EFFECT OF RICE HUSK ASH AS PARTIAL REPLACEMENT OF CEMENT ON CONCRETE STRENGTH
ABSTRACT
The global construction industry is increasingly seeking sustainable alternatives to ordinary Portland cement (OPC) due to the significant environmental burdens associated with its production, particularly carbon dioxide (CO₂) emissions which account for approximately 8% of global greenhouse gas emissions. Rice husk ash (RHA), a byproduct of rice milling and combustion, has emerged as a highly promising supplementary cementitious material (SCM) owing to its rich amorphous silica content, which typically ranges from 85% to 98% when properly processed under controlled temperatures. This study investigates the effect of rice husk ash as a partial replacement of cement on the compressive strength, workability, flexural strength, and durability characteristics of concrete. The experimental program involved the systematic replacement of ordinary Portland cement with RHA at proportions of 0%, 5%, 10%, 15%, 20%, and 25% by weight, with water-to-binder ratios maintained at 0.45. Standard 150 mm × 150 mm cube specimens were cast, cured under water at ambient temperature, and tested for compressive strength at 7, 14, 28, and 56 days in accordance with BS EN 12390-3:2019 and ASTM C39 standards. Slump tests were conducted in accordance with ASTM C143 to assess workability. Flexural strength tests were conducted on 100 mm × 100 mm × 500 mm prism specimens at 28 days. The findings indicate that RHA replacements of up to 10–15% yield comparable or marginally superior compressive strengths relative to the control mix at 28 days, attributed to the pozzolanic reaction between the amorphous silica in RHA and calcium hydroxide (Ca(OH)₂) liberated during cement hydration, resulting in additional calcium silicate hydrate (C-S-H) gel formation. Beyond 15% replacement, a progressive decline in compressive strength was observed, primarily due to the dilution effect on cement content and incomplete pozzolanic reactivity. This study concludes that RHA at 10–15% replacement is a viable, environmentally sustainable, and economically attractive partial substitute for OPC in structural concrete applications. The results contribute meaningful data toward promoting circular economy practices in the construction sector and the utilization of agricultural waste as value-added construction materials.
Keywords: Rice husk ash (RHA), supplementary cementitious material, pozzolanic reaction, compressive strength, sustainable concrete, cement replacement, ordinary Portland cement, agricultural waste utilization, concrete durability.
CHAPTER ONE
INTRODUCTION
1.1 Background of the Study
The construction industry stands as one of the most resource-intensive sectors in the global economy, with ordinary Portland cement (OPC) serving as the foundational binder material for concrete production worldwide. The production of OPC is, however, associated with extensive environmental consequences, particularly the emission of approximately 0.8–0.9 tonnes of carbon dioxide (CO₂) for every tonne of clinker produced (Lothenbach et al., 2011; Mehta & Monteiro, 2014). According to the Intergovernmental Panel on Climate Change (IPCC, 2022), the cement industry collectively contributes approximately 8% of global anthropogenic CO₂ emissions, placing it among the most polluting industrial sectors. As global construction demand continues to surge particularly in developing economies across Sub-Saharan Africa, South and Southeast Asia, and Latin America—the environmental pressures of escalating cement production have intensified, necessitating the exploration of low-carbon, sustainable, and locally available supplementary cementitious materials (SCMs). Among the many agricultural and industrial byproducts evaluated as potential SCMs, rice husk ash (RHA) has attracted significant research attention owing to its exceptional pozzolanic properties, widespread availability in rice-producing regions, and its potential to address dual challenges: agricultural waste management and environmental pollution. Rice is one of the most widely cultivated cereal crops globally, with world rice production exceeding 520 million metric tonnes of milled rice per annum (Food and Agriculture Organization [FAO], 2022). For every tonne of milled rice, approximately 200 kilograms of rice husk are generated, representing about 20% of the total grain weight (Givi et al., 2010; Chao-Lung et al., 2011). Global rice husk production is therefore estimated at approximately 120–150 million tonnes annually, a substantial proportion of which is either openly burned, discarded in landfills, or used inefficiently as low-grade fuel, contributing to environmental degradation and significant loss of potential value (Nehdi et al., 2003; Safiuddin et al., 2011).The chemical composition of RHA is predominantly silica (SiO₂), with content typically ranging from 85% to 98% depending on combustion conditions, temperature, and the origin of the rice husk (Rodríguez de Sensale, 2006; Zain et al., 2011; Sua-iam & Makul, 2013). When combustion is controlled at temperatures between 500°C and 700°C, the silica in RHA remains in an amorphous, highly reactive state, which confers superior pozzolanic activity upon the ash. In this reactive form, RHA silica reacts with calcium hydroxide (Ca(OH)₂) a byproduct of OPC hydration to produce secondary calcium silicate hydrate (C-S-H) gel, the compound primarily responsible for the strength and density of concrete (Cordeiro et al., 2009; Ganesan et al., 2008). Conversely, uncontrolled combustion at elevated temperatures (above 800°C) results in crystalline silica phases such as