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Effects of rice husk ash prepared from charcoal-powered incinerator on the strength and durability properties of concrete
Construction and Building Materials 196 (2019) 386–394
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effects of rice husk ash prepared from charcoal-powered incinerator on the strength and durability properties of concrete Festus A. Olutoge a, Peter A. Adesina b,⇑ a b
Department of Civil and Environmental Engineering, The University of West Indies, St Augustine, Trinidad and Tobago Civil and Industrial Engineering Dept., University of Liverpool, UK
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The properties of the concrete were
compromised with the inclusion of the ash. The effect of the ash on concrete durability was significant especially at higher replacement levels. Despite limitations, the method yields ash of a quality which is applicable in structural concrete. The incineration method being cheap and accessible encourages rice husk use in construction. A modification of the incineration method in future research could yield better quality ash.
a r t i c l e
i n f o
Article history: Received 12 March 2018 Received in revised form 13 November 2018 Accepted 16 November 2018
Keywords: Rice husk Rice husk ash (RHA) Charcoal-powered incinerator RHA-concrete Compressive strength Split-tensile strength Saturated water absorption Apparent porosity Bulk density
a b s t r a c t The use of rice husk ash (RHA) as alternative to conventional cement in concrete, mortar and masonry requires that the ash be of high quality if improved properties are to be benefited. High quality RHA are obtainable when rice husk from which the ash is derived is pre-treated before combustion and incinerated under controlled temperature. However, these methods with which high quality RHA is achievable are not only expensive but also difficult to access especially in rural areas where rice husk remains in abundance. To overcome these limitations, and encourage the increasing utilization of RHA, this research employs a cheap and accessible charcoal-powered incinerator to produce RHA, and thereafter investigate the effects of the RHA on the strength and durability properties of concrete. For this purpose, conventional cement was replaced with RHA at 0%, 5%, 7.5%, 10%, 12.5% and 15% by weight of the binder to produce RHA-Concrete. Strength and durability tests were conducted on the RHA-Concrete. Results from the tests show that replacing conventional cement with the RHA reduced both the compressive and splittensile strength of the resulting concrete. The saturated water absorption and apparent porosity were found to increase while the bulk density was found to reduce especially at higher content of the RHA. Though the quality of the RHA produced may not have been the best, the ash when incorporated in concrete yielded RHA-Concrete with properties applicable as structural concrete. The incineration method being cheap and accessible especially to rural dwellers encourages the increasing utilisation of rice husk in construction. Ó 2018 Elsevier Ltd. All rights reserved.
⇑ Corresponding author. E-mail address: [email protected] (P.A. Adesina). https://doi.org/10.1016/j.conbuildmat.2018.11.138 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
F.A. Olutoge, P.A. Adesina / Construction and Building Materials 196 (2019) 386–394
1. Introduction The production of cement which emit CO2 in quantities posing great hazard to the environment has necessitated the use of alternative materials. Various agricultural and industrial waste such as fly ash, silica fume, blast furnace slag, RHA, corn cob ash, etc. have been considered alternative supplementary cementitious materials blended in cement and incorporated in concrete to improve properties and as a way of mitigating the threat these wastes pose to the environment. When these supplementary cementitious materials, also known as pozzolans, are blended in suitable quantities with cement, they react with lime liberated during cement hydration which is in itself deleterious, to produce beneficial strength giving calcium-silicate-hydrate (C-S-H) [1,2]. RHA, metakaolin, and silica fume are regarded as highly reactive pozzolans especially because of their extremely high surface area. RHA in particular is a supplementary cementitious material with great potential due to its wide availability and its high pozzolanicity. It is derived from combusting rice husk, an agricultural waste material derived from rice paddy produced in millions of tons. During rice milling, rice husk accounts for about 20–22% of the weight of the rice paddy [3]. This husk which contains 75% organic volatile matter produces about 25% of the weight of the husk as ash during combustion [4]. Cook [5] reported that 200 kg of ash was realised from 1000 kg of rice husk burnt. Globally, about 120 million tonnes of rice husk are said to be possibly available for pozzolana production on an annual basis, meaning there are about 24 million tonnes of RHA that can be potentially realised from the husk yearly [6]. This rice husk is largely disposed of by uncontrolled burning since it is considered of low nutritional value. Whereas, research has demonstrated that replacing cement with high quality RHA at suitable quantities yields benefits such as increased compressive strength, decreased porosity, improved resistance to sulphate attack, acid attack and chloride penetration. RHA therefore stands an alternative material which can reduce the amount of Ordinary Portland cement (OPC) used in concrete. Many efforts had been put into blending of RHA with Portland cement at different percentages by previous researchers. RHA cement were initially recommended mainly for mortar and rendering applications [7]. However, Malhotra and Mehta [8] reported that substituting cement with finer RHA resulted in concrete with increased compressive strength while it also lowers water absorption at higher substitution amounts through enhanced particle packing. This is similar to findings reported by Nagrale, Hajare and Modak [6]; Mauro et al., [9] which stated that water absorption of concrete was lowered at higher substitution of cement with RHA and this was attributed to the higher fineness of the RHA. The RHA was however not found to result in a significant influence in the split-tensile strength of the resulting concrete, though it increased the compressive strength as RHA reacts with the calcium hydroxide liberated during the hydration of cement to form secondary cementitious compounds. RHA-Concrete and mortar were again found to exhibit higher compressive strength than the conventional mortar and concrete by [10]. Replacing conventional cement with RHA was found to improve the tensile and compressive strength including the durability properties of concrete [11– 13]. The effects of silica fume and RHA on the strength of heavy weight concrete investigated by Sakr [14] showed that the pozzolans gave higher strength than OPC at 28 days of curing. Investigating the impact of RHA average particle size on the properties of concrete, finer RHA was found to result in a concrete with higher strength when compared with the coarser RHA at 28 days of curing, while the particle size made no significant difference at earlier days of curing [15]. A study conducted on Brazilian RHA and rice straw ash (RSA) by Cordeiro, Filho and Fairbairn [16] indicated that grinding increases the pozzolanicity of RHA produced by uncon-
387
trolled combustion. Reducing the particle size of the RHA was anticipated to improve the pozzolanic reactivity by reducing the adverse effect of the high-carbon content and by making the material more homogenous. This is similar to the claims of Rukzon and Chindaprasirt [17]. It was reported that the high strength exhibited by the RHA, RSA concrete enables the production of blocks with good bearing strength applicable in a rural area. They showed that combining RHA and RSA with lime gave a weak cementitious material applicable in the stabilization of laterite. RHA has been found to improve the properties of concrete, however, some findings show that replacing conventional cement with RHA brought some deleterious effect on the properties of the resulting concrete. Krishna, Sandeep and Mini [18] studied the behaviour of concrete with partial replacement of cement with RHA. Conventional cement was replaced with RHA at 5%, 10%, 15% and 20% by weight of the binder and compressive strength test was conducted after 7, 14 and 28 days of curing. The compressive strength of the RHA-Concrete with 10% RHA content was found as the optimum. At 28 days, in comparison with the conventional concrete, the concrete with 5% and 10% RHA content resulted in 8% reduction, while those with 15% and 20% RHA resulted in 34.81% and 40.62% reduction in strength respectively. However, the split tensile strength of the concrete with 5% and 10% RHA were higher than the conventional concrete. Water absorption was also found to increase with the RHA content. Research conducted by Essien [19] showed that open heap combustion of rice husk from Enyong Creek, Akwa Ibom, Nigeria gave RHA with 94.47% total silica and 2.11% loss on ignition. This RHA when incorporated in concrete did not yield increase in compressive strength for all concrete made at the different mix proportions and curing days. Kartini et al. [20], investigated the effect of the high silica content of RHA on the strength and durability properties of high strength concrete of target strength 60 and 70 N/mm2. RHA was considered an alternative to the expensive silica fume in reducing the liberated heat associated with high strength concrete. The higher percentage of amorphous silica in RHA was expected to serve as a filler which will yield denser concrete with higher strength and durability. Amorphous RHA used in this research was produced by incinerating rice husk for 24 h in a ferrocement furnace and thereafter left to cool for another 24 h before it was fetched for grinding. Conventional cement was then replaced with RHA at 10%-50% by weight of the binder. Results from the investigation showed that replacing cement with RHA resulted in a reduction in strength. However, the RHA-concrete with 10% of RHA gave a strength which is higher than the target strength at 28-days and beyond. The reduction in strength was attributed to lesser amount of cement which is needed to produce the primary cementitious compound responsible for cement strengthening, since the replacement was done at higher amounts. Though the pozzolanic reaction of RHA produces secondary cementitious compounds, this was not enough to compensate the reduced primary compounds. At higher replacement of RHA with cement, chloride resistant was however found to increase while the water absorption was found to reduce. To get the best out of RHA-Concrete, RHA of high quality is needed. Nehdi, Duquette and El Damatty [21], De Sensale [13] reported that the composition and reactivity of RHA is influenced by the origin and incineration method of the rice husk. The particular method used to incinerate rice husk influences the carbon content and also the reactiveness of the silica in the resulting ash. Several incineration methods exist for RHA production. According to Nagrale, Hajare and Modak [6], they include open air burning, and controlled burning: fluidized bed and Torbed technologies used for the industrial production of RHA. While open air (uncontrolled) burning converts the silica in the ash into crystalline form and results in a less reactive ash with high carbon content, controlled incineration gives a more reactive ash with lower
