash as cement replacement agents not only enhances the sustainability of construction materials but also reduces the material cost, gives equal strength and reduces environmental impact of cement production.
It is not enough, we have 3D printers now which can do construction works in days. Similarly, the emergence of an idea to reduce the cost of constructions materials came up with unique thought to use recyclable materials in construction to reduce the cost with relatively greater strength. Whether it is a 3D printer working on it or conventional ways of construction, the use of such materials help preserve energy for a greener future.
What are Pozzolanic Materials?
To enhance the strength or other properties of the concrete or mortar, Pozzolanic materials are now used in advanced construction practices. It is relatively sophisticated powdery substance which is added to the cement in the mixture that reacts with the lime released by moistening the cement, resulting in relatively greater strength.
Pozzolanic materials are cementitious materials used in the construction industry to reduce the consumption of Ordinary Portland Cement (OPC) in cement-based products like Mortar and Concrete.
The consumption of a large amount of natural resources like lime, silica and Gypsum in production of cement for construction of Buildings and infrastructures is leading to great challenges to sustainable development. So, the cement consumption is increasing year by year. The table shows the projected consumption of cement for 2023-2030.
This is an very important waste of rice plant. Silica is absorbed by the soil and collected in the straw, where it is made of a structure and filled with cellulose. Here if cellulose is burned, it will only leave silicic acid, which is then turned into a fine powder, which is used as pozzolan. Rice straw can yield 15% ash after burning and thus every 1000 kg of rice straw burning produces 150 kg of ash.
Role of Cement Industry in carbon Emissions
During the process of manufacturing cement, the calcination (heating materials to relatively high temperatures) of limestone and other raw materials results in emission of CO2 which account for approximately 40% of the total cement production process and total impact of CO2 emissions of the cement industry amounts to 8% of the total global emissions.
A large amount of CO2 is discharged into the air during this process, causing great pressure on the ecosystem. Therefore, alternative cement products, carbon capture and secondary utilization, and actively looking for cement raw material substitutes can help reduce environmental impact of CO2 emissions as much as possible.
On the contrary pozzolan is a siliceous or siliceous and aluminous material which does not actually possess any cementitious value or properties but in the presence of moisture, chemically reacts with calcium hydroxide at ordinary temperatures to form compounds having cementitious properties.
Having quite lower in cost as compared to ordinary Portland cement, it can, not only reduce the consumption of cement and lower the production and construction cost but will help sustainability of the waste materials used in Pozzolanic materials.
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Understanding Pozzolanic Reaction
Chemically, when the iron oxide (Fe2O3), and silica (SiO2) and aluminum oxide (Al2O3) exceed 70% of the mass, it is known as the material substance Pozzolanic. Many researchers have observed that the compressive strength of the mortar increases with pozzolan materials. It increases the performance of cement due to the compression of low water content in concrete and mortar resulting from high pozzolanic reaction.
The fine weight of the pozzolan had a stronger pozzolanic reaction because the small molecules fill the blanks of the mortar or concrete mix and therefore increase the compressive strength of the mortar.
There are Many other Supplementary Materials
Recently, the use of additional cement materials, such as fly ash, silica fumes, slags and blast furnaces has increased in the construction and sophisticated concrete products. These complementary cement materials significantly improve the strength and durability of concrete.
Many other relatively new complementary cement materials, such as rice husk ash, sewage sludge ash and fly ashes, are being extensively studied in recent times.
The development of use of natural materials to produce concrete composite construction materials for construction has been continuous for many years. India is one of the largest rice producers where per capita rice consumption is higher than in any other country.
In United States and Canada the use of such materials has not yet emerged to a considerable extent and if there are any such developments in future, there will be substantial evidence of reduction in cost of cementitious materials and sustainability to the waste materials.
Conventional Uses of Wheat Straw and Rice Husk
There are three main products of biomass that come from rice such as Rice Straw, Rice Hulls and Rice Bran. Rice straw, Rice husk and rice bran feed have been used for cattle, poultry, fish etc. In the rural areas, rice straw is also used for cooking and other purposes. In Asian countries, a large amount of rice husk and wheat straw is burnt, where ash is produced and dumped, resulting in an environmental problem.
Although sometimes, if not burnt, it is consumed on agricultural land as composting material. To reduce the cost of building materials and raise environmental issues, significant efforts are being made worldwide to use local waste and secondary materials to improve the performance of building materials.
Impact of Increased Cost of Building Materials
Traditional building materials are getting out of reach of most of the world's population because of their affordability. Rice and Wheat straws are one of the most important agricultural products available worldwide.
During growth, rice and wheat plants pick up silicic acid from the soil and accumulate in their structures. This silica concentrated burning of rice husk at high temperatures removes other elements, making ash very valuable.
