Abstract

Pollution and waste management and control are global issues, particularly in the industrial and agricultural sectors. Pollution and health dangers, particularly those linked with the cement and clay-brick industries, are concerning and demand immediate action from authorities. One method of addressing the pollution problem associated with these sectors is the possible use of some agricultural waste that has pozzolanic properties. Based on available literature, RHA is widely accessible and has a great potential as a pozzolanic material. As a result, the current study was performed to examine in depth the chemical composition of non-ground RHA collected from various rice mills in Nigeria using X-ray fluorescence (XRF) technology. The RHs were collected from four (4) separate places in Nigeria, comprising seven (7) samples. In this study, Portland limestone cement of the UNICEM brand was used as a control sample and was acquired at a cement merchant store in Nsukka, Enugu State. The husks were carefully separated from the bran before being burned in the open air. Following that, the ashes were collected and stored in a dry area of the laboratory for analysis. Chemical analysis of the ashes and cement was performed to identify the elemental composition of each ash. This study also looked at the physical characteristics of RHA and cement, such as density, specific gravity, and particle size distribution. Non-ground RHA particle size distribution findings were comparable to cement. The ashes produced by open-air burning were milky-white in color, with a percentage difference in chemical composition. These compositional differences may be due to differences in soil chemistry at the sampling sites, paddy types, and the type of chemical fertilizers used. Finally, the findings indicated that the pozzolanic properties of RHA vary based on where they are located.

1. Introduction

There is a global problem of pollution and waste management and control, especially in the industrial and agricultural sectors. Pollution and health hazards specifically associated with the cement and clay-bricks industries are alarming and require serious attention from environmentalists and governments. Tackling the problem of pollution associated with these industries in developed countries is the alternative use of some agricultural waste that are compatible ¹. These agricultural wastes are mostly the by-products of rice mill, coconut refinery, groundnut mill, corn mill, etc. Every year, millions of tons of these waste materials are readily available and dumped across the world, posing environmental issues such as air pollution and deadly chemical leakage into soils and underground aquifers ².

Rice Husk (RH) is an agricultural waste product, and waste managers are faced with the problem of its proper disposal. In Nigeria, RH is found in abundance around the rice mills and parboiling plants of the areas where rice is produced. This waste is generated during the rice milling process and accounts for around twenty percent (22%) of the global annual gross rice output of 678 million metric tons, as estimated in 2010. ³. This RH comprises around 75% organic volatile materials and the remainder is inorganic. The rough RH is made up of approximately 40% cellulose, 30% lignin, and 20% silica ⁴. During the combustion process, 25% of the weight is transformed into ash known as rice husk ash (RHA). This RHA, in turn, contains around 85-90% silica, which is generally amorphous but varies depending on the burning temperature and duration. However, such thermal treatment of the silica in the husks results in structural transformations that influence both the pozzolanic activity of the ash and its grind-ability. This means that in 2010, 149.16 million tons of rice husk were produced, from which 37 million tons of RHA could be extracted ⁵.

In the absence of its utilisation, this huge quantity of RHA goes waste and becomes a great threat to the environment causing damage to the land and the surrounding areas in which it is dumped. If most of the RHA was utilized as a partial cementitious material in concrete, it would not only eliminate the need for discarding RHA, but it would also reduce CO₂ emissions into the environment by reducing cement production. Using RHA in concrete has numerous advantages, including enhanced concrete strength and durability. The yearly global output of RHA is insignificant in contrast to the vast amount of cement produced globally, thus it will not be able to fully eliminate cement ². However, by replacing cement partially with RHA, it will bring a substantial reduction in the amount of CO₂ emitted every year in the atmosphere.

Due to variations in paddy type, crop year, climate, and geographical conditions, the chemical composition of rice husk varies from sample to sample. Burning the husk at a temperature below 800°C can also create ash containing mostly amorphous silica ^{6, 7}. According to Ismail and Waliuddin, the fineness of RHA can affect the pozzolanic characteristics of RHA in crystalline form ⁸.

The pozzolanic reactivity of residual RHA can be enhanced by grinding it up to a suitable particle size, according to Zerbino et al. and Cordeiro et al. ^{14, 15}. In a similar search, Chopra also had good results when he ground crystalline RHA ¹⁶. However, Mehta advised against finely grinding amorphous RHA since its pozzolanic activity is mostly derived from the interior surface area of the particles ¹⁷.

