The Effects of Rice Husk Ash as Bio-Cementitious Material in Concrete

Abstract

Concrete is one of the most commonly used materials in civil engineering construction, and it continues to have increased production. This puts pressure on the consumption of its constituent materials, including Portland cement and aggregates. There are environmental consequences related to the increased emission of CO₂ that are associated with the production process of Portland cement. This has led to the development and use of alternative cementitious materials, mainly in the form of condensed silica fume, pulverised fuel ash, and ground granulated blast furnace slag. All of these are by-products of the silicon, electrical power generation, and iron production industries, respectively. In recent years, attention has turned to the possible use of sustainable bio-waste materials that might contribute to the replacement of Portland cement in concrete. This research investigates the effects of using rice husk ash as cement replacement material on the 1 to 28-day concrete properties, including the compressive strength, workability, and durability of concrete. The findings indicate that including rice husk ash in concrete can improve its strength at 3–28 days for percentage replacements of 5% to 20% (ranging from 2.4% to 18.7% increase) and improvements from 1 day for 20% replacement (with 11.1% increase). Any percentage replacement with rice husk ash also reduced the air permeability by 21.4% and therefore improved the durability, while there was a small reduction in the workability with increased replacement.

1. Introduction

Concrete has always been one of the most widely used construction materials in the civil engineering industry. Concrete’s popularity stems from a number of positive characteristics, including its affordability, adaptability, fire resistance, ease of forming, ease of production, availability of raw materials everywhere, and high compressive strength [1]. As a result, over 10 billion tons of concrete are manufactured per year to satisfy the global demand [2]. Concrete consists of cement paste as a bindery medium and aggregate as a mineral filler. Cement is a fine powder that, when mixed with water, changes from a paste into a hard material that gains strength over time. The hydration mechanism, which occurs when cement and water react chemically, results in increased strength. Aggregates, on the other hand, are rocks of various types, such as sand and fine and coarse rocks. As a result, the mineral fillers are bound into concrete by the cement paste formed by the hydration process [3].

The production and use of cement in the form of Portland cement (PC) is a source of environmental concern. Portland cement is considered to be one of the significant contributors to CO₂ emissions in the construction industry. Globally, cement accounts for 2.5 Gt of CO₂ emissions, which is equivalent to about 7% of the world’s total CO₂ emissions [4]. The release of CO₂ arising from the decomposition of limestone in the cement kiln is a process at present inherent to the production of cement [4]. As a result, the industry searched for ways to reduce the reliance on Portland cement. This resulted in the use of supplementary cementitious materials (SCMs) to replace Portland cement [5,6]. These include condensed silica fume (a by-product of the silicon-production industry), pulverised fuel ash (a by-product of the coal-powered electricity-generation industry), and ground granulated blast furnace slag (a by-product of the iron-production industry) [7].

In recent years, there has been increased interest is the use of bio-produced by-products as sustainable replacement materials for Portland cement. These include rice husk ash (RHA), which is a by-product of the agricultural industry [8,9].

Rice is an important essential food for nearly half of the world’s population [10]. More than seventy countries produce rice, including China, India, and Indonesia. In many rice-producing countries, the most common agricultural waste is the rice husk. As part of the milling process (Figure 1), rice husks (rice hulls) are strong protective coatings for rice grains, and they are removed from the rice seeds as a by-product [11]. Around one hundred million tonnes of this by-product are sourced from rice field processing worldwide. They have a very low bulk density ranging from 90 to 150 kg/m³. This results in a larger value of dry volume. The rice husk itself has a very rough surface and is abrasive in nature. Therefore, it is resistant to natural degradation. This would lead to environmental challenges with improper disposal. The three polymers cellulose, hemicellulose, and lignin make up the rice husk biomass [11].

When rice husk is burned, rice husk ash (RHA) is obtained [13]. Rice husk contains about 50% cellulose, 25–30% lignin, and 15–20% silica. Then, after burning, one-fifth to one-fourth of the rice husk will change to ash.

Global paddy output figures for 2010 are 678 million tons, implying 149.16 million tons of rice husk production, from which 37 million tons of RHA can be obtained [14]. After production, this massive amount of RHA goes to waste, posing a serious environmental problem by causing harm to the soil and nearby areas where it is deposited.