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carbon content which is very suitable to be applied in limepozzolana mixes and also for Portland cement replacement [22]. For the ash to be more reactive, there is need to keep it in a noncrystalline state, and this has been reported achievable at an ideal temperature between 500 °C and 700 °C [22]. Change from amorphous to crystalline silica in RHA starts at 800 °C, and at 900 °C, the transition to crystalline silica is completed [23]. Mehta [24] reported that to produce totally amorphous silica from rice husk, the combustion temperature should either be maintained at a temperature below 500 °C under oxidizing conditions for prolonged duration or at a temperature up to 680 °C. A temperature of up to 900 °C is also allowed provided the combustion will be for a duration of less than an hour and grinding of the ash to suitable particle size will be made to last for about 30 min. Salas et al., [25] chemically treated rice husk prior to incineration to enhance its reactivity. The pre-treated RHA was found to offer better mechanical and durability properties in comparison to the ordinary RHA when used in concrete. This better performance was attributed to the high content of amorphous silica in the pretreated RHA. It is evident that RHA produced in controlled combustion gives good quality, however, rice husk incinerated as fuel in brick kiln has also been reported as of good pozzolanic activity and of similar chemical composition to that produced from controlled combustion, since the incineration in the kiln occurs at a high temperature which is within 550 °C to 750 °C [22]. Nehdi, Duquette and El Damatty [21] reported that rice husk incinerated at temperatures of 750 °C and 830 °C also gave reactive RHA. Irrespective of the incineration method adopted, the RHA must meet the minimum requirement of loss on ignition (LOI-less than 6% by ASTM C618 [26]) in order to avoid coloration in concrete and also reduce the anti-surfactant effect brought about by the unburnt carbon in concrete. Though quality RHA with low LOI are effectively achieved with controlled incineration methods sometimes powered by liquified natural gas, these methods are costly and may be difficult to access especially in rural areas where rice husk is in abundance. Since these methods through which quality RHA is derived are both costly and inaccessible to communities with abundance of RHA, it is imperative to develop a cheap and readily available method to drive the utilisation of rice husk in construction. 2. Research objective This research which is part of a larger research conducted by Adesina [27] is focused at preparing RHA through a cheap and accessible charcoal-powered incinerator and then investigating how applicable the RHA produced could be in concrete. To achieve this aim, concrete made by partially replacing conventional cement with the RHA was subjected to strength and durability tests to determine its properties. The properties tested include compressive strength, split-tensile strength, water absorption, apparent porosity and bulk density. The overall contribution aimed in this research is to drive the utilisation of rice husk in construction, by
presenting the performance of the RHA prepared from charcoalpowered incinerator in concrete. This research also expands the scope of existing research literature on the application of RHA as an alternative to conventional cement in construction. 3. Experimental program The effects of RHA produced in an incinerator powered with charcoal on the strength and durability properties of concrete is the focus of this research. RHA was used to partially replace cement at 5%, 7.5%, 10%, 12.5% and 15% replacement levels while the conventional concrete has 0% content of RHA. The concrete from which the test specimens were cast was in the ratio 1:2:4, and with water to binder ratio of 0.65. Table 1 summarises the proportion of the constituent materials that makes up the concrete at each replacement level. The strength properties for which the specimens were tested include compressive strength and split tensile strength while the durability indicating properties include bulk density, saturated water absorption and apparent porosity. 108 concrete cubes were cast to test the compressive strength of the concrete at 7, 14, 21, 28, 56 and 90 days while 18 concrete cylinders were cast for the split tensile strength at 28 days. The durability tests were conducted on concrete cubes after 28 days of curing. 3.1. Materials RHA: The rice husk from which the RHA was made was collected from rice mill at Bodija market, Ibadan, Nigeria. Particularly the rice paddy from which the rice husk was derived was locally grown at Ido local government, Oyo State, Nigeria. The chemical composition of typical RHA prepared by open air combustion is as presented in Table 2. Cement: The cement used in this research was Portland Limestone Cement CEM II Grade 42.5R. The chemical composition of the cement used is as presented in Table 3. The designation R given to the cement shows that it is a high early strength cement. Water: The water used was clean drinking water suitable for concrete mixing and free from organic matter, visible impurities or any other substances harmful to cement hydration and concrete durability. Aggregates: The fine aggregate used was clean sharp river sand, free of clay, loam, dirt and any organic or chemical matter and sourced from a local supplier. It was mainly sand passing through 4.75 mm mesh of British Standard test sieves. The coarse aggregate was granite with maximum size of 16 mm. The sieve analyses of the fine and coarse aggregate are as presented in Table 4 and Table 5 respectively. 3.2. Methods RHA Processing: The rice husk was processed based on the design principles of Allen [28] which was adopted in research by Abalaka [29]. About 20% RHA was derived from the rice husk incin-
Table 1 Mix proportion of RHA-Concrete. Replacement (%)
W/B
0 5 7.5 10 12.5 15
0.65 0.65 0.65 0.65 0.65 0.65
Cementitious materials (kg/m3) Cement
RHA
342.86 325.71 317.14 308.57 291.43 291.43
0 17.14 25.71 34.29 51.43 51.43
Water (kg/m3)
Natural Sand (kg/m3)
Coarse Aggregate (kg/m3)
222.86 222.86 222.86 222.86 222.86 222.86
685.71 685.71 685.71 685.71 685.71 685.71
1371.42 1371.42 1371.42 1371.42 1371.42 1371.42
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F.A. Olutoge, P.A. Adesina / Construction and Building Materials 196 (2019) 386–394 Table 2 Typical chemical composition of RHA produced using different incinerator. Chemical analysis
[29]a (%)
[20]b (%)
[26] (%)
SiO2 Al2O3 Fe2O3 SiO2 + Al2O3 + Fe2O3 CaO MgO SO3 Na2O K2O LOI
95.41 0.00 0.82 96.23 0.00 1.24 0.07 0.22 1.65 0.77% (800 °C for 6 min), 3.88% (1050 °C for 2 h)
96.7 1.01 0.05 97.67 0.49 0.19 – 0.26 0.91 4.81
_ _ _ 70.0 _ 5.0 5.0
Table 4 Sieve analysis of the sharp sand used (mass of sample = 500 g).
6.0c
a The RHA was produced in an open-air combustion using an incinerator powered with charcoal as solid fuel and at a temperature of 758 °C. b The RHA was produced by allowing it to burn in a basket inside a furnace for 24 h and allowed to cool for another 24 h. c Class F pozzolan containing up to 12% loss on ignition may be approved if either performance records or laboratory test results are made available.
Sieve size (mm)
Mass of soil retained (g)
Percentage of soil retained (%)
Cumulative percentage of soil retained (%)
Percentage passing (%)
6.70 4.75 2.36 1.18 0.85 0.60 0.425 Pan Total
0.0 4.3 19.3 59.3 52.5 89.2 96.7 178.2 499.4
0.0 0.8 3.9 11.9 10.5 17.9 19.3 35.7 100.0
0.0 0.8 4.7 16.6 27.1 45.0 64.3 100.0 _
100.0 99.2 95.3 83.4 72.9 55.0 35.7 _ _
Table 5 Sieve analysis of the granite used (mass of sample = 2350 g).
erated. This agrees with the amount of ash derived from burning rice husk as reported by Karim et al., [30]. The incinerator used for producing the ash was powered using charcoal as solid fuel. The incinerator (shown in Fig. 1a) consists of two concentric steel mesh baskets with the larger basket being 625 mm in diameter and 950 mm high while the inner basket is of 250 mm diameter and 750 mm high. Both baskets were conveniently made from steel mesh. Dry rice husks were placed in the large basket until a layer of 150 mm is formed. The smaller basket was then placed and space between the baskets was filled with the husk. The baskets were supported with some pieces of concrete cube 150 mm above the ground to let air get underneath them. It was ensured that the ground upon which the concrete cubes support for the baskets were placed is level and free of plant growth for the same reason of giving room for sufficient air needed for the combustion. Red hot charcoal contained in an expanded steel metal basket was then placed in the smaller inner basket and allowed to burn out. Air was frequently blown into the basket containing the red-hot charcoal for the first 5 h for the needed air to be supplied. About ten minutes after ignition, flames were seen to come out of the incinerator. Then after about an hour, the side surfaces smouldered and turned black as shown in Fig. 1b. This smouldering spread slowly up the sides and the bed height reduces. The incineration took 24hr before the ash emerged with a whitish grey colour and some few black char (Fig. 1c). Another 24 h was allowed for the ash to cool in air. The ash derived was thereafter sieved with a BS sieve of aperture size 425 lm. This was to ensure the removal of the unwanted black char in the ash. After sieving, the RHA was then ground using a commercial ball mill to increase its fineness and reactivity as encouraged by Cordeiro, Filho and Fairbairn [16]; Malhotra and Mehta [8]. This method of RHA processing was adopted because it is affordable, accessible and also to check how the RHA produced under this condition affects the properties of concrete. Aggregate grading: Aggregate grading (sieve analysis) was conducted as specified by BS EN 933–1:2012 [31]. 500 g and 2350 g of fine aggregate and coarse aggregate respectively were used as the quantity of each test portion. Concrete matrix and specimen preparation: RHA was thoroughly mixed with cement in the planned proportion until the mix
Sieve size (mm)
Mass of soil retained (g)
Percentage of soil retained (%)
Cumulative percentage of soil retained (%)
Percentage passing (%)
16.0 13.2 6.70 4.75 2.36 Pan Total
0.0 459.1 1798.6 88.7 4.5 0.0 2350
0.0 19.5 76.5 3.8 0.2 0.0 100.0
0.0 19.5 96.1 99.8 100.0 100.0 _
100.0 80.5 3.9 0.2 0.0 _ _
became homogenous before it was used in concrete. The concrete specimens were cast and cured in accordance to BS EN 12390–2 [32]. Casting was done in three in layers for effective compaction. Compaction was manually done using a compaction rod which was used to apply not less than 35 S uniformly on each layer. After casting, test specimens were left in the mould for about 24 h at a temperature of (25 ± 5)°C. Curing in water for the intended number of days was done after specimens were removed from the mould. The mix ratio for the concrete as earlier mentioned was 1:2:4 (binder: fine aggregate: coarse aggregate) while the water to binder ratio was 0.65. Batching of the concrete materials was by weight. Moulds of size 100 100 100 mm were used to cast the concrete cubes while moulds of 200 100 mm were used for the concrete cylinders. Fig. 2 shows some of the concrete cubes after curing. Compressive and split-tensile strength test: Compressive strength test was done in accordance to (BS EN 12390–3 [33]. Specimens were loaded to failure at a constant loading rate of 6KN/sec in a compression testing machine which conforms to BS EN 12390–4 [34]. The cube specimens were centrally positioned in the machine so that the load applied is perpendicular to the direction of casting. The failure mode of the specimens was assessed to ensure they were rightly loaded. The split tensile strength test was done in conformity to BS EN 12390–6 [35]. For the test, specimens were placed centrally in the testing machine such that the upper and lower platen are parallel during loading. Compressive force is then applied to a narrow region along the length of the cylindrical specimen thereby causing an orthogonal tensile force which makes the specimen fail in tension. Bulk density, Saturated Water Absorption and Apparent Porosity tests on hardened concrete: For these tests which gives an indication of how durable the specimens could be, the mass of each specimen was first determined after which the specimens were dried in an oven at a temperature of 100 °C for not less than 24 h. During dry-
Table 3 Chemical composition of typical 42.5R Portland cement [37] Oxide
SiO2
TiO2
Al2O3
Fe2O3
SO3
CaO
MgO
Na2O
K2O
MnO
V2O5
BaO
LOI
Composition (%)
17.16
0.37
5.53
2.60
2.48
68.47
1.43
0.54
0.32
0.14
0.02
0.05
1.00
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F.A. Olutoge, P.A. Adesina / Construction and Building Materials 196 (2019) 386–394
Fig. 1. (a) Baskets used for combusting rice husk (b) Smouldered Surfaces of husk during incineration (c) Rice Hush Ash after incineration.
determined. This apparent mass was designated D. The surface moisture of the specimens was then removed with a towel and the mass of the specimen determined. The soaked, boiled, surface-dried mass was designated C. Calculations to obtain the test parameters were done using equations 1 to 3 below. These tests are patterned after the specifications of ASTM C642 [36].