Under controlled combustion and/or grinding, the improved RHA might be utilized as a pozzolanic material in cement and lime. A large proportion of silica (SiO₂) is present in RHA produced via controlled combustion ¹⁸. Cement hydration product Ca(OH)₂ interacts with RHA silica to create a secondary kind of Calcium-Silicate-Hydrated (C-S-H) gel. This C-S-H gel is mainly responsible for hardening of cement mortar. In cement, Ca(OH)₂ is only formed by the hydration of a siliceous compound. Tri-calcium silicate (C₃S) and di-calcium silicate are the only siliceous chemicals found in cement (C2S). There are certain proportions of C₃S and C₂S in Portland limestone cement (PLC), which produce a certain quantity of Ca(OH)₂. C-S-H gel is formed when a certain quantity of RHA interacts with a certain amount of Ca(OH)₂ ¹⁹.

Andres et al. (2013) research on the durability of RHA concrete with water absorption, chloride ion penetration, carbonation and sulphate resistance and the results compared favourably with that of control concrete with pozzolanic admixtures ²⁰. They showed that environmental pollution caused by the emission of CO₂ into the atmosphere during the processing of clinker or limestone can be mitigated through the use of alternative fuels, the use of energy efficiency improvements in the cement plants, and the replacement of limestone-based clinker with other materials such as supplementary cementing materials (SCM) to reduce the use of Portland cements. Various types of supplementary cementing materials (SCM) such as metakaolin (MK), fly ash (FA), silica fume (SF). RHA and others have been widely used in concrete, their utilisation reports not only economic and environmental benefits but also means increasing mechanical and durability properties, preserving the environment at the same time. They also posited that RHA unlike other supplementary materials can be used for architectural type applications because of its white colour. RHA has been produced by many researchers under controlled conditions and it has been used as a highly reactive pozzolanic material, leading to a significant improvement on strength and durability of normal concretes ^{21, 22, 23, 24, 25, 26}. In India, agricultural by-products such as Rice Husk Ash (RHA) have been assimilated extensively in the building industry ²⁷. This has brought about a remarkable improvement in her economy. It has been established from literature that RHA is available in plenty and it has very high potential as a pozzolanic material. Therefore, this present study has been undertaken to assess in detail the chemical composition of non-ground RHA obtained from different rice mills in Nigeria using X-ray fluorescence (XRF) technology.

The emission of distinctive "secondary" (or fluorescent) x-rays from a material that has been stimulated by high energy x-rays or gamma rays is known as x-ray fluorescence. XRF technology is one of the most straightforward, accurate, and cost-effective analytical methods for determining the chemical properties of a wide range of materials, especially metals, glass, ceramics, and building materials, as well as study in, archaeology, forensic science and geochemistry. No sample preparation is required and it is non-destructive and dependable. It is appropriate for solid, liquid and powdered materials. In addition to providing detection limits at the sub-ppm level, it can also test concentrations of up to 100% conveniently and concurrently ²⁸.

In the X-ray region, a photon incident on an inner shell electron causes it to be excited. An electron moves from a higher energy level to fill the vacancy during de-excitation. By emitting an X-ray, the atom's energy gap is visible. An examination of the spectrum obtained via the aforementioned procedure reveals several distinct peak patterns. An element can be identified by its energy (qualitative analysis) or by its intensity (semi-quantitative and quantitative analysis) ²⁹. This involves the use of either a radioisotope or an X-ray tube as a main source of radiation and equipment for detecting secondary X-rays from that source ³⁰. Materials subjected to short wavelength x-rays or gamma rays may ionize their constituent atoms. A secondary fluorescence x-ray emission occurs when atoms in a sample are excited by an x-ray beam and then revert to their original states. When short wavelength x-rays are diffracted in an analytical crystal with specified d-spacing, they are sorted.

2. Methodology

The RHs used in this research were gotten from four (4) different locations in Nigeria, these are: Adani in Enugu state, Ogoja in Cross River State, Abakaliki in Ebonyi and Adikpo in Benue State. In these locations, rice cultivation is predominant and RHs are readily available in large quantum. The husks were properly separated from some broken bran by sieving and winnowing. They were burnt in open air and the ashes collected. The ashes were kept in a dry section of the laboratory until they could be analysed. Portland limestone cement of UNICEM brand was also used in this research and was purchased in a cement merchant shop in Nsukka, Enugu State. Chemical analysis was conducted on the ashes and cement to determine the elemental composition of each ash. The following physical properties of RHA and cement were also investigated in this research work; density, specific gravity and particle size analysis.

