Concrete is the most widely used construction material in the world due to its strength, versatility, durability, workability and availability. Primary components of concretes include cement, water, fine aggregate, and coarse aggregate. Large quantity of natural resources is used in the production of the billion tons of concrete used each year. Apart from the depletion of these natural resources, the production of cement which is the key binding material in concrete, is an energy-intensive process that significantly contributes to global carbon dioxide emissions (Amitkumar et al., n.d.). According to the International Energy Agency, cement production is responsible for approximately 7% of the world's carbon dioxide emissions making it one of the largest single sources of industrial greenhouse gases (IEA, 2018). More carbon emission from cement production is expected due to the exponential increase in the demand for concrete predicted for coming years as a result, there is growing interest in reducing the environmental footprint of concrete production through various strategies, including the use of supplementary cementitious materials (Górecki et al., 2018).

The use of large quantities of supplementary cementitious materials (SCMs) derived from agricultural by-products and industrial wastes has been found to be one of the most practical and economical approaches to reducing CO2 in the concrete industry. Various research studies reported that using agricultural by-products as a partial replacement for cement in concrete production can improved the overall performance of concrete properties and enhanced their sustainability properties by lowering costs and improving environmental protection.(Endale et al., 2023)

Rice husk ash is an agricultural waste product which is produced by the incineration of rice husk at a certain temperature level, because of its high amorphous silica concentration, specific surface area and pozzolanic activity, it is regarded by many researchers as a cementitious material for increasing the properties of concrete (Zhang et al., 2022a). Usage of RHA in concrete as partly replacements of cement minimize the environmental effects of land fill disposals, minimizes the cost of concrete per cubic meter and helps to easy recycling of waste generated from incinerations or combustions of rice husk used in industrial projects and also helps to reduce the CO2 pollution from cement production process (Siddika et al., 2018). Recent studies have demonstrated that the incorporation of rice husk ash into concrete mixes can lead to a significant improvement in compressive strength, flexural strength and splitting tensile strength. This review article aims to provide a comprehensive analysis of the impact of using rice husk ash (RHA) as a supplementary cementitious material on the mechanical properties of concrete. The review paper explores different experimental research findings of concretes incorporating rice husk ash as a partial replacement of cement and research findings which uses rice husk ash and other materials. The review further examines the optimal replacement proportions and provide a suitable conclusion offering guidance for future research and practical implementations.

LITERATURE REVIEW 

This review article outlines relevant experimental research topics related to the potential used of Rice Husk Ash as a partial replacement of cement in concrete and their subsequent impact on mechanical properties of the concrete. Thirty-five relevant research papers from different research journals have been selected using keywords search in the google scholar however sixteen experimental research papers out of these papers were summarized briefly outlining the author names, title of the study, research aims, methodology used and findings. while the remaining paper were read understood and used in the development of the article.

Experimental study: Partial Replacement of Cement by Rice Husk Ash

1. (Kulkarni et al., 2014) study the effect of rice husk ash (RHA) on properties of concrete. the work was carried out to study the suitability of the Rice Husk Ash as a partial replacement of cement in concrete. IS-Code method of mix design was used for mix design of M30 grade of concrete. The quantities of ingredient materials and mix proportions as per design are: 1: 1.2: 2.73 and 0.45 for W/C ratio. Test for compressive strength on concrete cubes for different mixes M0= 0% RHA, M1= 10% RHA, M2= 20% RHA, M3= 30% RHA for 7- and 28-days curing was conducted. It was observed from the experimental results that early strength gain is slightly increasing with addition of 10%, 20% & 30% RHA in normal concrete at 7 days but in 28 days tests results it was found that with addition of 20% RHA in normal concrete strength is running parallel or more than of normal concrete. Thus 20% RHA is the optimum content for getting nearly equal strength at 28 days. It was concluded that replacement of cement with Rice Husk Ash leads to increase in the compressive strength improved the workability, impermeability and achieved the target strength at 20% replacement for the grade of concrete.