cristobalite and tridymite, which exhibit significantly reduced pozzolanic reactivity (Muthadhi et al., 2007; Chindaprasirt et al., 2007). The study of rice husk ash as a cement replacement material has a relatively long scholarly history, with foundational work tracing back to Mehta (1977, 1992), who first systematically documented the pozzolanic potential of RHA and advocated for its integration into concrete technology. Since then, a growing body of empirical literature has confirmed and expanded upon these early observations. Rao (2001) demonstrated that concrete mixes incorporating 10–20% RHA exhibited 28-day compressive strengths comparable to or exceeding those of OPC control mixes, while simultaneously improving resistance to chloride ion penetration and sulfate attack. Ganesan et al. (2008) conducted an extensive experimental investigation and reported that 30% replacement of cement with RHA achieved optimum results in terms of compressive, split tensile, and flexural strengths, alongside significantly improved resistance to aggressive chemical environments. More recent empirical studies have further refined our understanding of RHA’s mechanical and durability-enhancing properties in concrete. Kolawole et al. (2021) evaluated the influence of RHA on the compressive strength and durability of concrete in tropical climatic conditions and reported that 10–15% RHA replacement produced concrete with 28-day compressive strengths superior to the OPC control, with strength improvements attributed to the pozzolanic filling effect and densification of the interfacial transition zone (ITZ) between cement paste and aggregates. Similarly, Owolabi et al. (2022) investigated the combined effects of RHA and metakaolin as binary SCM blends and observed synergistic strength improvements at early and late curing ages, demonstrating the potential of multi-component blended cements in sustainable construction. In a comprehensive meta-analysis involving data from 42 experimental studies, Singh et al. (2023) confirmed that RHA replacements in the range of 10–20% consistently yielded compressive strength values within 90–110% of OPC control values, with notable improvements in concrete microstructure as evidenced by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses.
The workability of RHA-blended concrete has also received considerable scholarly attention, as it directly affects the practicality of RHA use in construction sites. Due to its high specific surface area ranging from 40,000 to 100,000 cm²/g depending on grinding conditions RHA tends to absorb more water and reduce the workability of fresh concrete compared to plain OPC mixes (Nehdi et al., 2003; Zain et al., 2011). Several researchers have recommended the use of superplasticizers or water-reducing admixtures to mitigate this effect and maintain adequate workability at higher RHA replacement levels (Chao-Lung et al., 2011; Cordeiro et al., 2009). Nevertheless, within moderate replacement levels of 5–15%, workability reductions are generally manageable and within acceptable limits for most structural applications. From a sustainability and circular economy perspective, the utilization of RHA as an SCM offers compelling advantages beyond its pozzolanic performance. The partial substitution of cement with RHA reduces the demand for clinker production, thereby lowering energy consumption and greenhouse gas emissions associated with cement manufacturing (Lothenbach et al., 2011; Mehta & Monteiro, 2014). RHA also contributes to the valorization of a low-value agricultural byproduct that would otherwise constitute an environmental burden, aligning with the principles of sustainable development and resource efficiency (Safiuddin et al., 2011; Owolabi et al., 2022). In developing countries where rice cultivation is a dominant agricultural activity including Nigeria, Ghana, India, Bangladesh, Vietnam, and Thailand the local availability of rice husk presents an opportunity to produce lower-cost concrete by reducing the proportion of imported or expensive cement (Kolawole et al., 2021; Mehta, 1992). Despite the extensive body of literature on RHA as a cement replacement material, there remain gaps in knowledge regarding the long-term durability performance of RHA concrete under specific exposure conditions, the influence of RHA fineness on mechanical properties, and the optimum replacement levels for different concrete grades and structural applications. Furthermore, variations in combustion methods, source material quality, and processing techniques result in RHA products of varying reactivity, making it important to establish locally specific data for the particular RHA source under investigation. This study therefore seeks to contribute to the evidence base by systematically evaluating the effect of locally sourced RHA as a partial replacement of cement on the compressive strength, workability, and flexural strength of concrete, using rigorous experimental protocols and standardized testing procedures. The importance of this research is further underscored by the escalating cost of cement in many developing nations, driven by supply chain disruptions, inflation, and high energy costs of clinker production. Partial replacement of cement with RHA a locally available, low-cost material has the potential to reduce concrete production costs by 10–25% depending on replacement levels and local market conditions (FAO, 2022; Kolawole et al., 2021). When coupled with the environmental and sustainability benefits, RHA emerges as an ideal candidate for integration into national standards and construction guidelines for concrete production in rice-producing regions.
