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CHAPTER 5

Properties of Rice Husk Ash Blended Cement

Mu’izzuddin Herman1, Kell Moryane1*, Shahiron Shahidan1

1 Jamilus Research Center, Faculty of Civil and Environmental Engineering, Universiti Tun Hussein Onn Malaysia, 86400, Batu Pahat, Malaysia

* kellmoryane@hotmail.com

Abstract

Rice husk is the remaining residue after the grain is removed. Rice milling produced 78% weight of rice rest 22% is husk, rice husk is firing using furnace with certain temperature to produce rice husk ash (RHA). The rice husk is an unnecessary waste of rice, with many opinion of research about rice hush is can enhance the strength and properties of concrete, hence this proposed research. The purpose of this paper is to investigate the properties of rice husk ash (RHA) as blended cement. The effect of grinding on the particle size and the surface area was first investigated, then, to verify the characteristic of rice husk ash on physical and chemical properties to know their density, color, fineness effective, shape and the chemical element involved. Furthermore, the effect of RHA average particle size and percentage on concrete workability, superplasticizer content and the compressive strength influence the properties of cement. Although grinding RHA would reduce its average particle size. Incorporation of RHA in concrete increased water demand and gave excellent improvement in strength. Another interesting observation from this paper is the relationship that exists among water absorption and chloride penetration. Increasing RHA fineness enhanced the strength of blended concrete compared to coarser RHA and control OPC mixtures. The rice husk ash is an agricultural waste has plenty of used. It is hope this paper statement will improve the cement industry properties and strength using rice husk ash as blended cement.

Keywords— characteristic of RHA, concrete workability, superplasticizer, compressive strength, water absorption, chloride penetration

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1.0 INTRODUCTION

Rice husk is agricultural waste generated from outer shell of rice from milling process and produces 20% of the 500 million tons of rice produced annually in the world. In Malaysia, the government aims to achieve a self-sufficiency of rice production at 100% in 2020 [1]. In modern rice mills, the disposal rice husk is usually a serious problem because of the extremely low bulk density and requires a large space for storage and transportation. Since their protein contain is inevitable, they are not suitable for animal feed. The husk burns from milling plant when used as a fuel also contributes to pollution and efforts are being made to address the environmental problem by using this material as an additional cementing material [2]. For example of rice husk in agricultural waste is shown in Figure 1.

Figure 1: Example of rice husk.

The chemical composition of rice husk is varied from one sample to another due to the difference in rice type, year crops, climate and geography [3]. The digital and SEM image of difference type of rice such sticky, red and brown rice shown in Figure 2.

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Figure 2: Digital and SEM images of raw rice, rice, rice husk ash and synthesized silica Nano powders from sticky, red and brown rice [4].

2.0 RHA AS CEMENT REPLACEMENT MATERIALS

2.1 PRODUCTION OF RHA

RHAs are produced in two ways: open burning and controlled combustion. Open burning is a long way of combustion and also takes a long time to complete the combustion. The rate of time depends on the thickness and also the height of the pile or heap of rice husk to be burned. Controlled combustion is the combustion using a special combustion chamber which is a ferro-cement furnace as shown in Figure 3 that controls the combustion of the rice husk. This way will produce a quality RHA and can get 10% - 15% of the original weight of the rice husk. It can also burn 35kg of rice husk in 24 hours.

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Figure 3: The RHA Furnace.

After the combustion process the RHA produced is required to go through the crushing process to produce a more refined RHA. This refined RHA is suitable for additives. It also has a large surface area and allows it to react with water and cement more effectively. The shape of the crystal and the surface area of the comparator affects the reactivity of the RHA. Besides, the reactivity of the RHA is affected by the silica content of the armor. According to Cook (1979), rice husks roasted at 450°C for 4 hours will produce RHA containing active armor silica. Other things such as combustion, oxidation method and rice husk morphology also affect the activity of RHA [5].

2.2 THERMAL TREATMENT OF RHA

RHA is an important source of silica. With heat controlled, the decomposition of rice husk, is possible to produce ashes containing silica reactive. In the conversion to rice husk ash, rooting process removes organic material and produce structural changes husk which affects both pozzolanic ash activity and its advantages. Recently, the pozzolanic RHA activity using a variety of techniques to detect temperature cuts and combustion times, the sample was burned at 500 or 700°C and burned more than 12 hours to produce high

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volatile ash without much crystalline material. Short burning time (15 - 360 minutes) produces high carbon content for RHA produced even when high temperatures burn 500°C to 700°C [6].