Saturated Water Absorption; % ¼ ððB AÞ=AÞ 100ð1Þ Bulk density; dry ¼ ðA=ðC DÞÞ:qð2Þ Apparent porosity ðv oidsÞ; % ¼ ððC AÞ=ðC DÞÞ 100 Fig. 2. Concrete cubes after curing.
ð3Þ
where: q is Density of water. 4. Experimental result and discussions
ing, the specimens mass was intermittently checked until total dryness was achieved. The specimens were considered dry when the difference between any two successive values is less than 0.5% of the lowest value obtained. The specimens were thereafter allowed to cool in dry air and the dry mass designated as A. After this, the specimens were immersed in water for not less than 60 h and until two successive mass of surface-dried sample taken at intervals of 24 h show an increase which is less than 0.5% of the larger value. The specimens were again surface-dried and the mass was determined. The final surface-dry mass after immersion was designated B. The specimen processed as described above were then suspended in a receptacle at a minimum of 1/4 of the container’s height from the bottom. The specimens were thereafter boiled for 5 h in water after which they were allowed to cool by natural loss of heat for not less than 24 h. After this, the specimens were suspended by a wire and the apparent mass in water was
4.1. Effect of RHA on the compressive strength of concrete Tables 6a–6c shows the mass and compressive strength of the RHA-Concrete specimens at the different curing days. It was observed that all RHA-Concrete specimens had lower compressive strengths in comparison with the conventional concrete. However, except at 14-day, more than 70% of the conventional concrete strength was recorded at each curing day for the 5% RHAConcrete. At 7.5% replacement level, more than 60% of the conventional concrete strength was recorded at each curing day. At 10%, 12.5% and 15% replacement levels, more than 62%, 51% and 47% of the conventional were achieved respectively at all curing days with the exception of the 14-day. This is as indicated in Fig. 3. It was also observed as indicated in Fig. 3 that though the highest strength for the RHA-Concrete specimens was achieved at the 5%
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F.A. Olutoge, P.A. Adesina / Construction and Building Materials 196 (2019) 386–394 Table 6a Compressive strength of RHA-Concrete at 7 and 14 days.
7 days
Specimen
Mass (Kg)
Average Mass (Kg)
Strength (MPa)
Average strength (MPa)
0%
2.53 2.52 2.54 2.48 2.47 2.50 2.60 2.49 2.59 2.60 2.48 2.52 2.52 2.52 2.50 2.50 2.42 2.47
2.53
25.0 25.4 27.5 20.5 21.9 20.6 16.2 16.2 16.6 17.8 19.3 17.8 15.1 15.1 15.4 11.4 13.5 12.6
26.0
5%
7.5%
10%
12.5%
15%
2.48 2.56
2.53
2.51 2.46
Mass (Kg) 14 days
21.0 16.3
18.3
15.2 12.5
2.61 2.44 2.61 2.56 2.63 2.54 2.48 2.54 2.66 2.57 2.58 2.58 2.60 2.46 2.56 2.53 2.52 2.64
Average Mass (Kg) 2.55
2.58
2.56
2.58
2.54
2.56
Strength (MPa) 38.4 40.8 38.4 23.2 20.8 25.6 21.9 18.3 17.4 17.1 19.6 19.4 16.2 17.8 17.8 17.1 19.7 16.7
Average Strength (MPa) 39.2
23.2
19.2
18.7
17.3
17.8
Table 6b Compressive strength of RHA-Concrete at 21 and 28 days.
21 days
Specimen
Mass (Kg)
Average Mass (Kg)
Strength (MPa)
Average strength (MPa)
0%
2.62
2.60
41.5
40.9
5%
7.5%
10%
12.5%
15%
2.64 2.54 2.56 2.55 2.50 2.50 2.52 2.51 2.58 2.54 2.49 2.46 2.49 2.48 2.48 2.50 2.38
2.54
2.51
2.54
2.48
2.45
40.2 41.0 28.1 27.7 31.1 26.0 25.8 22.8 25.3 25.1 25.9 26.3 26.3 22.1 22.8 23.8 22.8
28 days
Mass (Kg)
Average mass (Kg)
Strength (MPa)
2.48
2.49
45.6
2.49 2.50 2.56 2.57 2.56 2.50 2.50 2.60 2.57 2.54 2.58 2.49 2.53 2.51 2.62 2.53 2.56
29.0
24.9
25.4
24.9 23.1
2.56
2.53
2.56
2.51
2.57
38.9 38.6 31.5 33.8 38.6 34.1 32.6 32.6 25.9 25.9 26.5 25.5 20.8 21.7 24.3 26.9 25.4
Average Strength (MPa)
41.0 34.6
33.1
26.1
22.7
25.5
Table 6c Compressive strength of RHA-Concrete at 56 and 90 days.
56 days
Specimen
Mass (Kg)
Average Mass (Kg)
Strength (MPa)
Average strength (MPa)
0%
2.54
2.61
45.9
45.9
5%
7.5%
10%
12.5%
15%
2.72 2.56 2.64 2.52 2.52 2.58 2.58 2.58 2.54 2.56 2.54 2.53 2.56 2.52 2.52 2.42 2.50
2.56
2.58
2.55
2.54
2.48
48.7 43.2 37.8 36.5 34.3 34.3 34.3 36.5 29.0 30.3 30.3 23.4 25.4 26.5 26.8 24.7 24.8
36.2
35.0
29.9
25.1
25.4
90 days
Mass (Kg)
Average Mass (Kg)
Strength (MPa)
AverageStrength (MPa)
2.60
2.62
55.4
53.6
2.72 2.54 2.70 2.58 2.66 2.58 2.52 2.62 2.53 2.52 2.62 2.52 2.56 2.39 2.58 2.54 2.46
2.65
2.57
2.56
2.49
2.53
51.6 53.8 43.7 44.5 40.9 39.0 35.0 35.0 35.2 34.3 38.7 29.9 26.5 26.6 22.2 26.8 26.7
43.0
36.3
36.1
27.7
25.2
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Compressive Strength (N/mm2)
120
Relative Strength
100 80 60 40 20 0 7 days (%)
14 days (%) 21 days (%) 28 days (%) 56 days (%) 90 days (%) 0%
5%
7.50%
10%
12.50%
60 50 40 30 0% 5% 7.5% 10.0% 12.5%
20 10 0 7 days
14days
21days
28 days
15%
Fig. 3. Relative comparison of RHA-Concrete to conventional concrete.
15.0%
12.5%
10.0%
15.0% 56 days 7.5%
5%
90 days 0%
Fig. 4. Graphical representation of the strength development of RHA-concrete.
replacement level, the difference in the specimens is largely within 25% of the highest value. Reduction in strength as a result of replacing cement with RHA was also reported by Krishna, Sandeep and Mini [18]; Kartini et al.[20]. The reduction in strength could be as a result of the lesser amount of the cement needed to produce the primary cementitious compound responsible for cement strengthening in the mixes. Though the pozzolanic reaction of RHA could have produced secondary cementitious compounds, this might not be enough to compensate the reduced primary compounds. The fact that the reduction in strength generally increases as the percentage of conventional cement replaced increases supports this claim. The reduction in strength could also be as a result of the incineration method used. The RHA used in this research was processed in open air burning which might have increased the quantity of unburnt carbon in the pozzolan. When large quantities of carbon are present in the ash, the strength of the resulting concrete is greatly compromised [6]. In comparison, all specimens achieved about one-half and double of the 7-day strength at 28day and 90-day respectively. Despite the reduction in strength, at 28 days and beyond, the RHA-Concrete at all replacement levels generally gave compressive strength values higher than the 25 N/mm2 typical of structural concrete made with mix ratio 1:2:4, which indicates they can be used as structural concrete. This implies that the replacement of cement with RHA up to 15% as shown by this research did not sufficiently compromise the compressive strength of the resulting concrete to the point that that it debars it from being used as structural concrete. Taking 25 N/mm2 as the intended design strength, while structures which makes use of the conventional concrete can be fully loaded after 7 days of casting, structures involving the application of the RHA-Concrete can only be fully loaded after 28 days. 4.2. Effect of RHA on the strength development of concrete Fig. 4 presents the strength development of the RHA-Concrete over the curing days. It was observed that the strength of all specimens generally increased from one curing day to another which indicates they all experienced strength development. While the RHA-Concrete specimens generally achieved more than 80% of the 90-day strength at 28-day, the conventional concrete only achieved 76.6%. At 7-day, all specimens including the conventional were largely at similar percentage of their respective 90-day strength while at 14 and 21-day, the conventional largely gave higher strength than the RHA-Concrete specimens. It can therefore be said that RHA-Concrete developed its strength slower before 28day and thereafter experienced a faster strength development up till the 90-day. As indicated by Cook, Pamat and Paul [38]; Pushpakumara and Silva [39], this late strength development can be because the RHA needs a considerable longer time to react with the lime liberated during cement hydration to form cementitious
compounds. The generally insignificant difference in the percentage of the 90-day strength achieved at the 7-day by both the RHA-Concrete and the conventional concrete could be as a result of the Portland Limestone cement used which is a high early strength cement. This likely did not make the slower reaction of the pozzolana have effect on the early strength development of the RHA-Concrete. As indicated in Fig. 4, the strength development of the specimens did not follow a linear path. 4.3. Effect of RHA on the Mass and density of concrete Since the cement from which the concrete was made is denser than RHA, it was important to investigate the impact of replacing cement with RHA on the mass and density of the resulting concrete. It was observed at all curing days with the exception of the 28-day and 56-day, that replacing RHA with cement generally led to a reduction in the mass and consequently the density of the resulting concrete. The greatest reduction was generally observed in the RHA-Concrete at the 12.5% and 15% replacement levels. This reduction in density which occurred as RHA content increased was also reported by Omondi [40]. The reduction in strength especially at the 12.5% and 15% replacement levels can be attributed to this reduction in density. 4.4. Effect of RHA on the split-tensile strength of concrete Table 7 presents the split-tensile strength of the RHA-Concrete specimens. All RHA-Concrete specimens were found to exhibit lower split-tensile strength in comparison to the conventional concrete. However, the specimens at 5%, 7.5% and 10% replacement level where within 75% of the conventional, while the remaining two specimens were within 67% of the conventional concrete strength. This reduction in split-tensile strength does not agree with the findings of Ganesan, Rajagopal and Thangavel [11]; Giannotti da Silva, Liborio and Helene [12] who reported higher tensile strength when RHA is used to replace conventional cement. The reduction in tensile strength can also be attributed to the lesser amount of the cement present in the RHA-Concrete. It was also observed that though the highest value resulted at the 5% replacement level, no significant difference existed between the splittensile strength of the RHA-Concrete specimens. Comparing the compressive strength of the concrete specimens to their splittensile strength, Table 8 presents the percentage ratio of the split-tensile strength to the compressive strength of the specimens. It was observed that specimens at 5% and 7.5% replacement level resulted in lower ratios compared to the conventional while specimens at 10%, 12.5% and 15% replacement levels resulted in higher ratios. It is important to note that higher ratios observed in the