If the majority of RHA is used in concrete, it would not only eliminate the need to landfill RHA, but it would also reduce CO₂ release to the atmosphere by reducing the reliance on Portland cement. The annual global production of RHA, which is insignificant in relation to the immense amount of cement manufactured (4.1 billion tons in 2022) [15], cannot totally exclude cement. However, partly replacing cement with RHA would result in a major decrease in the volume of CO₂ released into the atmosphere per year [16]. The rice husk can be used more effectively by the concrete manufacturing industry [17,18]. The inclusion of RHA in concrete tends to increase the workability of fresh concrete [19]. The concrete can become more plastic and cohesive, and therefore, it can be easier to place and finish [20]. The inclusion of RHA in concrete tends to result in a reduction in strength [20,21], except at low water/cement ratios [21], with the concrete exhibiting more brittle behaviour [22].

Most work by other investigators in this area has established the benefits of RHA on durability and investigated the strength at 28 days. The aim of this research is to investigate RHA’s effects on the properties of concrete and its potential use as a low-cost building material by partially replacing Portland cement. The effects of 5–20% cement replacement on the compressive strength and air permeability as an indication of the durability, workability, and density of concrete are investigated during a period of 1 to 28 days. Improvements in the strength and durability of concrete as a result of incorporating RHA would further establish the use and application of RHA as a cement replacement in the concrete industry.

Such application and use of RHA, as an agricultural waste, would provide a practical and efficient method of disposal. This could boost the rural economy, converting rice husk from a waste product to a resource for the manufacture of a highly efficient supplementary cement replacement material.

2. Materials and Method

2.1. Materials

2.1.1. Cement

Cement is manufactured by combining eight primary ingredients in a chemical reaction during the manufacturing process: limestone, clay, marl, shale, chalk, sand, bauxite, and iron ore. Cement is the binding agent in concrete, and it is also the most active and expensive constituent. By volume, cement accounts for 10 to 15% of the concrete mix. The cement and a small amount of water react and harden in a process called hydration, where the materials are bonded into a rock-like mass. Portland cement (PC) of 42.5 grade conforming to BS EN 197-5:2021 [23] with a specific gravity of 3.15 [24] was used in the experiments.

Table 1. The chemical composition of PC.

Material	Specific Gravity	Chemical Analysis (wt%)
		SiO2	Fe2O3	Al2O3	CaO	MgO	LOI
Portland cement	3.15	21.54	3.63	5.32	63.33	1.08	2.2

shows the composition and specific gravity of cement.

2.1.2. Rice Husk Ash

Rice husk from Punjab province, Pakistan (Khan and Company Ltd., Cardiff, UK), was chosen to test its suitability as an ash substitute for PC in concrete. In the laboratory, rice husk was burned at 800 °C in a stable environment. After reaching the necessary temperature of 800 °C, the ash was cooled by spreading it in trays at laboratory ambient temperatures of 20 ± 1 °C. After burning the rice husk, only 22% of the ash was collected. It was sieved into 200 or 325 μm mesh after being ground in a milling machine (CF420 electric rice husk milling machine, by Laizhou Chengda Co., Ltd., Yantai, China). The particle size distribution is shown in Figure 2. The size ranges from 3 to 18 μm and average of 7 μm. The specific gravity of the substance was 2.1. The chemical analysis of the RHA is presented in

Table 2. The chemical composition of RHA.

Materials	Specific Gravity	Chemical Analysis (wt%)
		SiO2	Fe2O3	Al2O3	CaO	MgO	Na2O	K2O	LOI
RHA	2.1	84.00	2.01	1.39	0.85	0.60	1.83	3.09	5.85

.

Table 2 shows the following:

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Silica content is high in RHA samples compared with other constituents.

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RHA was observed to have high loss on ignition (LOI). This can be attributed to the carbonisation in the range of 400–600 °C. [25].

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There is low alkali content, with obtained Na₂O value being much lower than 3, indicating no extensive cracking and, consequently, no adverse effects on the mechanical properties of the concrete [26].

2.1.3. Aggregates and Sand

Fine Aggregate

Fine aggregates are any natural sand particles extracted from the ground through the mining process. They are made up of natural sand or crushed stone particles with a diameter of less than 5 mm. Fine aggregate with a specific gravity of 2.93 was acquired to meet the necessary properties of fine aggregate for experimental work, and the sand satisfied the BS EN 12620:2013 [27] requirements.

Coarse Aggregate

As coarse aggregate, crushed granite with a maximum size of 20 mm was used. The combined aggregates’ sieve analysis shows that they meet the requirements of BS 1881-125: 2013 [28] for graded aggregates with a specific gravity of 2.96. It was supplied by the Central Quarry & Mining LLC, Ras Al Khaimah, United Arab Emirates.