When rice husk is first heated, weight loss occurs up to 100°C as a result evaporation of polluted air. The husk burned into ash at temperature range between 300°C to 450°C,if the husk burned below 500°C the ignition was not complete, and amount of unburned carbon was found in ash, from 400°C to 500°C, residual carbon oxides and the majority of weight loss occur during this period [7]. At 350°C, the volatile is on, causing further weight and chaff start to burn.

Thermal decomposition of rice husks prone to higher of 300°C is shown in TABLE 1. This process can be classified into two stage, carbonization and decomposition [8]. Carbonization Rice husk occurs when rice husk is treated at temperatures higher than300°C and releases combustible gas and tar. Instead, decarbonation is considered as a permanent carbon burning in rice husk char. Decarbonation occurs when char is treated at higher temperatures in the presence of sufficient oxygen. The silica phase in RHA is clear influenced by the temperature of burning rice husk and it is important factor in reactive chemical silica at RHA. If it burns rice hull temperature is lower than 700°C, silica in RHA remain in the form of 3-5-5 amorphous [9]. If the temperature burns rice husk is higher than 800°C, crystalline silica in RHA as cristobalite or tridymite which has a lower Chemical reactivity compared with amorphous silica conditions.

Table 1: Thermal decomposition of rice husk.

Heat O2

Combustible

Gas tar Heat CO2

Carbonization

[Decomposition of volatile matters] [Carbonization of fizzed carbon] Decarbonization

In addition, chemical silica reactivity is directly proportional to specific surface area of RHA. Heating rate is also another factor in burning rice husk.

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Rice husks contain little potassium, which come from fertilizers. If the rate of heating is high, potassium rice husk unalterable and react with silica, potassium burned to polysilicate united with carbon. Rapid combustion of rice husks causes high residuals carbon in RHA. Therefore, the combustion process should be designed considering the decomposition properties of the rice chaff [10].

3.0 CHARACTERISTIC OF RHA

The characteristic of rice husk ash depend on geological and geographic factors related to the types of the rice, duration of burning, burning temperature, grinding method and collecting devices. Because of this factor, which will influence the effective use of ash in concrete [11].

3.1 PHYSICAL PROPERTIES

The physical properties of rice husk ash like colour, fineness, shape and size, density are mainly depend upon the type of burning the rice husk and grinding time of the ashes.

3.1.1 COLOUR

The colour of rice husk ash may vary from white, light to grey and almost black depending on the type rice and temperature burning and the duration of burning. If the burning occurs between 450°C and 550°C, carbon will remain in the ash which is black [12]. Figure 4 shows the burnt rice husk ash with certain temperature. As the temperature of processing becomes higher, the ash becomes whiter. However, ash recovered from the interior of large masses of burnt husks where air access is restricted, such as in heap burning, is a lilac pink colour and the best colour for cement mix is bright gray [13].

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Figure 4: Burnt rice husk ash.

3.1.2 FINENESS

The fineness of rice husk ash will have an influence on the pozzolanic reactivity and workability of concrete. The use of rice husk ash increase the water demand due to fineness of ashes [14]. The refinement of this ash is usually measured by the BET nitrogen collection method or the Blaine air usability method in m2/Kg. RHA has specific surface area (Blaine's) varies from 300 m2/ Kg to 2000 m2/Kg. Specific surface area (Blaine's) BA may vary from 450 m2/Kg to 1000 m2/kg. The RHA's efficiency increases with increased grinding time for all combustion temperatures. Generally for the grinding of the given time, there is a large reduction in the specific RHA surface area as the temperature combustion increases. The influence of fineness, as determined by Blaine's permeability air, is determined on the strength compression mortar and indicates that strength compression increases when fineness increases [15].

3.1.3 DENSITY

The density of rice husk ash depends on the constituents (iron, silica, aluminium and calcium) and higher carbon content tents to lower the density. The compacted unit mass of rice husk ash ranges from 200 to 600kg/m3 while the value of concrete incorporating rice husk ash ranging from 2000 to 2300kg/m3. The specific gravity of rice husk ash varies from 2.02 to 2.08 and 2.00 to 2.06 respectively [16].

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3.1.4 SHAPE AND SIZE

The shape and size of the particle ash mainly depends on the phase of mineralogy and pyroprocessing. When rice husk ash was scanned by electron microscope, usually the form of RHA particles is an irregular and cellular texture [17]. Figure 5 shows the SEM of RHA from three different researchers. The RHA was analyzed to be multilayered, angular, microporous surface and honeycombed. This justifies the RHA high specific area [18, 19, 20].