F.A. Olutoge, P.A. Adesina / Construction and Building Materials 196 (2019) 386–394 Table 7 Split tensile strength of RHA-Concrete at 28 days Strength. Specimen
Mass (Kg)
Average Mass (Kg)
Strength (MPa)
Average strength (MPa)
0%
4.18 4.45 4.31 4.24 4.26 4.24 4.23 4.27 4.38 4.30 4.28 4.22 4.25 4.25 4.30 4.18 4.22 4.25
4.31
3.28 3.22 3.15 2.52 2.56 2.47 2.52 2.34 2.52 2.42 2.43 2.40 2.32 2.28 2.15 2.20 2.15 2.16
3.22
5%
7.5%
10%
12.5%
15%
4.25
4.29
4.27
4.27
4.22
2.51
2.46
2.42
5. Conclusions 2.17
Compressive Strength (A)
Split-Tensile Strength (B)
B/A (%)
41 34.6 33.1 26.1 22.7 25.5
3.22 2.51 2.46 2.42 2.25 2.17
7.9 7.3 7.4 9.3 9.9 8.5
mentioned specimens does not indicate higher split-tensile strength, the higher ratios were largely as a result of the lower compressive strength of the specimens. 4.5. Effect of RHA on the bulk density, water absorption and apparent porosity of RHA-Concrete Table 9 presents the bulk density, saturated water absorption and apparent porosity of the RHA-Concrete specimens which indicates their durability. The bulk densities of the RHA-Concrete were within 0.1% difference from the conventional concrete at replacement levels 5%, 7.5% and 10% which indicates no significant difference in bulk density resulted from replacing cement with RHA at these levels. However, at replacement levels 12.5% and 15% a more significant reduction in bulk density was observed. The saturated water absorption and apparent porosity of the RHA-Concrete were found higher when compared with the conventional concrete. The increase in water absorption can be related to the apparent porosity behaviour. Since the apparent porosity of the RHA-Concrete increased, it is expected that they will absorb more water as they are more porous. The increase in water absorption reported by
Table 9 Bulk density, Saturated water absorption, and Apparent porosity of RHA-Concrete. Specimen
0 5% 7.5% 10% 12.5% 15%
Abalaka [29] was attributed to the hygroscopic nature of RHA as a material. The specimen at 5% replacement level had the least water absorption and apparent porosity among the RHA-Concrete specimen. Specimens at replacement levels 5%, 7.5% and 10% had water absorption which differs from that of the conventional concrete within 5% while specimens at replacement levels 12.5% and 15% differs from that of the conventional by 20% and 23% respectively. Similar trend was observed in apparent porosity. This behaviour indicates that the effect of replacing conventional cement with RHA on bulk density, water absorption and apparent porosity is more significant at higher replacement levels. The saturated water absorption and apparent porosity were also observed to generally increase as the bulk density decreased over the replacement levels.
2.25
Table 8 Relationship between Compressive and Split tensile strength of RHA-Concrete.
0% 5% 7.5% 10% 12.5% 15%
393
Property Bulk density (kg/m3)
Saturated water absorption (%)
Apparent porosity (%)
2590 2610 2580 2610 2520 2480
3.38 3.42 3.53 3.56 4.08 4.16
8.75 8.95 9.12 9.29 9.87 10.32
The effects of RHA produced in charcoal-powered incinerator on the strength and durability properties of concrete was the focus of this research. Concrete specimens were cast replacing conventional cement with the RHA and tests were conducted to investigate the effect of this replacement on the properties of the resulting concrete. The following are the conclusions derived from this investigation: (1) Replacing conventional cement with the RHA prepared from charcoal-powered incinerator in this research resulted in decreased compressive strength. However, the strength of the RHA-Concrete at all replacement level was not compromised to the point that they did not meet the strength requirement of normal strength structural concrete. At 28 days, the reduction in compressive strength was 15.6%, 19.3%, 36.4%, 44.8% and 37.8% at replacement levels 5%, 7.5%, 10%, 12.5% and 15% respectively. (2) The strength development of the RHA-Concrete differs from that of the conventional concrete. While the RHA-Concrete achieved more of its 90-day strength after 28 days of curing, the conventional concrete achieved a higher percentage of its 90-day strength at 14 and 21 days of curing. The RHAConcrete was found to exhibit late strength development. (3) The split-tensile strength of the RHA-Concrete were found lower in comparison to the conventional concrete. However, the reduction was found within 25% for RHA-Concrete at replacement level 5%, 7.5% and 10% and within 33% at 12.5% and 15% replacement level. Also, no significant difference was found in the split-tensile strength of the RHAConcrete at the different replacement level. (4) At replacement levels 5%, 7.5% and 10%, no significant difference in bulk density was found between the RHA-Concrete and the conventional concrete. A more significant difference was however observed at replacement levels 12.5% and 15%. RHA-Concrete were found more porous and therefore absorb more water in comparison with the conventional concrete. Replacing conventional cement with RHA had a more significant effect on bulk density, water absorption and apparent porosity at higher replacement levels. (5) RHA made from charcoal-powered incinerator as shown in this research did not result in improved performance when incorporated in concrete. This is not perfectly consistent with findings from Abalaka [29] who employed similar incineration method. Certain factors which include the quality of the rice husk used, the fuel used for the combustion, the duration of the combustion, the processing of the ash after combustion, etc. could be responsible for the disparity observed in this research.
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(6) Though the method of preparing RHA presented in this research did not yield best quality RHA when compared to the industrial production methods found in the literature, the method however yields ash of a quality which is applicable in structural concrete. In addition, the method is cheap and can be easily achieved by people in areas with abundance of risk husk. This encourages increasing utilisation of rice husk in construction. (7) Future research effort will be needed to modify the charcoalpowered incineration method for improved performance. This must however be without compromising on delivering a cheap and accessible incinerator to people in places where rice husk remains in abundance.
Conflict of interest I acknowledge there is no conflict of interest in the research and the manuscript presented. Acknowledgement A special acknowledgement is due to Engr. Dr. Ogundipe, a scholar of the Department of Agricultural and Environmental Engineering, University of Ibadan who provided immense support towards the completion of the research experiment. References [1] V.N. Dwivedia, N.P. Singh, S.S. Das, N.B. Singh, A new pozzolanic material for cement industry: bamboo leaf ash, Int. J. Phys. Sci. 1 (2006) 106–111. [2] C.S. Poon, S.C. Kou, L. Lam, Compressive strength, chloride diffusivity and pore structure of High Performance metakaolin and silica fume concrete, Constr. Build. Mater. 20 (2006) 858–865. [3] A. Muthadhi, S. Kothandaraman, Optimum production conditions for reactive rice husk ash, Mater. Struct. 43 (2010) 1303–1315. [4] S.M. Agus, Utilization of uncontrolled burnt rice husk ash in soil improvement, Dimens. Tek. Sipil. 4 (2002) 100–105. [5] D.J. Cook, Using rice husk for making cement-like materials, Appropr. Technol. 6 (1980) 9–11. [6] S.D. Nagrale, H. Hajare, P.R. Modak, Utilization of rice husk ash, Int. J. Eng. Res. 2 (2012) 1–5. [7] A. Dass, R. Mohau, Prospects and problems in the production of cementitious materials from rice husk, in: Proc. UNIDO/ESCAPIRCTT Work. Rice-Husk Ash Cem, 1979: pp. 49–56. [8] V.M. Malhotra, P.K. Mehta, Pozzolanic and Cementitious Materials, London Taylor Fr. 2004. [9] M.T. Mauro, C.A.R. Da Silva, L.A. Jorge, M.B. Barbosa, The possibility of adding rice husk ash to the concrete, 2007. [10] I. Wada, T. Kawano, N. Mokotomaeda, Strength properties of concrete incorporating highly reactive rice-husk ash, Trans. Japan Concr. Inst. 21 (2000) 57–62. [11] K. Ganesan, K. Rajagopal, K. Thangavel, Rice husk ash blended cement: assessment of optimal level of replacement for strength and permeability properties of concrete, Constr. Build. Mater. 22 (2008) 1675–1683. [12] F. Giannotti da Silva, J.B.L. Liborio, P. Helene, Improvement of physical and chemical properties of concrete with Brazilian silica rice husk (SRH), Rev. Ing. Construcción 23 (2008) 18–25. [13] G.R. De Sensale, Strength development of concrete with rice-husk ash, Cem. Concr. Compos. 28 (2006) 158–160.