Washed Sand

Washed sand is light off-white in colour and was screened and washed to eliminate impurities, including silt, dust, and clay, before draining. The sand was left to drain until the excess materials were extracted. Washed sand is great for rendering and mixing concrete and has a less malleable mortar for products like flagstone with a specific gravity of 2.7.

2.2. Instrumentation

2.2.1. Pan-Type Mixer

Pan-type concrete mixer, which can be seen in Figure 3, is designed to mix both dry and wet materials efficiently. The mixing pans are removable and tilt to allow for easier access to the pan and emptying after the mixing process is completed. The pan has a total volume of 108 L, but the mixers have a size of 56/40 L.

2.2.2. Slump Cone Test

Slump testing is a laboratory or on-site method of determining concrete consistency/workability and the water/cement ratio indirectly for approval purposes or to record mixture characteristics. The slump test indicates the uniformity of concrete in various batches. The shape of the concrete slumps reveals details about the concrete’s workability and quality. The workability of a fresh concrete mix is measured using Gilson slump cones (Gilsonco, Lewis Center, OH, USA), which are shown in Figure 4, and follows the relevant BS EN 12350-2:2000 [29] requirements.

2.2.3. Cube Moulds

The concrete cubes used are cast in a one-piece mould with dimensions of 150 mm inside that are used to form specimens for concrete compressive strength testing in compliance with BS EN 12390: Part-1: 2000 [30].

2.2.4. Compressive Strength Test

Compressive strength test (CST) is a mechanical test that determines the amount of compressive strength a material can withstand before fracturing. A gradually applied load compresses the test piece, which is in the shape of a cube, between the platens of a compression-testing machine at a rate of 0.23 MPa/s and in accordance with BS EN 12390-3:2002 [31].

2.3. Procedure

2.3.1. Sample Preparation

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The mix design, as per the specifications and aggregate gradation report, was prepared and calculated as per batch.

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The aggregate, cement, water, and admixture were weighed as per batch size (0.02 m³ to 0.045 m³).

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The moisture correction and absorption were considered as per the testing of aggregate samples, and the water content in mix was adjusted.

2.3.2. Mixing Procedure

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Half quantity of coarse aggregate was added in the mixer.

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Total fine aggregates was added to coarse aggregate.

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After fine aggregate, remaining coarse aggregate was added and spread evenly with scoop.

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Aggregates (fine and coarse) were mixed for 15–30 s.

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Approximately half quantity of water was added and continued mixing for next 15–30 s.

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Mixer was stopped, and contents were left for 3–5 min to allow water absorption by aggregates.

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All cementation materials (PC and RHA) were added and spread evenly over the aggregate and mixed for 15–30 s.

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Mixer was stopped and removed the material adhering from the sides and blades of mixer.

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The remaining water was added together with admixture and mixed it for 2–3 min.

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The mixer was stopped, and the concrete was checked with a trowel for uniformity.

2.3.3. Mix Proportions

Table 3. Mix proportions used for the production of the different concretes.

Material Description		Quantity (Final Weight) kg/m3	Water Absorption (wt%)	Moisture Content (wt%)	Moisture Correction kg/m3	Corrected Quantity kg/m3	Batch 0.035 m3 (kg)
20 mm Coarse Aggregate		615	0.5	0	−3.08	611.9	21.42
10 mm Fine Aggregate		365	0.5	0	−1.83	363.2	12.71
0–5 mm Washed Sand		635	1.4	3	10.16	645.2	22.58
Water		160	-	-	-	154.4	5.4
Test A	100% PC	400	-	-	-	400.0	14
	0% RHA	0	-	-	-	0.0	0
Test B	95% PC	380	-	-	-	380.0	13.3
	5% RHA	20	-	-	-	20.0	0.7
Test C	90% PC	360	-	-	-	360.0	12.6
	10% RHA	40	-	-	-	40.0	1.4
Test D	85% PC	340	-	-	-	340.0	11.9
	15% RHA	60	-	-	-	60.0	2.1
Test E	80% PC	320	-	-	-	320.0	11.2
	20% RHA	80	-	-	-	80.0	2.8

below includes the material proportions used to produce the different concretes.

2.3.4. Testing

Slump Cone Test

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The workability/consistency test (slump cone) was carried out in accordance with BS EN 12350-2:2000 [29].

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The concrete was poured in three layers; each layer was tapped twenty-five times.

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The cone was removed after tapping; then, the concrete slumped, and the amount of the minimum concrete slump was measured in millimetres.