Figure 5: RHA image of SEM, (a) [18], (b) [19], c [20].

3.1.5 AMORPHOUS SIO2

Reactivity of RHA as pozzolanic material depends on the crystalline or amorphous chain. Therefore, for the characterization of RHA, the assessment of the amount of amorphous silica has become very important. For this Purpose, there are certain methods. The silica phase in the RHA is clearly influenced by the temperature of rice husk burning and it is an important factor in reactive silica chemistry in RHA.

3.2 CHEMICAL PROPERTIES

RHA is a fine particulate material with major constituents of SiO2, Al2O3, Fe2O3 and CaO responsible for pozzolanic activity. They also contain MgO, K2O, N2O, SO3 and unburnt carbon. The general changes in the three basic principles in RHA are as follows, which RHA - SiO2 (80-98%), Al2O3 (0.10-0.6%), Fe2O3 (0.15-0.60%). There are some differences in standard requirements in

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the case of SO3 and loss on ignition. RHA has some amount of crystalline elements such as quartz, crystobalite and tridymite and free calcium oxide (up to 10%) [21].

4.0 PERMEABILITY

4.1 WATER ABSORPTION

Concrete blended with RHA leads to a lower water absorption and sorptivity rate which is a vital factor for concrete durability [22, 23]. The inside pores of the concrete are minimized as well as makes the concrete uniformly arranged by finer RHA particles [24]. The water absorption co-efficient are reported lower for ultrafine RHA with an average particle size of 5 mm. However, RHA with average particle size of 95 mm showed greater water absorption value than OPC [25]. This concludes that the fineness of RHA is accountable to reduce water absorption in the concrete.

4.2 CHLORIDE RESISTANCE

In evaluating the capability of RHA blended concrete to resist chloride penetration, various methods were applied by investigators including ASTM C1152 (2003), AASHTO T259 (1980), AASHTO T277 (1983), ASTM T1202 (rapid chloride penetration test) [22, 26, 28, 29]. Their results acknowledged that at replacement levels up to 40%, the concrete proved to be more resistance. It also concluded that the risk of concrete deterioration is reduced because the RHA in concrete are considered as a protection for the steel in reinforced concrete.

5.0 PROPERTIES OF RHA

Rapid pollution lead to the expanding interest for creating reliable development materials. Supplementary cementitious materials turn out to be the most compelling way to meet the requirements of a durable concrete. Rice husk ash is observed to be better than other supplementary materials such as slag, silica fume and fly ash [29]. As a result of its high

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pozzolanic activity, both strength and durability of concrete are improved [30]. Contrary to all other industrial by-products rice husk ash must be produced out of the raw agricultural waste which is husk. The quality of the ash is hugely affected by its method of production. To transform this ash into an active pozzolanic material, certain controlled requirement of production and processing techniques must be taken after, which are yet to be completely comprehended and developed. Table 2 below shows the chemical and physical properties of rice husk ash by various researchers.

Table 2: Chemical properties of RHA.

Components Gastaldini

et. al. [31] Kunchariyakun et. al. [32] Huang et. al. [33] Raman et. al. [34] Dakroury and Gasser [35] Chindaprasirt et al. [36] Silica (SiO2) 93.54 92.80 91.56 89.87 87.20 93.20 Aluminium oxide (A12O3) 0.52 0.15 0.19 0.14 0.15 0.40 Iron oxide (Fe2O3) 0.20 0.17 0.17 0.94 0.16 0.10 Calcium oxide (CaO) 0.79 0.70 1.07 0.49 0.55 1.10 Magnesium oxide (MgO) 0.49 0.77 0.65 - 0.35 0.10 Sodium oxide (Na2O) 0.03 0.08 0.16 0.25 1.12 0.10 Potassium oxide (K2O) 1.65 3.35 3.76 2.16 3.68 1.30 Sulfur oxide (SO3) 0.05 - 0.47 - 0.24 0.90 LOI 2.32 - - 4.81 8.55 3.70

5.1 CONCRETE WORKABILITY

Concrete is notable as a heterogeneous blend of concrete, water and aggregates. The admixtures might be added in concrete with a specific amount to upgrade the properties. In its simplest form, concrete is a mixture of paste and aggregates. Different materials are added, for example, fly ash, rice husk, and admixture to acquire concrete of desired properties. The aspect of the concrete is controlled by the quality of the paste. The key to accomplishing a strong, durable concrete rests in the meticulous proportioning, blending and compacting of the ingredients. The outcome by various researches [37, 38, 39] on the impact of RHA on the workability of

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concrete concluded that concrete continuously wind up impractical as the percent replacement of concrete with RHA rises unless water-reducing admixtures are utilized. The slump and compacting factor workability’s were both measured and showed a decrease in value with an increase in RHA. Hence, more water is necessary to make the concrete workable with increase in RHA.