2. (Noaman et al., 2019) conducted a comparative study of pozzolanic and filler effect of rice husk ash on the mechanical properties and microstructure of brick aggregate concrete (BAC). The focus was to investigate and compare the pozzolanic effect (PE) and filler effect (FE) of Rice Husk Ash (RHA) on the mechanical properties and microstructure of Brick Aggregate Concrete (BAC). The study revealed that the mean particle size of RHA decreases with increasing grinding time. The compressive strength (f'c) of BAC due to filler effect are 59-95% less compared to the pozzolanic effect of RHA for the 10%–25% replacement of cement respectively. Meanwhile, the 15% RHA showed the maximum compressive strength of BAC due to pozzolanic effect of RHA. The tensile strength (fsp) and flexural strength (fr) of BAC due to pozzolanic effect are 60%–150% and 25%–150% higher than that of filler effect of RHA for the 10%–25% replacement of cement respectively. The modulus of elasticity (Ec) and Poisson's ratio ð?) of BAC due to pozzolanic effect are 2%–29% and 27%–43% greater than that of filler effect of RHA for the 10%–25% replacement of cement respectively. BAC with 10–20% RHA shows a dense and homogeneous microstructure. Therefore, it was concluded that inclusion of RHA as a partial replacement of cement possesses a significant pozzolanic effect than the filler effect on the mechanical properties and microstructure of BAC.
3. (Abolhasani et al., 2022) study the effect of rice husk ash on mechanical properties, fracture energy, brittleness and aging of Calcium Aluminate Cement CAC concrete. Samples of concrete mixtures with different contents of Rice Husk Ash (0%, 2.5%, 5%, 7.5%, and 10%) as a cement replacement material were examined. In addition to the workability properties of the fresh concrete, the microstructural and mechanical properties of hardened concrete are characterized at the ages of 7, 28 and 90 days. The findings indicate that, at 90 days, the mechanical strengths of the mixes containing RHA were higher than those of the control mix, with the maximum improvement occurring at the substitution percentage of 5%. In accordance with Thermogravimetric Analysis TGA the substitution of 5% RHA in CAC concrete led to a higher hydration level, which in turn improved the mechanical properties relative to the specimen without RHA at 90 days. It was observed from the study result that adding RHA to the CAC concretes with different water-cement ratios reduced the workability of concrete, due to the porous microstructure of RHA and its high specific surface area. Therefore, maintaining the desired workability required an increment in the water content or using additional superplasticizer. Also, it has been highlighted that addition of RHA to CAC concrete decreased both characteristic length (Lch) and fracture energy (GF) at different ages; indicating that the substitution of RHA leads to a more brittle concrete.
4. (Zhang et al., 2022) investigate the Impact of rice husk ash on the mechanical characteristics and freeze–thaw resistance of Recycled Aggregate Concrete the research aims to investigate the impact of partially replacing (0%, 10%, 20%, 30% of binder) of ordinary Portland cement with RHA by equal mass on recycled concretes. Workability, compressive strength, mass loss and dynamic elastic modulus of recycled concrete were tested and the hydration products and microstructure were analyzed using scanning electron microscope (SEM) tests. 100mm × 100mm × 100mm cubic specimens was used to test the compressive strength at the age of 7d, 28d, 90d, and 360d using YAW-3000 universal loading machine. Concrete specimens were put into the TDR-28 concrete freeze–thaw cycle testing machine for a rapid freeze–thaw cycle experiment. Its observed that gradual increase in the RHA replacement ratio increased the cementitious material’s total specific surface area and porosity, which will increase the mixtures’ friction resistance and discourage the flow, which in turn leads to the decrease in the concrete slump and the reduced workability. Compressive strength of recycled concrete increased with the growth of the curing age and the optimum compressive strength of recycled concrete is observed with a 20% RHA replacement ratio. It was further observed that the degree of freeze–thaw damage deterioration grows as the RHA replacement ratio increases.