1.2 Statement of the Problem
Despite the well-documented pozzolanic properties of rice husk ash (RHA) and the growing body of global research supporting its use as a supplementary cementitious material, its systematic integration into concrete production remains limited, particularly in developing countries where both the construction demand and rice production are highest. Several inter-related problems motivate this study.
Firstly, ordinary Portland cement production is environmentally unsustainable at current scales. The cement industry generates approximately 2.9 billion tonnes of CO₂ annually (IPCC, 2022), and without significant intervention through the adoption of low-carbon SCMs such as RHA, this figure is projected to increase in parallel with urbanization and infrastructure development needs, particularly across Africa and Asia (Mehta & Monteiro, 2014).
Secondly, the open burning and indiscriminate disposal of rice husk represent significant environmental challenges in rice-producing regions. In Sub-Saharan Africa, South Asia, and Southeast Asia, millions of tonnes of rice husk are burned in open fields annually, releasing particulate matter, greenhouse gases, and toxic compounds into the atmosphere (Safiuddin et al., 2011; Nehdi et al., 2003). The conversion of this agricultural waste into a value-added construction material would simultaneously address waste management and environmental pollution issues. Thirdly, while a broad body of international literature exists on RHA concrete, much of this research has been conducted using RHA derived from specific geographic sources, processed under particular combustion conditions, and tested under specific climate and exposure regimes. There is, therefore, a recognized need for locally specific experimental data to validate the applicability of findings to local RHA sources, local aggregate types, and local climatic conditions, particularly in tropical environments where curing conditions and temperature can significantly influence concrete strength development (Kolawole et al., 2021; Owolabi et al., 2022).
Fourthly, the optimum replacement level for RHA in concrete remains a subject of ongoing debate in the literature, with reported values ranging from 5% to 30% depending on the study’s methodology, the RHA’s physical and chemical characteristics, and the performance criteria applied. A rigorous experimental investigation is therefore warranted to establish evidence-based optimum replacement levels for locally sourced RHA.
1.3 Aim and Objectives of the Study
The aim of this study is to investigate the effect of rice husk ash (RHA) as a partial replacement of ordinary Portland cement (OPC) on the compressive strength, workability, and flexural strength of concrete.
1.3.2 Specific Objectives
The specific objectives of this study are to:
1. Determine the physical and chemical properties of the rice husk ash used in this study through proximate analysis, X-ray fluorescence (XRF) analysis, and specific surface area measurements.
2. Evaluate the effect of varying proportions of RHA (0%, 5%, 10%, 15%, 20%, and 25%) as partial cement replacement on the workability of fresh concrete using the slump test.
3. Determine the compressive strength development of RHA-blended concrete at 7, 14, 28, and 56 days of curing and compare results with the OPC control mix.
4. Assess the flexural strength of RHA-blended concrete beams at 28 days of curing to determine the effect of RHA on tensile behavior.
5. Identify the optimum RHA replacement level that achieves the best balance between mechanical performance, workability, and sustainability.
1.4 Research Questions
This study is guided by the following research questions:
1. What are the physical and chemical characteristics of the locally sourced RHA, and do they meet the requirements of ASTM C618 for pozzolanic materials?
2. How does increasing the proportion of RHA as a cement replacement affect the workability of fresh concrete?
3. What is the effect of RHA replacement levels on the compressive strength development of concrete at different curing ages?
4. What is the optimum percentage of RHA that can replace cement while still meeting or exceeding the target compressive strength of 25 N/mm²?
1.5 Significance of the Study
The significance of this research is multi-dimensional, encompassing environmental, economic, technical, and policy-related dimensions. Environmentally, this study contributes to the development of low-carbon construction practices by providing rigorous empirical evidence for the use of RHA as a partial cement replacement. Every tonne of cement replaced by RHA reduces CO₂ emissions by approximately 0.8 tonnes (Mehta & Monteiro, 2014; IPCC, 2022), representing a meaningful contribution to climate change mitigation efforts. Furthermore, the productive utilization of rice husk waste reduces the environmental burden associated with its open burning and landfill disposal. Economically, the study supports the reduction of concrete production costs in rice-producing regions by partially substituting expensive cement with locally available, low-cost RHA. This is particularly significant in developing countries where cement constitutes a substantial proportion of construction costs and where affordable housing and infrastructure development are national priorities (Kolawole et al., 2021; FAO, 2022).From a technical standpoint, this research generates locally specific data on the properties of RHA concrete, which can be used to inform mix design recommendations, construction guidelines, and specifications for RHA-blended concrete in structural applications. The study also contributes to the broader body of knowledge on supplementary cementitious materials and sustainable concrete technology.
At the policy level, the findings of this study can serve as a scientific basis for the inclusion of RHA in national building codes, standards, and specifications for concrete production, thereby facilitating the mainstreaming of this sustainable material in the construction sector.