The chemical compositional analysis of RHA was carried out as employed by ³¹ using X-ray fluorescence spectrometer (Model QX 1279). The physical properties of the samples were obtained using set of sieves, gas jars rubber Bunge, ground glass cover plates, weighing balance, thermometer, etc.

To perform XRF analysis, it is necessary to keep the tube-sample-detector system at a consistent geometry. Selected samples were placed on the tube's window at a standardized, modest distance from it. So, in order for the flux to be reproducible and repeatable since X-ray intensity follows an inverse square law, it's important to have very tight tolerances for placement and surface flatness. Ashes were shaped, finely powdered, and then pressed into a tablet using a milling machine. As a plus, secondary X-rays from lighter elements generally only emanate from the sample's top few micrometers, which is why it was important to achieve a smooth and representative sample surface. 5-20 rpm was used to decrease the effect of surface imperfections. We had to make sure the samples were thick enough to absorb the main beam in their entirety ³².

3. Results and Discussion

The results of the particle size distribution analysis carried out for cement and non-ground RHA are presented in Table 2. The results show that the percentage passing of non-ground RHA is comparable to cement and as such should be expected to perform a pozzolanic role as well as a microfiller effect to improve the particle packing density of concrete. RHA's physical properties, such as particle size, are affected by processing factors which include the burning technique, separation process, and grinding ³³. The particle size results were influenced by open field burning and the fact the ash was not ground.

The ashes obtained after open air burning were milky-white with percentage variation in chemical composition (Table 3). This variation in composition may be attributed to the difference in soil chemistry of the locations of collection and paddy variety.

Eight (8) elements were detected in all by the XRF method with varying concentrations. The RHA from Ogoja had the highest SiO₂ content, whereas the Vandikiya sample had a lower SiO₂ content but was higher than that found in cement. Furthermore, the total percentage composition of Silicon dioxide (SiO₂) and Iron III Oxide (Fe₂O₃) in RHA samples from Adani, Ogoja, Abakaliki and Adikpo are above the minimum pozzolanic requirement of 70% by the American standard for testing materials (ASTM, 1978) ³⁴. The results also reveal similar composition of cement with RHA but in varying proportions. Mehta and Real confirmed in their search that the variations in RHA could be linked to difference in soil chemistry at the sites where the samples were collected, paddy types, and the type of chemical fertilizers utilized ^{35, 36}. Apart from the above factors, the amorphous silica and carbon concentrations of RHA vary with incineration time and temperature. Ideally, the incineration temperature ranges between 500°C - 700°C to have high concentration of amorphous silica with the maximum specific surface area of up to 150 m²/g of RHA particles ³⁷. RHA containing approximately 85-95 percent amorphous silica was usually generated by controlled combustion and grinding ³⁸. Concentration of SiO₂ below 85% witnessed in the results is because the samples were not incinerated under controlled temperature.

In order to ascertain that the variation in the nature of RHA obtained from various geographical locations are significant and not merely by chance, the results were subjected to analysis of variance (ANOVA). The calculated F-value at 0.05 level of significance for particle size is 622.4 which is much greater than the critical F value of 2.1 (Table 4). This confirms that the variation in particle size is significant. On the contrary, the calculated F-value for variation in particle size of RHA due to geographical location is 1.5 which is less than the critical F-value of 2.17. This shows that particle size distribution of RHA is independent of geographical location. The particle size distribution is more of a function of the process of ashing.

Table 5 shows that though there are variations in the chemical composition of RHA obtained from various locations, the variations are not significant because the F-value of 0.2 is less than the critical F value of 2.2 at 0.05 confidence level.

4. Conclusions

Rice husk ash is a suitable pozzolanic material due to its high concentration of active silica. This research work has shown that Rice Husk Ash (RHA) varies in pozzolanic properties depending on the location they are found. The locations considered for variability in the chemical composition in this research has a silica content of 70.1, 84.6, 76.3, 70.1, 64.7, 55.6 and 48.4 compared to OPC with 23.5 percent. The variability in the chemical or pozzolanic characteristics of RHA will however impact the strength qualities of concrete.

After establishing RHA as a pozzolana, its use would prevent indiscriminate husk disposal while also reducing environmental pollution associated with cement production.

References