[14] K. Sakr, Effects of silica fume and rice husk ash on the properties of heavy weight concrete, J. Mater. Civ. Eng. 18 (2006) 367–376. [15] G.A. Habeeb, M.M. Fayyadh, Rice husk ash concrete: the effect of RHA average particle size on mechanical properties and drying shrinkage, Aust. J. Basic Appl. Sci. 3 (2009) 1616–1622. [16] G.C. Cordeiro, R.D.T. Filho, E.D.R. Fairbairn, Use of ultrafine saw dust ash with high-carbon content as pozzolan in high performance concrete, Mater. Struct. 42 (2009) 983–992. [17] S. Rukzon, P. Chindaprasirt, Strength and chloride resistance of blended portland cement mortar containing palm oil fuel ash and fly ash, Int. J. Miner. Metall. Mater. 16 (2009) 475–481. [18] N.K. Krishna, S. Sandeep, K.M. Mini, Study on concrete with partial replacement of cement by rice husk ash Study on concrete with partial replacement of cement, IOP Conf. Ser. Mater. Sci. Eng. (2016), https://doi.org/ 10.1088/1757-899X/149/1/012109. [19] O.E. Essien, A comparative characterization of rice husk ash and Portland cement mixtures for construction purposes, in: Proc. 7th Int. Conf. Niger. Inst. Agric. Eng., 2006: pp. 404–414. [20] K. Kartini, N. Nazierah, M.Z. Zaidahtulakmal, G. Siti Aisyah, Effects of silica in rice husk ash (RHA) in producing high strength concrete, Int. J. Eng. Technol. 2 (2012) 1951–1956. [21] M. Nehdi, J. Duquette, A. El Damatty, Performance of rice husk ash produced using a new technology as a mineral admixture in concrete. cement and concrete research, Cem. Concr. Res. 33 (2003) 1203–1210. [22] S. De Silva, R. Uduweriya, Investigation of the properties of concrete containing rice-husk-ash, Trans. Inst. Eng. Sri Lankax. (2011) 71–79. [23] L. Howlett, Rice husk ash market study, Http//Www.Berr.Gov.Uk/Files/ File15138.Pdf. (accessed 20.02.13), 2003 [24] P.K. Mehta, The chemistry and technology of cements made from rice husks ash, in: Proc. UNIDO/ESCAP/RCTT Work. RiceHusk Ash Cem. Peshawar, Pakistan, 1979: p. 1113–122. [25] A. Salas, S. Delvasto, R.M. Gutierrez, D. Lange, Comparison of Two Processes for Treating Rice Husk Ash for Use in High Performance Concrete, Cem. Concr. Res. 39 (2009) 773–778. [26] ASTM C618, Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Concrete 1, n.d. [27] P.A. Adesina, Effects of RHA and Lime contents on the strength and durability properties of Concrete, University of Ibadan, 2014. [28] M.L. Allen, The manufacture of a cement extender from rice-husks using a basket-burner., Http://Www.Journeyto Forever.Org/Farm_library/RiceHusks, 2008. [29] A.E. Abalaka, Effects of method of incineration on rice husk ash blended concrete, J. Environ. Technol. 5 (2012) 34–47. [30] M.R. Karim, M. Jamil, F.C. Lai, M.N. Islam, Strength of mortar and concrete as influenced by rice husk ash: a review, World Appl. Sci. 19 (2012) 1501–1513, https://doi.org/10.5829/idosi.wasj.2012.19.10.533. [31] BS EN 933-1, Fine and Coarse Aggregate Grading, 2012. [32] BS EN 12390-2, Testing hardened concrete; Part 2: Making and curing specimens for strength test, 2009. [33] BS EN 12390-3, Testing hardened concrete; Part3: Compressive strength of test specimens, 2009. [34] BS EN 12390-4, Testing hardened concrete Part 4: Compressive strengthSpecification for testing machines, 2000. [35] BS EN 12390-6, Testing hardened concrete Part 6: Tensile splitting strength of test specimens, 2009. [36] ASTM, Standard Test Method for Density, Absorption, and Voids in Hardened Concrete, 2006. [37] M. Joel, I.D. Mbapuun, Comparative analysis of the properties of concrete produced with portland limestone cement (PLC) grade 32. 5N AND 42. 5R for use in rigid pavement work, Glob. J. Eng. Res. 15 (2016) 17–25. [38] D.J. Cook, R.P. Pamat, B.K. Paul, Rice husk ash-lime-cement mixes for use in masonry units, Build. Environ. 12 (1977) 281–288. [39] B.H.J. Pushpakumara, G.H.M.J.S. De Silva, Characteristics of masonry blocks manufactured with rice husk ash (RHA) and lime, Engineer XXXXV (2012) 1– 10. [40] O.B. Omondi, Investigation into the Cementitous Properties of a Mixture of Rice Husks Ash with Building Lime, University of Nairobi, 2013.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effects of rice husk ash prepared from charcoal-powered incinerator on the strength and durability properties of concrete Festus A. Olutoge a, Peter A. Adesina b,⇑ a b
Department of Civil and Environmental Engineering, The University of West Indies, St Augustine, Trinidad and Tobago Civil and Industrial Engineering Dept., University of Liverpool, UK
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The properties of the concrete were
compromised with the inclusion of the ash. The effect of the ash on concrete durability was significant especially at higher replacement levels. Despite limitations, the method yields ash of a quality which is applicable in structural concrete. The incineration method being cheap and accessible encourages rice husk use in construction. A modification of the incineration method in future research could yield better quality ash.
a r t i c l e
i n f o
Article history: Received 12 March 2018 Received in revised form 13 November 2018 Accepted 16 November 2018
Keywords: Rice husk Rice husk ash (RHA) Charcoal-powered incinerator RHA-concrete Compressive strength Split-tensile strength Saturated water absorption Apparent porosity Bulk density
a b s t r a c t The use of rice husk ash (RHA) as alternative to conventional cement in concrete, mortar and masonry requires that the ash be of high quality if improved properties are to be benefited. High quality RHA are obtainable when rice husk from which the ash is derived is pre-treated before combustion and incinerated under controlled temperature. However, these methods with which high quality RHA is achievable are not only expensive but also difficult to access especially in rural areas where rice husk remains in abundance. To overcome these limitations, and encourage the increasing utilization of RHA, this research employs a cheap and accessible charcoal-powered incinerator to produce RHA, and thereafter investigate the effects of the RHA on the strength and durability properties of concrete. For this purpose, conventional cement was replaced with RHA at 0%, 5%, 7.5%, 10%, 12.5% and 15% by weight of the binder to produce RHA-Concrete. Strength and durability tests were conducted on the RHA-Concrete. Results from the tests show that replacing conventional cement with the RHA reduced both the compressive and splittensile strength of the resulting concrete. The saturated water absorption and apparent porosity were found to increase while the bulk density was found to reduce especially at higher content of the RHA. Though the quality of the RHA produced may not have been the best, the ash when incorporated in concrete yielded RHA-Concrete with properties applicable as structural concrete. The incineration method being cheap and accessible especially to rural dwellers encourages the increasing utilisation of rice husk in construction. Ó 2018 Elsevier Ltd. All rights reserved.
⇑ Corresponding author. E-mail address: [email protected] (P.A. Adesina). https://doi.org/10.1016/j.conbuildmat.2018.11.138 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
F.A. Olutoge, P.A. Adesina / Construction and Building Materials 196 (2019) 386–394
1. Introduction The production of cement which emit CO2 in quantities posing great hazard to the environment has necessitated the use of alternative materials. Various agricultural and industrial waste such as fly ash, silica fume, blast furnace slag, RHA, corn cob ash, etc. have been considered alternative supplementary cementitious materials blended in cement and incorporated in concrete to improve properties and as a way of mitigating the threat these wastes pose to the environment. When these supplementary cementitious materials, also known as pozzolans, are blended in suitable quantities with cement, they react with lime liberated during cement hydration which is in itself deleterious, to produce beneficial strength giving calcium-silicate-hydrate (C-S-H) [1,2]. RHA, metakaolin, and silica fume are regarded as highly reactive pozzolans especially because of their extremely high surface area. RHA in particular is a supplementary cementitious material with great potential due to its wide availability and its high pozzolanicity. It is derived from combusting rice husk, an agricultural waste material derived from rice paddy produced in millions of tons. During rice milling, rice husk accounts for about 20–22% of the weight of the rice paddy [3]. This husk which contains 75% organic volatile matter produces about 25% of the weight of the husk as ash during combustion [4]. Cook [5] reported that 200 kg of ash was realised from 1000 kg of rice husk burnt. Globally, about 120 million tonnes of rice husk are said to be possibly available for pozzolana production on an annual basis, meaning there are about 24 million tonnes of RHA that can be potentially realised from the husk yearly [6]. This rice husk is largely disposed of by uncontrolled burning since it is considered of low nutritional value. Whereas, research has demonstrated that replacing cement with high quality RHA at suitable quantities yields benefits such as increased compressive strength, decreased porosity, improved resistance to sulphate attack, acid attack and chloride penetration. RHA therefore stands an alternative material which can reduce the amount of Ordinary Portland cement (OPC) used in concrete. Many efforts had been put into blending of RHA with Portland cement at different percentages by previous researchers. RHA cement were initially recommended mainly for mortar and rendering applications [7]. However, Malhotra and Mehta [8] reported that substituting cement with finer RHA resulted in concrete with increased compressive strength while it also lowers water absorption at higher substitution amounts through enhanced particle packing. This is similar to findings reported by Nagrale, Hajare and Modak [6]; Mauro et al., [9] which stated that water absorption of concrete was lowered at higher substitution of cement with RHA and this was attributed to the higher fineness of the RHA. The RHA was however not found to result in a significant influence in the split-tensile strength of the resulting concrete, though it increased the compressive strength as RHA reacts with the calcium hydroxide liberated during the hydration of cement to form secondary cementitious compounds. RHA-Concrete and mortar were again found to exhibit higher compressive strength than the conventional mortar and concrete by [10]. Replacing conventional cement with RHA was found to improve the tensile and compressive strength including the durability properties of concrete [11– 13]. The effects of silica fume and RHA on the strength of heavy weight concrete investigated by Sakr [14] showed that the pozzolans gave higher strength than OPC at 28 days of curing. Investigating the impact of RHA average particle size on the properties of concrete, finer RHA was found to result in a concrete with higher strength when compared with the coarser RHA at 28 days of curing, while the particle size made no significant difference at earlier days of curing [15]. A study conducted on Brazilian RHA and rice straw ash (RSA) by Cordeiro, Filho and Fairbairn [16] indicated that grinding increases the pozzolanicity of RHA produced by uncon-
387