5. (Al-Alwan et al., 2024) conducted an experimental study which investigates the impact of using rice husk ash as a replacement material in concrete. In this study, the impact of adding rice husk ash RHA in different ratios (0%, 7%, and 14%) as a replacement of ordinary Portland cement with different water–cement ratios (0.3, 0.5, and 0.7) both on the durability, strength and corrosion of the concrete was investigated. Compressive strength test was performed according to IS:516-1959 after 10, 20, and 30 days. Flexural strength test was performed for all specimens using beam specimens of 125 mm × 125 mm × 650 mm according to IS:516- 1959, Split tensile strength test was done under a compression testing machine according to IS:5816-1999 for all the specimens. The rapid chloride ion penetration test (RCPT) was performed for the specimens after curing them for 30 days according to ASTM C1202-94. The experiment results showed that increasing the RHA content increases the flexural, compressive, and tensile strength values with time and decreases the slump value and the chloride ion penetration rate. However, an increase in the w/c ratio has an adverse impact, as it decreases the values of all strength tests and increases the slump value and the chloride ion penetration rate. The experimental study used fine particles of RHA, which decreased the pore structure of the resulting concrete and increased its bulk density. Thus, RHA with fine particles is a promising partial replacement for Portland cement. Its addition can enhance the concrete durability, strength, and resistance to harsh environment conditions. It also provides economic and ecological advantages over using ordinary Portland cement.
6. (Saravanan & Sivaraja, 2016) conducted an experimental study on the mechanical behavior of concrete modified by replacement of cement by Rice Husk Ash. the study was carried out for concrete mixes with 10%, 20% and 30% replacement of cement by RHA to determine the mechanical properties of concrete with w/c ratio of 0.45. Concrete specimens (150 x150 x150) mm were made for testing the compressive strength of concrete. All the cubic samples of the conventional concrete and RHA replaced concrete were tested in a HEICO Compression Testing Machine with references to IS: 516 –1959 at an age of 7 days and 28 days. Splitting tensile strength tests was carried out on cylindrical specimens of diameter (150 x 300) mm at an age of 7 days and 28 days using HEICO Compression Testing Machine. The flexural strength tests were carried out in Universal Testing Machine by two-point loading method as per as IS: 516 – 1959 guidelines. The specimens were of size 500 mm x 100 mm x 100mm. It was observed that concrete with RHA shows significant effects on the mechanical properties of concrete. Out of all the mixes considered, Concrete with 10% and 20% replacement of cement by RHA was found to be optimum.
7. (Siddika et al., 2018) conducted a study on concrete with rice husk ash, the mechanical behavior of concrete specimens was tested with different percentages of rice husk ash RHA as a partial replacement of cement content and with different w/c ratio and then compared with control concrete. The mechanical behavior of concrete specimens was tested with different percentages of RHA as replacement of cement content and with different w/c ratio and then compared with control concrete. Slump test was done to evaluate workability as per IS: 1199- 1959, The water absorption tests were carried out for concrete specimen (cylindrical) with 0, 10 and 15% RHA replacement, Specimens for compressive strength test were casted using cylindrical molds of (100 ×200) mm. Tests were done according to IS: 516-1959 after 7, 14 and 28-days curing, Beam specimens of size (150 × 150 × 700) mm were prepared for testing the flexural strength of plain cement concrete and RHA-used concrete beams according to IS:516-1959, Consequently, cylindrical specimens of (150 × 300) mm were casted for tensile strength test. The specimens were hardened for 24 h after casting and cured in water for 28 days and then tested under compression testing machine as per IS:5816-1999. It was observed that, Slump decreases and the water demand increases with increase in cement replacement with RHA. This phenomenon occurs due to RHA used concrete is more porous than control concrete. When water–cement ratio increases, concrete porosity also increases. The compressive strength, flexural strength and tensile strength of concrete specimens with 10?ment replacement with RHA are comparable to the control specimens. As Replacement of 10?ment by RHA is optimum and considerable with respect to compressive strength of concrete.