Fresh Concrete Tests

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The plastic density test was carried out in accordance with BS EN 12350-6:2000 [32] 60 min after the batching time.

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The air content was checked in accordance with BS EN 12350-7:2019 [33] 60 min after the batching time.

Compressive Strength Test

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The compressive strength of the concrete cubes was measured after 1 day, 3 days, 7 days, and 28 days of the casting day by placing them in compressive strength machine conforming to BS EN 12390-3:2002 [31] and applying compressive load at a rate of 0.23 MPa/s. Compression load was applied to the concrete samples until cracking failure of the concrete. The load was recorded in kN, and strength was recorded in MPa.

Each of the mentioned tests was carried out for all the concrete mixes in this research using 5 concrete samples.

3. Results and Discussion

3.1. Effects of RHA on Concrete Workability and Air Content

Table 4. Workability and air content variation with the percentage of RHA.

% OPC	% RHA	Workability mm	Temperature °C	Air Content %
100	0	210	25	1.4
95	5	220	25	1.1
90	10	210	25	1.1
85	15	190	25	1.1
80	20	190	25	1.1

and Figure 5 show a slight increase in the workability of concrete with a 5% replacement level of RHA, where the mix was cohesive in contrast to the control sample. This agrees with the literature, which shows that a low level of replacement for RHA (at 5%) increases concrete workability [34,35]. However, increasing the percentage of RHA to 10% and beyond to 15% and 20% maintains a similar level of workability of the RHA concrete. This also complies with some of the published work that shows the workability of concrete and mortar decreasing as cement is replaced with RHA [36,37].

Also, the inclusion of RHA at 5–20% appears to reduce the air content of the concrete by 21.4% (Table 4). This might relate to the efficient void-filling ability of the RHA particles, which can improve the durability of the concrete.

3.2. Effects of RHA on the Compressive Strength of Concrete

The compressive strength test results are shown in Figure 6, which shows that the control (100% PC) mix had a lower compressive strength than the other mixes at all curing ages (1, 3, 7, and 28 days). Only the 5% RHA mix had a lower compressive strength than the control sample on day 1. Initially, RHA samples of 5% and 10% replacement levels at the early curing age (day 3) have weaker compressive strength than the control mix [37]. The 5% and 10% RHA mixes, on the other hand, demonstrated improved compressive strength after 3 days of curing. For all curing ages, the mixes of 15% and 20% RHA replacement achieved even higher compressive strength compared to control. The RHA appears to improve the packing quality of the cement paste in the concrete mix, which might relate to its particle size (average of 7 μm, and Figure 2) and properties that when RHA replaces Portland cement, it provides improved void filling of the cement paste. Also, a contributor to this would be the pozzolanic reaction between RHA and the calcium hydroxide Ca(OH)₂ (portlandite) [38]. These findings are similar to those achieved using 10% microsilica with RHA by Zareei et al. [39] to enhance the concrete mix on days 7 and 28. However, they are contrary to those by Kamau et al. [40] and Rasoul et al. [41], who demonstrated a reduction in strength for RHA concrete from that of control (PC concrete) on days 7 and 28, which can be due to variations in the RHA properties as well as the mixing and testing conditions [42].

The higher replacement levels of RHA can have a greater impact on the compressive strength of concrete than pure PC.

3.3. The Effects of RHA on the Density of Concrete

The effects of replacing cement with RHA at 20% on the concrete density as it matures are demonstrated in Figure 7, which also includes the standard deviation of the density measurements. Up to 7 days of curing, the density of RHA concrete is the same as that for control (100% PC). However, beyond that, the concrete tends to have a slightly lower density for RHA replacement. This would be due to the lower density of rice husk ash (specific gravity of around 2.1) compared to Portland cement (specific gravity of 3.1). This indicates that while the RHA can produce lighter concrete, it also increases the strength of the concrete, as shown in Figure 6.

4. Conclusions

From the obtained findings of this study, the following conclusions can be made:

• The inclusion of rice husk ash in concrete at 10–20% partial replacement of Portland cement improved the strength of concrete starting 1 day after curing and up to 28 days.
• There is also a reduction in the air content of the concrete, indicating improvement in the durability of concrete containing rice husk ash.
• Rice husk ash can help produce lighter concrete without impairing its strength property. Therefore, lighter concrete can be produced with similar or improved compressive strengths to that of 100% PC concrete.
• Concrete can maintain its workability despite including rice husk ash in the cementitious content.
• The improvement in concrete strength might provide the potential for the increased use and application of concretes containing RHA that would be of further benefit to the environment.