trolled combustion. Reducing the particle size of the RHA was anticipated to improve the pozzolanic reactivity by reducing the adverse effect of the high-carbon content and by making the material more homogenous. This is similar to the claims of Rukzon and Chindaprasirt [17]. It was reported that the high strength exhibited by the RHA, RSA concrete enables the production of blocks with good bearing strength applicable in a rural area. They showed that combining RHA and RSA with lime gave a weak cementitious material applicable in the stabilization of laterite. RHA has been found to improve the properties of concrete, however, some findings show that replacing conventional cement with RHA brought some deleterious effect on the properties of the resulting concrete. Krishna, Sandeep and Mini [18] studied the behaviour of concrete with partial replacement of cement with RHA. Conventional cement was replaced with RHA at 5%, 10%, 15% and 20% by weight of the binder and compressive strength test was conducted after 7, 14 and 28 days of curing. The compressive strength of the RHA-Concrete with 10% RHA content was found as the optimum. At 28 days, in comparison with the conventional concrete, the concrete with 5% and 10% RHA content resulted in 8% reduction, while those with 15% and 20% RHA resulted in 34.81% and 40.62% reduction in strength respectively. However, the split tensile strength of the concrete with 5% and 10% RHA were higher than the conventional concrete. Water absorption was also found to increase with the RHA content. Research conducted by Essien [19] showed that open heap combustion of rice husk from Enyong Creek, Akwa Ibom, Nigeria gave RHA with 94.47% total silica and 2.11% loss on ignition. This RHA when incorporated in concrete did not yield increase in compressive strength for all concrete made at the different mix proportions and curing days. Kartini et al. [20], investigated the effect of the high silica content of RHA on the strength and durability properties of high strength concrete of target strength 60 and 70 N/mm2. RHA was considered an alternative to the expensive silica fume in reducing the liberated heat associated with high strength concrete. The higher percentage of amorphous silica in RHA was expected to serve as a filler which will yield denser concrete with higher strength and durability. Amorphous RHA used in this research was produced by incinerating rice husk for 24 h in a ferrocement furnace and thereafter left to cool for another 24 h before it was fetched for grinding. Conventional cement was then replaced with RHA at 10%-50% by weight of the binder. Results from the investigation showed that replacing cement with RHA resulted in a reduction in strength. However, the RHA-concrete with 10% of RHA gave a strength which is higher than the target strength at 28-days and beyond. The reduction in strength was attributed to lesser amount of cement which is needed to produce the primary cementitious compound responsible for cement strengthening, since the replacement was done at higher amounts. Though the pozzolanic reaction of RHA produces secondary cementitious compounds, this was not enough to compensate the reduced primary compounds. At higher replacement of RHA with cement, chloride resistant was however found to increase while the water absorption was found to reduce. To get the best out of RHA-Concrete, RHA of high quality is needed. Nehdi, Duquette and El Damatty [21], De Sensale [13] reported that the composition and reactivity of RHA is influenced by the origin and incineration method of the rice husk. The particular method used to incinerate rice husk influences the carbon content and also the reactiveness of the silica in the resulting ash. Several incineration methods exist for RHA production. According to Nagrale, Hajare and Modak [6], they include open air burning, and controlled burning: fluidized bed and Torbed technologies used for the industrial production of RHA. While open air (uncontrolled) burning converts the silica in the ash into crystalline form and results in a less reactive ash with high carbon content, controlled incineration gives a more reactive ash with lower
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carbon content which is very suitable to be applied in limepozzolana mixes and also for Portland cement replacement [22]. For the ash to be more reactive, there is need to keep it in a noncrystalline state, and this has been reported achievable at an ideal temperature between 500 °C and 700 °C [22]. Change from amorphous to crystalline silica in RHA starts at 800 °C, and at 900 °C, the transition to crystalline silica is completed [23]. Mehta [24] reported that to produce totally amorphous silica from rice husk, the combustion temperature should either be maintained at a temperature below 500 °C under oxidizing conditions for prolonged duration or at a temperature up to 680 °C. A temperature of up to 900 °C is also allowed provided the combustion will be for a duration of less than an hour and grinding of the ash to suitable particle size will be made to last for about 30 min. Salas et al., [25] chemically treated rice husk prior to incineration to enhance its reactivity. The pre-treated RHA was found to offer better mechanical and durability properties in comparison to the ordinary RHA when used in concrete. This better performance was attributed to the high content of amorphous silica in the pretreated RHA. It is evident that RHA produced in controlled combustion gives good quality, however, rice husk incinerated as fuel in brick kiln has also been reported as of good pozzolanic activity and of similar chemical composition to that produced from controlled combustion, since the incineration in the kiln occurs at a high temperature which is within 550 °C to 750 °C [22]. Nehdi, Duquette and El Damatty [21] reported that rice husk incinerated at temperatures of 750 °C and 830 °C also gave reactive RHA. Irrespective of the incineration method adopted, the RHA must meet the minimum requirement of loss on ignition (LOI-less than 6% by ASTM C618 [26]) in order to avoid coloration in concrete and also reduce the anti-surfactant effect brought about by the unburnt carbon in concrete. Though quality RHA with low LOI are effectively achieved with controlled incineration methods sometimes powered by liquified natural gas, these methods are costly and may be difficult to access especially in rural areas where rice husk is in abundance. Since these methods through which quality RHA is derived are both costly and inaccessible to communities with abundance of RHA, it is imperative to develop a cheap and readily available method to drive the utilisation of rice husk in construction. 2. Research objective This research which is part of a larger research conducted by Adesina [27] is focused at preparing RHA through a cheap and accessible charcoal-powered incinerator and then investigating how applicable the RHA produced could be in concrete. To achieve this aim, concrete made by partially replacing conventional cement with the RHA was subjected to strength and durability tests to determine its properties. The properties tested include compressive strength, split-tensile strength, water absorption, apparent porosity and bulk density. The overall contribution aimed in this research is to drive the utilisation of rice husk in construction, by
presenting the performance of the RHA prepared from charcoalpowered incinerator in concrete. This research also expands the scope of existing research literature on the application of RHA as an alternative to conventional cement in construction. 3. Experimental program The effects of RHA produced in an incinerator powered with charcoal on the strength and durability properties of concrete is the focus of this research. RHA was used to partially replace cement at 5%, 7.5%, 10%, 12.5% and 15% replacement levels while the conventional concrete has 0% content of RHA. The concrete from which the test specimens were cast was in the ratio 1:2:4, and with water to binder ratio of 0.65. Table 1 summarises the proportion of the constituent materials that makes up the concrete at each replacement level. The strength properties for which the specimens were tested include compressive strength and split tensile strength while the durability indicating properties include bulk density, saturated water absorption and apparent porosity. 108 concrete cubes were cast to test the compressive strength of the concrete at 7, 14, 21, 28, 56 and 90 days while 18 concrete cylinders were cast for the split tensile strength at 28 days. The durability tests were conducted on concrete cubes after 28 days of curing. 3.1. Materials RHA: The rice husk from which the RHA was made was collected from rice mill at Bodija market, Ibadan, Nigeria. Particularly the rice paddy from which the rice husk was derived was locally grown at Ido local government, Oyo State, Nigeria. The chemical composition of typical RHA prepared by open air combustion is as presented in Table 2. Cement: The cement used in this research was Portland Limestone Cement CEM II Grade 42.5R. The chemical composition of the cement used is as presented in Table 3. The designation R given to the cement shows that it is a high early strength cement. Water: The water used was clean drinking water suitable for concrete mixing and free from organic matter, visible impurities or any other substances harmful to cement hydration and concrete durability. Aggregates: The fine aggregate used was clean sharp river sand, free of clay, loam, dirt and any organic or chemical matter and sourced from a local supplier. It was mainly sand passing through 4.75 mm mesh of British Standard test sieves. The coarse aggregate was granite with maximum size of 16 mm. The sieve analyses of the fine and coarse aggregate are as presented in Table 4 and Table 5 respectively. 3.2. Methods RHA Processing: The rice husk was processed based on the design principles of Allen [28] which was adopted in research by Abalaka [29]. About 20% RHA was derived from the rice husk incin-
Table 1 Mix proportion of RHA-Concrete. Replacement (%)
W/B
0 5 7.5 10 12.5 15
0.65 0.65 0.65 0.65 0.65 0.65
Cementitious materials (kg/m3) Cement
RHA
342.86 325.71 317.14 308.57 291.43 291.43
0 17.14 25.71 34.29 51.43 51.43
Water (kg/m3)
Natural Sand (kg/m3)
Coarse Aggregate (kg/m3)
222.86 222.86 222.86 222.86 222.86 222.86
685.71 685.71 685.71 685.71 685.71 685.71
1371.42 1371.42 1371.42 1371.42 1371.42 1371.42
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F.A. Olutoge, P.A. Adesina / Construction and Building Materials 196 (2019) 386–394 Table 2 Typical chemical composition of RHA produced using different incinerator. Chemical analysis
[29]a (%)
[20]b (%)
[26] (%)
SiO2 Al2O3 Fe2O3 SiO2 + Al2O3 + Fe2O3 CaO MgO SO3 Na2O K2O LOI
95.41 0.00 0.82 96.23 0.00 1.24 0.07 0.22 1.65 0.77% (800 °C for 6 min), 3.88% (1050 °C for 2 h)
96.7 1.01 0.05 97.67 0.49 0.19 – 0.26 0.91 4.81
_ _ _ 70.0 _ 5.0 5.0
Table 4 Sieve analysis of the sharp sand used (mass of sample = 500 g).
6.0c
a The RHA was produced in an open-air combustion using an incinerator powered with charcoal as solid fuel and at a temperature of 758 °C. b The RHA was produced by allowing it to burn in a basket inside a furnace for 24 h and allowed to cool for another 24 h. c Class F pozzolan containing up to 12% loss on ignition may be approved if either performance records or laboratory test results are made available.
Sieve size (mm)
Mass of soil retained (g)
Percentage of soil retained (%)
Cumulative percentage of soil retained (%)
Percentage passing (%)
6.70 4.75 2.36 1.18 0.85 0.60 0.425 Pan Total
0.0 4.3 19.3 59.3 52.5 89.2 96.7 178.2 499.4
0.0 0.8 3.9 11.9 10.5 17.9 19.3 35.7 100.0
0.0 0.8 4.7 16.6 27.1 45.0 64.3 100.0 _
100.0 99.2 95.3 83.4 72.9 55.0 35.7 _ _
Table 5 Sieve analysis of the granite used (mass of sample = 2350 g).