8. (Mostafa et al., 2022) conducted an experimental study and theoretical prediction of mechanical properties of ultra high performance concrete incorporated with Nano rice husk ash burning at different temperature treatments. the research aimed to investigate the effect of Nano rice husk ash (NRHA) prepared using different thermal treatment methods on ultra-high-performance concrete (UHPC) behavior. NRHA was prepared by two methods: (1) burning for 3 h at 300, 500, 700 and 900 °C and (2) burning for different durations (9, 7, 5 and 3 h) at 300, 500, 700 and 900 °C. NRHA was added to UHPC to make 25 mixtures with three dosages (1%, 3% and 5%). Density, compressive strength, tensile strength, flexure strength and ultrasonic pulse velocity tests were performed at the experimental level. Moreover, full microstructure analysis was performed. It was observed from the experimental results that, Using NRHAs improved the mechanical and microstructure properties of UHPC. At a constant burning duration, the optimum strengths and superior properties were obtained using 1% NRHA burned at 900 °C, which had the largest amount of amorphous silica. At different burning durations, the optimum strengths and the best performance were obtained using 3% NRHA burned at 700 °C, which had the highest quantity of amorphous silica. It was further observed from the experimental results that, Burning RHA at 900 °C for 3 h and at 700 °C for 5 h has similar performances when added to UHPC at 1% and 3% dosages, respectively. The obtained experimental results were used to build an artificial neural network (ANN) to predict UHPC properties. The ANN model was used as a validation tool to determine the correlation between results. Results showed a remarkable improvement in the mechanical properties of UHPC incorporating NRHA for all mixtures.
9. (Martinez Urtecho et al., n.d.) conducted an experimental study on Sustainable Concrete with Rice Husk Ash (RHA) for Marine Structures In this investigation, cylindrical specimens of conventional concrete and sustainable concrete with cement replacement percentages of 5%, 10% and 15% RHA have been developed. Studying the breakdown of coarse and fine aggregate, the cost of concrete with RHA; as well as the influence of 5%, 10%, and15% of RHA on the rapid penetration by chloride ions, the resistance to compression and the depth of penetration of water under pressure. The results show that for a 5% RHA replacement, the compressive strength is greater than conventional concrete at an early age; the disintegration of the aggregates decreases, being for the fine of 9.86% and for the coarse aggregate of 4.34%, the penetrability of sulfate ions decreases as the percentage of RHA replacement increases, the permeability of conventional concrete is much greater with respect to RHA replacement concrete and, finally, the unit cost analysis per cubic meter of RHA concrete is less than conventional concrete.
10. (Botchway et al., 2020) investigates the effect of partial replacement of ordinary portland cement (OPC) with Ghanaian rice husk ash (RHA) on the compressive Strength of Concrete In this experimental investigation, percentile replacement (0%, 5%, 10%, 15%, 20% and 25%) of cement with RHA by weight to compare with conventional concrete experimental results for concrete mixes C20, C25, C30 and C35. A total of 128 concrete cubes were cast and their compressive strengths of RHAC in comparison with Conventional Concrete (CC) were assessed at 3, 7, 14, 21, 28, 56, 90 and 180 days. It was observed from the experimental result that, Addition of RHA to the mixes resulted in a slight increase in the compressive strength of the Ghanaian rice husk ash concrete but further increase in the RHA resulted in the reduction in the compressive strength with an increase in the demand for water which accounted for the low workability of the RHA concrete. The optimum replacement of OPC with RHA taken at 28 days strength for all the grades C20, C25, C30 and C35 was at 5%. The results for the chemical analysis showed a low percentage of metals in the samples tested. the RHA was found to have a high percentage of lignin which is not biodegradable and as such may add to the durability property of concrete not decaying when embedded.