1.6 Scope and Limitations of the Study
This study is limited to the experimental investigation of the effect of RHA as a partial replacement of ordinary Portland cement on the compressive strength, workability, and flexural strength of concrete. The RHA used in this study is sourced from a controlled combustion process, and its chemical and physical properties are characterized prior to use. The concrete mix design is based on a water-to-binder ratio of 0.45, with a target grade of C25 (25 N/mm²). RHA replacement levels are restricted to 0%, 5%, 10%, 15%, 20%, and 25% by weight of cement.
The study does not investigate the durability performance of RHA concrete under aggressive chemical environments, freeze-thaw cycles, or elevated temperatures, nor does it examine the structural behavior of full-scale reinforced concrete members. The influence of admixtures such as superplasticizers on RHA concrete is also outside the scope of this study. Additionally, while the economic analysis of cost savings associated with RHA use is acknowledged as significant, a detailed cost-benefit analysis is beyond the scope of this research.
1.7 Research Hypotheses
The following null and alternate hypotheses guide the study:
H₀₁: There is no statistically significant difference in the compressive strength of concrete with varying proportions of RHA as cement replacement compared to the OPC control mix at 28 days of curing.
Hₐ₁: There is a statistically significant difference in the compressive strength of concrete with varying proportions of RHA as cement replacement compared to the OPC control mix at 28 days of curing.
H₀₂: The workability of fresh concrete is not significantly affected by the partial replacement of cement with RHA at replacement levels of 5–25%.
Hₐ₂: The workability of fresh concrete is significantly reduced by the partial replacement of cement with RHA at replacement levels of 5–25%.
1.8 Definition of Key Terms
Rice Husk Ash (RHA): The residual ash obtained from the controlled combustion of rice husks, the outer protective covering of rice grains. RHA is characterized by a high silica (SiO₂) content (85–98%) in amorphous form, which confers pozzolanic reactivity.
Ordinary Portland Cement (OPC): The standard hydraulic binder used in concrete production worldwide, produced by grinding clinker with a small proportion of gypsum. OPC is characterized by its ability to set and harden by reacting with water through the process of hydration.
Pozzolanic Activity: The ability of a siliceous or aluminosiliceous material to react with calcium hydroxide in the presence of water at ambient temperature to form compounds with cementitious properties. The pozzolanic activity index is determined according to ASTM C311/C618.
Supplementary Cementitious Material (SCM): A material that, when used in conjunction with Portland cement, contributes to the properties of hardened concrete through hydraulic or pozzolanic activity, or both. Common SCMs include fly ash, ground granulated blast furnace slag (GGBS), silica fume, and RHA.
Compressive Strength: The measured maximum resistance of a concrete or mortar specimen to axial loading, expressed in megapascals (MPa) or N/mm². It is the most commonly used measure of concrete performance.
Workability: The ease with which freshly mixed concrete can be placed, consolidated, and finished without harmful segregation or bleeding. Workability is typically quantified using the slump test (ASTM C143) or the Vebe consistency test (BS EN 12350-3).
Calcium Silicate Hydrate (C-S-H): The principal binding phase formed during the hydration of Portland cement and the pozzolanic reaction of SCMs. C-S-H gel is responsible for the strength and density of hardened cement paste and concrete.
Interfacial Transition Zone (ITZ): The microstructural region approximately 10–50 µm thick around aggregate particles in concrete, characterized by higher porosity and lower strength compared to the bulk cement paste. SCMs such as RHA are known to densify the ITZ through pozzolanic reactions, improving bond strength and concrete performance.
Water-to-Binder Ratio (w/b): The ratio of the mass of water to the total mass of cementitious material (cement plus SCM) in a concrete mix. The w/b ratio is one of the most critical parameters governing concrete strength and durability.
1.9 Organization of the Study
This research is organized into five chapters as follows:
Chapter One presents the introduction to the study, encompassing the background of the study, statement of the problem, aims and objectives, research questions, significance of the study, scope and limitations, hypotheses, and definition of key terms.
Chapter Two provides a comprehensive review of relevant literature on rice husk ash, supplementary cementitious materials, and their effects on concrete properties. This chapter critically appraises empirical studies, theoretical frameworks, and standards relevant to the research.
Chapter Three describes the materials and methods adopted for the study, including the characterization of RHA, mix design procedures, specimen preparation, curing protocols, and testing methods for workability, compressive strength, and flexural strength.
Chapter Four presents the results of all experimental investigations, supported by tables, figures, and statistical analyses. The results are critically discussed in relation to existing literature and theoretical expectations.
Chapter Five provides the conclusions drawn from the study, highlights the contributions to knowledge, and presents recommendations for practice and future research.
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