erated. This agrees with the amount of ash derived from burning rice husk as reported by Karim et al., [30]. The incinerator used for producing the ash was powered using charcoal as solid fuel. The incinerator (shown in Fig. 1a) consists of two concentric steel mesh baskets with the larger basket being 625 mm in diameter and 950 mm high while the inner basket is of 250 mm diameter and 750 mm high. Both baskets were conveniently made from steel mesh. Dry rice husks were placed in the large basket until a layer of 150 mm is formed. The smaller basket was then placed and space between the baskets was filled with the husk. The baskets were supported with some pieces of concrete cube 150 mm above the ground to let air get underneath them. It was ensured that the ground upon which the concrete cubes support for the baskets were placed is level and free of plant growth for the same reason of giving room for sufficient air needed for the combustion. Red hot charcoal contained in an expanded steel metal basket was then placed in the smaller inner basket and allowed to burn out. Air was frequently blown into the basket containing the red-hot charcoal for the first 5 h for the needed air to be supplied. About ten minutes after ignition, flames were seen to come out of the incinerator. Then after about an hour, the side surfaces smouldered and turned black as shown in Fig. 1b. This smouldering spread slowly up the sides and the bed height reduces. The incineration took 24hr before the ash emerged with a whitish grey colour and some few black char (Fig. 1c). Another 24 h was allowed for the ash to cool in air. The ash derived was thereafter sieved with a BS sieve of aperture size 425 lm. This was to ensure the removal of the unwanted black char in the ash. After sieving, the RHA was then ground using a commercial ball mill to increase its fineness and reactivity as encouraged by Cordeiro, Filho and Fairbairn [16]; Malhotra and Mehta [8]. This method of RHA processing was adopted because it is affordable, accessible and also to check how the RHA produced under this condition affects the properties of concrete. Aggregate grading: Aggregate grading (sieve analysis) was conducted as specified by BS EN 933–1:2012 [31]. 500 g and 2350 g of fine aggregate and coarse aggregate respectively were used as the quantity of each test portion. Concrete matrix and specimen preparation: RHA was thoroughly mixed with cement in the planned proportion until the mix
Sieve size (mm)
Mass of soil retained (g)
Percentage of soil retained (%)
Cumulative percentage of soil retained (%)
Percentage passing (%)
16.0 13.2 6.70 4.75 2.36 Pan Total
0.0 459.1 1798.6 88.7 4.5 0.0 2350
0.0 19.5 76.5 3.8 0.2 0.0 100.0
0.0 19.5 96.1 99.8 100.0 100.0 _
100.0 80.5 3.9 0.2 0.0 _ _
became homogenous before it was used in concrete. The concrete specimens were cast and cured in accordance to BS EN 12390–2 [32]. Casting was done in three in layers for effective compaction. Compaction was manually done using a compaction rod which was used to apply not less than 35 S uniformly on each layer. After casting, test specimens were left in the mould for about 24 h at a temperature of (25 ± 5)°C. Curing in water for the intended number of days was done after specimens were removed from the mould. The mix ratio for the concrete as earlier mentioned was 1:2:4 (binder: fine aggregate: coarse aggregate) while the water to binder ratio was 0.65. Batching of the concrete materials was by weight. Moulds of size 100 100 100 mm were used to cast the concrete cubes while moulds of 200 100 mm were used for the concrete cylinders. Fig. 2 shows some of the concrete cubes after curing. Compressive and split-tensile strength test: Compressive strength test was done in accordance to (BS EN 12390–3 [33]. Specimens were loaded to failure at a constant loading rate of 6KN/sec in a compression testing machine which conforms to BS EN 12390–4 [34]. The cube specimens were centrally positioned in the machine so that the load applied is perpendicular to the direction of casting. The failure mode of the specimens was assessed to ensure they were rightly loaded. The split tensile strength test was done in conformity to BS EN 12390–6 [35]. For the test, specimens were placed centrally in the testing machine such that the upper and lower platen are parallel during loading. Compressive force is then applied to a narrow region along the length of the cylindrical specimen thereby causing an orthogonal tensile force which makes the specimen fail in tension. Bulk density, Saturated Water Absorption and Apparent Porosity tests on hardened concrete: For these tests which gives an indication of how durable the specimens could be, the mass of each specimen was first determined after which the specimens were dried in an oven at a temperature of 100 °C for not less than 24 h. During dry-
Table 3 Chemical composition of typical 42.5R Portland cement [37] Oxide
SiO2
TiO2
Al2O3
Fe2O3
SO3
CaO
MgO
Na2O
K2O
MnO
V2O5
BaO
LOI
Composition (%)
17.16
0.37
5.53
2.60
2.48
68.47
1.43
0.54
0.32
0.14
0.02
0.05
1.00
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Fig. 1. (a) Baskets used for combusting rice husk (b) Smouldered Surfaces of husk during incineration (c) Rice Hush Ash after incineration.
determined. This apparent mass was designated D. The surface moisture of the specimens was then removed with a towel and the mass of the specimen determined. The soaked, boiled, surface-dried mass was designated C. Calculations to obtain the test parameters were done using equations 1 to 3 below. These tests are patterned after the specifications of ASTM C642 [36].
Saturated Water Absorption; % ¼ ððB AÞ=AÞ 100ð1Þ Bulk density; dry ¼ ðA=ðC DÞÞ:qð2Þ Apparent porosity ðv oidsÞ; % ¼ ððC AÞ=ðC DÞÞ 100 Fig. 2. Concrete cubes after curing.
ð3Þ
where: q is Density of water. 4. Experimental result and discussions
ing, the specimens mass was intermittently checked until total dryness was achieved. The specimens were considered dry when the difference between any two successive values is less than 0.5% of the lowest value obtained. The specimens were thereafter allowed to cool in dry air and the dry mass designated as A. After this, the specimens were immersed in water for not less than 60 h and until two successive mass of surface-dried sample taken at intervals of 24 h show an increase which is less than 0.5% of the larger value. The specimens were again surface-dried and the mass was determined. The final surface-dry mass after immersion was designated B. The specimen processed as described above were then suspended in a receptacle at a minimum of 1/4 of the container’s height from the bottom. The specimens were thereafter boiled for 5 h in water after which they were allowed to cool by natural loss of heat for not less than 24 h. After this, the specimens were suspended by a wire and the apparent mass in water was
4.1. Effect of RHA on the compressive strength of concrete Tables 6a–6c shows the mass and compressive strength of the RHA-Concrete specimens at the different curing days. It was observed that all RHA-Concrete specimens had lower compressive strengths in comparison with the conventional concrete. However, except at 14-day, more than 70% of the conventional concrete strength was recorded at each curing day for the 5% RHAConcrete. At 7.5% replacement level, more than 60% of the conventional concrete strength was recorded at each curing day. At 10%, 12.5% and 15% replacement levels, more than 62%, 51% and 47% of the conventional were achieved respectively at all curing days with the exception of the 14-day. This is as indicated in Fig. 3. It was also observed as indicated in Fig. 3 that though the highest strength for the RHA-Concrete specimens was achieved at the 5%
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F.A. Olutoge, P.A. Adesina / Construction and Building Materials 196 (2019) 386–394 Table 6a Compressive strength of RHA-Concrete at 7 and 14 days.
7 days
Specimen
Mass (Kg)
Average Mass (Kg)
Strength (MPa)
Average strength (MPa)
0%
2.53 2.52 2.54 2.48 2.47 2.50 2.60 2.49 2.59 2.60 2.48 2.52 2.52 2.52 2.50 2.50 2.42 2.47
2.53
25.0 25.4 27.5 20.5 21.9 20.6 16.2 16.2 16.6 17.8 19.3 17.8 15.1 15.1 15.4 11.4 13.5 12.6
26.0
5%
7.5%
10%
12.5%
15%
2.48 2.56
2.53
2.51 2.46
Mass (Kg) 14 days
21.0 16.3
18.3
15.2 12.5
2.61 2.44 2.61 2.56 2.63 2.54 2.48 2.54 2.66 2.57 2.58 2.58 2.60 2.46 2.56 2.53 2.52 2.64
Average Mass (Kg) 2.55
2.58
2.56
2.58
2.54
2.56
Strength (MPa) 38.4 40.8 38.4 23.2 20.8 25.6 21.9 18.3 17.4 17.1 19.6 19.4 16.2 17.8 17.8 17.1 19.7 16.7
Average Strength (MPa) 39.2
23.2
19.2
18.7
17.3
17.8
Table 6b Compressive strength of RHA-Concrete at 21 and 28 days.
21 days
Specimen
Mass (Kg)
Average Mass (Kg)
Strength (MPa)
Average strength (MPa)
0%
2.62
2.60
41.5
40.9
5%
7.5%
10%
12.5%
15%
2.64 2.54 2.56 2.55 2.50 2.50 2.52 2.51 2.58 2.54 2.49 2.46 2.49 2.48 2.48 2.50 2.38
2.54
2.51
2.54
2.48
2.45
40.2 41.0 28.1 27.7 31.1 26.0 25.8 22.8 25.3 25.1 25.9 26.3 26.3 22.1 22.8 23.8 22.8
28 days
Mass (Kg)
Average mass (Kg)
Strength (MPa)
2.48
2.49
45.6
2.49 2.50 2.56 2.57 2.56 2.50 2.50 2.60 2.57 2.54 2.58 2.49 2.53 2.51 2.62 2.53 2.56
29.0
24.9
25.4
24.9 23.1
2.56
2.53
2.56
2.51
2.57
38.9 38.6 31.5 33.8 38.6 34.1 32.6 32.6 25.9 25.9 26.5 25.5 20.8 21.7 24.3 26.9 25.4
Average Strength (MPa)
41.0 34.6
33.1
26.1
22.7
25.5
Table 6c Compressive strength of RHA-Concrete at 56 and 90 days.
56 days
Specimen
Mass (Kg)
Average Mass (Kg)
Strength (MPa)
Average strength (MPa)
0%
2.54
2.61
45.9
45.9
5%
7.5%
10%
12.5%
15%
2.72 2.56 2.64 2.52 2.52 2.58 2.58 2.58 2.54 2.56 2.54 2.53 2.56 2.52 2.52 2.42 2.50
2.56
2.58
2.55
2.54
2.48
48.7 43.2 37.8 36.5 34.3 34.3 34.3 36.5 29.0 30.3 30.3 23.4 25.4 26.5 26.8 24.7 24.8
36.2
35.0
29.9
25.1
25.4
90 days
Mass (Kg)
Average Mass (Kg)
Strength (MPa)
AverageStrength (MPa)
2.60
2.62
55.4
53.6
2.72 2.54 2.70 2.58 2.66 2.58 2.52 2.62 2.53 2.52 2.62 2.52 2.56 2.39 2.58 2.54 2.46
2.65
2.57
2.56
2.49
2.53
51.6 53.8 43.7 44.5 40.9 39.0 35.0 35.0 35.2 34.3 38.7 29.9 26.5 26.6 22.2 26.8 26.7
43.0
36.3
36.1
27.7
25.2
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Compressive Strength (N/mm2)
120
Relative Strength
100 80 60 40 20 0 7 days (%)
14 days (%) 21 days (%) 28 days (%) 56 days (%) 90 days (%) 0%
5%
7.50%
10%
12.50%
60 50 40 30 0% 5% 7.5% 10.0% 12.5%
20 10 0 7 days
14days
21days
28 days
15%
Fig. 3. Relative comparison of RHA-Concrete to conventional concrete.