5.2 SUPERPLASTIZER (SP)

Superplasticizers are utilized as dispersants to keep away from particle segregation (rock, coarse and fine sands), and to enhance the flow attributes (rheology) of suspensions, for example, in concrete applications. Their addition to concrete or mortar enables the reduction of the water to cement ratio, not influencing the workability of the mixture, and enables the creation of self-consolidating concrete and high-performance concrete. This impact definitely enhances the performance of the hardening fresh paste. The strength of concrete was increases when the water cement ratio decreases. Nonetheless, due to a lack of understanding of their working mechanisms, it results in certain cases cement-superplasticizer incompatibilities [40].

5.3 COMPRESSIVE STRENGTH

Compressive strength is normally considered as very important properties of concrete and a major indicator of general quality control. Variables impacting the quality of concrete incorporate the sorts and quality of materials, the mixture proportion, the construction methods, the curing condition, and the test technique. Oversaw a research of the incorporation of rice husk ash as cement replacement and uncover the results [41]. After standard curing, the compressive strength of concrete specimen was determined at 7, 14 and 28 days. Based on the studies conducted by ZaimeeAzuar (1995) the use of RHA will increase the concrete strength of concrete compared to ordinary concrete after 7 days of age. At the age of 7 days the compressive strength is lower than the normal concrete because

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the RHA is slowly reactive and the rate of reactions is increasing over time [40].In investigating the incorporation of RHA in concrete, nearly all researchers included compressive strength as one of the parameters. In distinction to the investigation some of these researchers, the aspect affecting RHA blended concrete are water-cement ratio, the curing duration and RHA replacement level. At water-cement ratios of 0.30, 0.32 and 0.34 the compressive strength of RHA blended concrete up to 20% increased at the curing ranges of 7, 28 and 90 days [42,43,44]. Concrete incorporating RHA, without regard of other constituent’s percentage, by as much as 10% by weight, results in development of strength comparable to samples without RHA, irrespective of the water-binder ratio used. The analysis by varied researchers is shown in Table 3.

Table 3: Optimal RHA replacement for strength improvement.

Researchers % RHA Water-binder Ratio Remarks

Gastaldini et. al. 20 0.35 -

Mahmud et. al. 20 0.25 High strength high

performance concrete

Ganesan et. al. 15 - -

Kartini et. al. 20 0.68 Superplasticiser added

to the mix

Habeeb and Mahmud 10 0.53 -

6.0 CONCLUSIONS

The enhancement of new concrete properties resulting from this mixture of pozzolanic reactions is well understood. About 500 million tons of paddies is produced worldwide every year, from which 200 million tons of rice ash can be extracted.

Cement production is associated with large energy consumption with replacement of cement produces energy saving. Portland cement production is also associated with the release of carbon dioxide, which is a major source of global warming, and the use of Portland cement with substitutes for emission of carbon dioxide emissions. Each cement plant is released at an average of 1 tons of carbon dioxide to the atmosphere to produce 1 ton of liquid. RHA typically replaces 30% cement and therefore,

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the average replacement of 1 ton 25% will result in a reduction of 0.25 tons of carbon dioxide.

The use of RHA in cement mixtures is expected to increase the strength and durability of concrete such as reduced bleeding and separation, reduce the permeability of chloride ions, reducing hydration heat, lower permeability, less deficiencies, better early development, enhanced resistance to cracking, higher compression strength, lighter colors and increase the resistance to steel in steel structures.

RHA is rich in amorphous silica and the loss in ignition is relatively high based on other studies and increases RHA reactions. The permeability of the characteristics with varying differences and the milling time of rice husk ash were investigated. The composition of the chemical composition of rice husk ash with a better difference increases the density of concrete comparing to normal concrete.

ACKNOWLEDGEMENT

The authors would like to express gratitude to concrete technology class, Dr Shahiron Shahidan authors lecture, Faculty Civil engineering and environment and University Tun Hussein Onn Malaysia for the given supports towards this paper.

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