Experimental Study: Partial Replacement of Cement by Rice Husk Ash and Other Materials:

1. (Bheel et al., 2020) conducted an experimental research on Rice Husk Ash and Fly Ash effects on the mechanical properties of concrete The properties of fresh and hardened concrete were studied for five concrete mixtures: Conventional concrete mix 0%(0%RHA+0?), (52.5%RHA+2.5?), (5%RHA+5?), (7.5%RHA+7.5?) and 20% (10%RHA+10?). The samples were casted with mix proportions of 1:2:4 and 0.55 water/cement ratio and were cured for 7 and 28 days. In order to determine the characteristic strength of concrete samples, they were tested on a Universal Testing Machine (UTM), Concrete cubes (100×100×100) mm, were used to measure compressive strength, and cylindrical samples (200×100) mm were used for determining indirect tensile strength. Similarly, the density and the water absorption of the samples were measured after 28 days. Results from the experimental study showed that, after 28 days 10% (5% RHA+5?) sample’s compressive strength was enhanced by 16.14% and its indirect tensile strength was improved by 15.20% compared to the conventional sample. More over the sample’s slump value dropped as the content of RHA and FA increased.
2. (Hesami et al., 2014) study the effects of Rice Husk Ash and Fiber on mechanical properties of pervious concrete pavement. the experimental study investigates the physical and mechanical properties of pervious concrete containing Fibers and Rice Husk Ash. Rice husk ash (RHA) was used in order to strengthen pozzolanic cement paste and the effect of 0%, 2%, 4%, 6%, 8%, 10% and 12% weight percentages as a cement replacement in concrete mixtures on the mechanical properties was studied. Moreover, 0.2% Vf of glass (where Vf is the proportion of fiber volume to total volume of concrete), 0.5% Vf of steel and 0.3% Vf of polyphenylene sulfide (PPS) fibers were used to improve the mechanical properties of the pervious concrete. Also, several water to cement (w/c) ratios were made and then, physical and mechanical properties of hardened concrete including porosity, permeability, compressive strength, tensile strength and flexural strength were investigated. The results indicated a significant increase in compressive, tensile and flexural strengths It was observed from the experimental results that, the optimum percentage of RHA in the specimens without fibers (series D) is 8% while it is between 8% and 10% in the specimens with fiber series (A, B and C). This indicates that the addition of fibers to the concrete containing RHA does not considerably affect the optimal amount. The compressive, tensile and flexural strengths in concrete with 12% of RHA were found to be slightly lower than those of 10% of RHA, as the optimum RHA amount, while its permeability is considerably higher than that of 10% of RHA. Hence, it seems that 12% of RHA can be more suitable for concrete pavements.
3. (Varadharajan et al., 2020) conducted an experimental study which assess the mechanical properties and environmental benefits of using rice husk ash and marble dust in concrete. The research work evaluates the effect of replacement of Cement with Rice Husk Ash (RHA) and replacement of Fine Aggregate with Mable Waste Powder (MWP) with addition of 1.5% Hooked Steel Fibers on mechanical properties of concrete and also determines the environmental benefits of using the RHA and MWP in concrete by performing life cycle assessment using recipe midpoint and end point analysis. concrete samples were prepared by replacing the cement with RHA in quantity varying from 5% to 20% and fine aggregate with MWP in proportions of 10%, 20%, and 30%. The hooked steel fibers were used in the ratio of 1.5% by weight of cement. The concrete mixtures were produced and grouped into four families. RHA replaced the cement by (1) 0%, (2) 5%, (3) 10% (4) 20%. It was observed from experimental results that, the optimum combination was obtained for 15% RHA, 30% MWP and 1.5% hooked steel fibers. A maximum enhancement of 44.4%, 60%, and 46.13%, in compressive, tensile, and flexural strength for the optimum combination. In addition, porosity and water absorption reduced with the inclusion of RHA, MWP and steel fibers. The economic feasibility study showed a reasonable enhancement of cost in the inclusion of RHA, MWP & steel fibers, which is justified in return for an excellent improvement in mechanical and permeability properties along with the positive impact on the environment. The environmental impact assessment showed that utilization of RHA & MWP adds a valuable contribution and positive impact in minimizing the environmental pollution resulting in a sustainable and clean environment.