Among the agricultural wastes, rice straw has a very high potential to produce highly efficient secondary raw materials. This is mainly due to the availability of the rice husk where the silica content is very high and relatively low cost.
After burning of rice straws and husks with a temperature and duration controlled by small plants, the resulting substance is converted into high value-added ash, which is a secondary substance due to its high silica content.
Rice straw and husk consist of both organic and inorganic substances. Organic material consists of cellulose, lignin, hemicellulose, some proteins and vitamins, while the main element of inorganic metals is silica.
The actual configuration differs from rice straw and crust with rice type, and the intake of rice bran and broken in the bark, geographical factors, seasonal harvest, and the preparation of samples and relative humidity.
The safe disposal of wheat straw, aids the production of eco-friendly cement-based composites, introduces sustainable consumption of raw material expended during the production of cement, and thus results in reducing the costs of construction.
It would also add economic value to the wheat straw, thus creating an incentive for the farmer to refrain from open burning of wheat straw. Among the other crop residue, successful investigations have been done on rice husk ash, bagasse ash, etc., for their use in cement based composites.
Experimental Program of Concrete Mix Design with Pozzolanic Materials
Materials used in concrete mix or mortar are:- Ordinary Portland (OPC) 42.5 N with a specific gravity 3.15.
- Fine aggregate, Siliceous sand with a fineness modulus and a specific gravity 2.64 and 2.74, respectively.
- Coarse aggregate, crushed stone aggregate with a specific gravity 2.89.
- Tap (safely drinkable) water for mixing and curing procedures.
- Rice Straw Ash (RSA) & Wheat Straw Ash (WSA) being agricultural wastes as supplementary materials
- Superplasticizer with a 3.5% of the weight of the cement.
Preparation of Concrete Mix Design
The straws were first washed with tap water and then dried in an oven at 80°C for 24hrs. The straws were secondly grinded in grinding machine and then passed through different sieves using 20-200 micron mesh size sieves.
Silica dissolution from ashes powder was carried out using an alkali process using 1M sodium hydroxide (NaOH) and heated for 4hrs at 100°C.
The obtained solution was filtered to remove impurities. 10% sulfuric acid (6M H2SO4) at the pH=7 was used to precipitate silica from sodium silicate solution and left for 24 hours. Superplasticizer is added with amount of 3.5% of the cement weight.
The workability of the concrete mixes decreases with increase of the percentage of added ashes. That is because, concrete containing ashes need more water due to high fineness. So, a superplasticizer is added to the mix to improve the workability for a suitable slump.
Results and Analysis
Compressive strength is tested by digital compression testing machine of 200 KN capacity. Standard Cube specimens of dimensions 150x150x150mm or Cylinder Specimens of dimensions 12’’x6’’ are normally used to determine the compressive strength for concrete.
After 7 days, increasing RSA (Rise Straw Ash) & WSA (Wheat Straw Ash) from 5% to 15% of the cement respectively led to increase in compressive strength from 21.00 N/mm2 to 28.00N/mm2 and 27.00N/mm2 to 35.00N/mm2 respectively.
Higher compressive strengths were observed at 15% RSA & WSA with compressive strength of 28.00 N/mm2 & 35.00 N/mm2 and at 20% RSA & WSA compressive strength deceased to be 26.00 N/mm2 32N/mm2.
The same trend was found after 28 days, increasing RSA & WSA from 5% to 15% led to increase in tensile strength from 42.00 N/mm2 to 51.00 N/mm2 & 49.00 N/mm2 to 63.00 N/mm2 respectively.
Higher tensile strengths were observed at 15% RSA & WSA with tensile strength of 51.00 N/mm2 & 63.00 N/mm2. While at 20% RSA & WSA strength deceased to be 45.00 N/mm2 & 56.00 N/mm2. (Image: Cylinder/Cube Compressive Strength Testing Machine)
Note: The practical procedures and experiments were executed in a controlled environment in a University Lab. Readers and researchers are advised to execute their own experiments as the results may vary due to area, temperature and climate conditions.
Conclusion
Based on the results of experimental study, it is concluded that RSA and WSA are natural agricultural waste that can be used as a very good cement replacement material due to silica produced by chemical activation method. The production of silica from agricultural wastes provide an environmentally friendly result with exactly suitable and economically cost effective product. Using the RSA & WSA enhanced the compressive strength and tensile strength of concrete samples.
It is Therefore, recommended that the existence of WSA in higher grade concrete mix improves durability of concrete. Besides, the presence of Superplasticizer in RSA & WSA concrete mix improved the slump & the workability of the concrete. X-ray diffraction for WSA also indicated that the structure of silica is of amorphous material.
Thus the RSA and WSA are strong pozzolanic materials having the potential to be good cement replacement substances and could contribute to the sustainability of the construction material.
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