4. (Panda et al., 2020) study the effect of Rice Husk Ash on mechanical properties of concrete containing Crushed Seashell as fine aggregate. The experimental study reveals how mechanical properties of concrete containing crushed seashell is influenced by use of rice husk ash (RHA). the study comprises of twelve concrete mixes and 0.45 w/c ratio has been used for all the twelve mixes. First four mixes were made by replacing 0, 10, 20, and 30% of fine aggregate with crushed seashells and 0% of cement with RHA, second four mixes were prepared by replacing 0, 10, 20, and 30% of fine aggregate with crushed Seashells and 10% of cement with RHA and third four mixes were prepared by replacing 0, 10, 20, and 30% of fine aggregate with crushed seashells and 20% of cement with RHA. M30 grade mix design was chosen for the study. Fresh and hardened tests was conducted to determine workability and mechanical properties of the spacemen samples after 7 days, 28 days and 90 days of curing under water. The experimental observations indicate that use of RHA based concrete mixes improves the strength in comparison with that of crushed seashell based concrete mixes. In seashell-based concrete, the strength gradually decreases at all percentage of replacement of crushed seashells with fine aggregate. Further, it is observed that, partial substitution of cement with RHA increases strength in all the concrete mixes. The result indicates higher compressive, split-tensile and flexural strength at 10% partial replacement of fine aggregate with crushed seashells and 20% partial substitution of cement with RHA.
5. (Bheel et al., 2021) conducted an experimental study on mechanical properties of concrete incorporating Rice Husk Ash (RHA) and Wheat Straw Ash (WSA) as ternary cementitious materials. the experimental study aims to investigate the influence of RHA and WSA as TCM on the fresh (slump), physical (water absorption and density), and hardened properties (compressive strength, splitting tensile strength, and flexural strength) and drying shrinkage of concrete. the research work is performed on the concrete blended with 0%, 5%, 10%, 15%, and 20% of RHA and WSA as TCM in the mixture. Total of 240 concrete samples (cylinders, cubes, and beams) were prepared with 1: 2: 4 mix proportions at 0.50 water-cement ratio and cured at 7 and 28 days, respectively. It was observed from the experimental results that; the workability of green concrete is getting reduced as the quantity of TCM increases in the mixture. Besides, the compressive strength, splitting tensile strength, and flexural strength are enhanced by 12.65%, 9.40%, and 9.46% at 10% of TCM (5% RHA and 5% WSA) on 28 days consistently. Furthermore, the density and water absorption of concrete are reduced with the increase in the dosages of TCM on 28 days, respectively. In addition, the drying shrinkage is reduced with the increase in the quantity of TCM in concrete.
6. (Salem Al-Ahdal et al., 2018) conducted an experimental study on mechanical properties of concrete containing fly ash (FA), rice husk ash (RHA) and waste glass powder (WGP). the research work investigates the workability, compressive and tensile strength of the concrete. Standardly sized cubes (150× 150×150 mm) were utilized for this research program. The samples in the research study were divided into 4 groups namely (G1, G2, G3, and G4) respectively. Group G1 was the control group, G2 samples comprised the addition of 15% RHA and 39?. G3 samples used a substitute of Cement, and fine aggregate, by 10% of RHA and 10% of Waste Glass Powder (GWP) respectively. The last group G4 comprised samples with the addition of Steel Fibers in the samples for Group 3. 28 days target strength was adapted and w/c ratio was kept in the range of 0.40 to 0.45. Superplasticizer was added to the concrete cubes of Group 3 and 4 only. It was observed from the experimental results that, Maximum compressive strength and splitting tensile strength was obtained in the G3 with a combination of 10% Rice husk ash (RHA), 10% of Waste glass (WG) and 39% Silica fume as partial substitute of cement for various curing periods 28 days. It was also revealed that, Addition of Fly Ash in G2 increases the workability of concrete, but Rice Husk Ash has decreased the workability in G3. In G4, the addition of steel fibers did not affect the workability of concrete as compared to the controlled sample. The environment pollution during the disposal of excess Fly ash and Rice husk ash can be reduced by reusing rice husk ash, waste glass, and fly ash, therefore, the partial substitutes of these materials can decrease the cost of concrete.