15.0%
12.5%
10.0%
15.0% 56 days 7.5%
5%
90 days 0%
Fig. 4. Graphical representation of the strength development of RHA-concrete.
replacement level, the difference in the specimens is largely within 25% of the highest value. Reduction in strength as a result of replacing cement with RHA was also reported by Krishna, Sandeep and Mini [18]; Kartini et al.[20]. The reduction in strength could be as a result of the lesser amount of the cement needed to produce the primary cementitious compound responsible for cement strengthening in the mixes. Though the pozzolanic reaction of RHA could have produced secondary cementitious compounds, this might not be enough to compensate the reduced primary compounds. The fact that the reduction in strength generally increases as the percentage of conventional cement replaced increases supports this claim. The reduction in strength could also be as a result of the incineration method used. The RHA used in this research was processed in open air burning which might have increased the quantity of unburnt carbon in the pozzolan. When large quantities of carbon are present in the ash, the strength of the resulting concrete is greatly compromised [6]. In comparison, all specimens achieved about one-half and double of the 7-day strength at 28day and 90-day respectively. Despite the reduction in strength, at 28 days and beyond, the RHA-Concrete at all replacement levels generally gave compressive strength values higher than the 25 N/mm2 typical of structural concrete made with mix ratio 1:2:4, which indicates they can be used as structural concrete. This implies that the replacement of cement with RHA up to 15% as shown by this research did not sufficiently compromise the compressive strength of the resulting concrete to the point that that it debars it from being used as structural concrete. Taking 25 N/mm2 as the intended design strength, while structures which makes use of the conventional concrete can be fully loaded after 7 days of casting, structures involving the application of the RHA-Concrete can only be fully loaded after 28 days. 4.2. Effect of RHA on the strength development of concrete Fig. 4 presents the strength development of the RHA-Concrete over the curing days. It was observed that the strength of all specimens generally increased from one curing day to another which indicates they all experienced strength development. While the RHA-Concrete specimens generally achieved more than 80% of the 90-day strength at 28-day, the conventional concrete only achieved 76.6%. At 7-day, all specimens including the conventional were largely at similar percentage of their respective 90-day strength while at 14 and 21-day, the conventional largely gave higher strength than the RHA-Concrete specimens. It can therefore be said that RHA-Concrete developed its strength slower before 28day and thereafter experienced a faster strength development up till the 90-day. As indicated by Cook, Pamat and Paul [38]; Pushpakumara and Silva [39], this late strength development can be because the RHA needs a considerable longer time to react with the lime liberated during cement hydration to form cementitious
compounds. The generally insignificant difference in the percentage of the 90-day strength achieved at the 7-day by both the RHA-Concrete and the conventional concrete could be as a result of the Portland Limestone cement used which is a high early strength cement. This likely did not make the slower reaction of the pozzolana have effect on the early strength development of the RHA-Concrete. As indicated in Fig. 4, the strength development of the specimens did not follow a linear path. 4.3. Effect of RHA on the Mass and density of concrete Since the cement from which the concrete was made is denser than RHA, it was important to investigate the impact of replacing cement with RHA on the mass and density of the resulting concrete. It was observed at all curing days with the exception of the 28-day and 56-day, that replacing RHA with cement generally led to a reduction in the mass and consequently the density of the resulting concrete. The greatest reduction was generally observed in the RHA-Concrete at the 12.5% and 15% replacement levels. This reduction in density which occurred as RHA content increased was also reported by Omondi [40]. The reduction in strength especially at the 12.5% and 15% replacement levels can be attributed to this reduction in density. 4.4. Effect of RHA on the split-tensile strength of concrete Table 7 presents the split-tensile strength of the RHA-Concrete specimens. All RHA-Concrete specimens were found to exhibit lower split-tensile strength in comparison to the conventional concrete. However, the specimens at 5%, 7.5% and 10% replacement level where within 75% of the conventional, while the remaining two specimens were within 67% of the conventional concrete strength. This reduction in split-tensile strength does not agree with the findings of Ganesan, Rajagopal and Thangavel [11]; Giannotti da Silva, Liborio and Helene [12] who reported higher tensile strength when RHA is used to replace conventional cement. The reduction in tensile strength can also be attributed to the lesser amount of the cement present in the RHA-Concrete. It was also observed that though the highest value resulted at the 5% replacement level, no significant difference existed between the splittensile strength of the RHA-Concrete specimens. Comparing the compressive strength of the concrete specimens to their splittensile strength, Table 8 presents the percentage ratio of the split-tensile strength to the compressive strength of the specimens. It was observed that specimens at 5% and 7.5% replacement level resulted in lower ratios compared to the conventional while specimens at 10%, 12.5% and 15% replacement levels resulted in higher ratios. It is important to note that higher ratios observed in the
F.A. Olutoge, P.A. Adesina / Construction and Building Materials 196 (2019) 386–394 Table 7 Split tensile strength of RHA-Concrete at 28 days Strength. Specimen
Mass (Kg)
Average Mass (Kg)
Strength (MPa)
Average strength (MPa)
0%
4.18 4.45 4.31 4.24 4.26 4.24 4.23 4.27 4.38 4.30 4.28 4.22 4.25 4.25 4.30 4.18 4.22 4.25
4.31
3.28 3.22 3.15 2.52 2.56 2.47 2.52 2.34 2.52 2.42 2.43 2.40 2.32 2.28 2.15 2.20 2.15 2.16
3.22
5%
7.5%
10%
12.5%
15%
4.25
4.29
4.27
4.27
4.22
2.51
2.46
2.42
5. Conclusions 2.17
Compressive Strength (A)
Split-Tensile Strength (B)
B/A (%)
41 34.6 33.1 26.1 22.7 25.5
3.22 2.51 2.46 2.42 2.25 2.17
7.9 7.3 7.4 9.3 9.9 8.5
mentioned specimens does not indicate higher split-tensile strength, the higher ratios were largely as a result of the lower compressive strength of the specimens. 4.5. Effect of RHA on the bulk density, water absorption and apparent porosity of RHA-Concrete Table 9 presents the bulk density, saturated water absorption and apparent porosity of the RHA-Concrete specimens which indicates their durability. The bulk densities of the RHA-Concrete were within 0.1% difference from the conventional concrete at replacement levels 5%, 7.5% and 10% which indicates no significant difference in bulk density resulted from replacing cement with RHA at these levels. However, at replacement levels 12.5% and 15% a more significant reduction in bulk density was observed. The saturated water absorption and apparent porosity of the RHA-Concrete were found higher when compared with the conventional concrete. The increase in water absorption can be related to the apparent porosity behaviour. Since the apparent porosity of the RHA-Concrete increased, it is expected that they will absorb more water as they are more porous. The increase in water absorption reported by
Table 9 Bulk density, Saturated water absorption, and Apparent porosity of RHA-Concrete. Specimen
0 5% 7.5% 10% 12.5% 15%
Abalaka [29] was attributed to the hygroscopic nature of RHA as a material. The specimen at 5% replacement level had the least water absorption and apparent porosity among the RHA-Concrete specimen. Specimens at replacement levels 5%, 7.5% and 10% had water absorption which differs from that of the conventional concrete within 5% while specimens at replacement levels 12.5% and 15% differs from that of the conventional by 20% and 23% respectively. Similar trend was observed in apparent porosity. This behaviour indicates that the effect of replacing conventional cement with RHA on bulk density, water absorption and apparent porosity is more significant at higher replacement levels. The saturated water absorption and apparent porosity were also observed to generally increase as the bulk density decreased over the replacement levels.
2.25
Table 8 Relationship between Compressive and Split tensile strength of RHA-Concrete.
0% 5% 7.5% 10% 12.5% 15%
393
Property Bulk density (kg/m3)
Saturated water absorption (%)
Apparent porosity (%)
2590 2610 2580 2610 2520 2480
3.38 3.42 3.53 3.56 4.08 4.16
8.75 8.95 9.12 9.29 9.87 10.32
The effects of RHA produced in charcoal-powered incinerator on the strength and durability properties of concrete was the focus of this research. Concrete specimens were cast replacing conventional cement with the RHA and tests were conducted to investigate the effect of this replacement on the properties of the resulting concrete. The following are the conclusions derived from this investigation: (1) Replacing conventional cement with the RHA prepared from charcoal-powered incinerator in this research resulted in decreased compressive strength. However, the strength of the RHA-Concrete at all replacement level was not compromised to the point that they did not meet the strength requirement of normal strength structural concrete. At 28 days, the reduction in compressive strength was 15.6%, 19.3%, 36.4%, 44.8% and 37.8% at replacement levels 5%, 7.5%, 10%, 12.5% and 15% respectively. (2) The strength development of the RHA-Concrete differs from that of the conventional concrete. While the RHA-Concrete achieved more of its 90-day strength after 28 days of curing, the conventional concrete achieved a higher percentage of its 90-day strength at 14 and 21 days of curing. The RHAConcrete was found to exhibit late strength development. (3) The split-tensile strength of the RHA-Concrete were found lower in comparison to the conventional concrete. However, the reduction was found within 25% for RHA-Concrete at replacement level 5%, 7.5% and 10% and within 33% at 12.5% and 15% replacement level. Also, no significant difference was found in the split-tensile strength of the RHAConcrete at the different replacement level. (4) At replacement levels 5%, 7.5% and 10%, no significant difference in bulk density was found between the RHA-Concrete and the conventional concrete. A more significant difference was however observed at replacement levels 12.5% and 15%. RHA-Concrete were found more porous and therefore absorb more water in comparison with the conventional concrete. Replacing conventional cement with RHA had a more significant effect on bulk density, water absorption and apparent porosity at higher replacement levels. (5) RHA made from charcoal-powered incinerator as shown in this research did not result in improved performance when incorporated in concrete. This is not perfectly consistent with findings from Abalaka [29] who employed similar incineration method. Certain factors which include the quality of the rice husk used, the fuel used for the combustion, the duration of the combustion, the processing of the ash after combustion, etc. could be responsible for the disparity observed in this research.
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(6) Though the method of preparing RHA presented in this research did not yield best quality RHA when compared to the industrial production methods found in the literature, the method however yields ash of a quality which is applicable in structural concrete. In addition, the method is cheap and can be easily achieved by people in areas with abundance of risk husk. This encourages increasing utilisation of rice husk in construction. (7) Future research effort will be needed to modify the charcoalpowered incineration method for improved performance. This must however be without compromising on delivering a cheap and accessible incinerator to people in places where rice husk remains in abundance.
Conflict of interest I acknowledge there is no conflict of interest in the research and the manuscript presented. Acknowledgement A special acknowledgement is due to Engr. Dr. Ogundipe, a scholar of the Department of Agricultural and Environmental Engineering, University of Ibadan who provided immense support towards the completion of the research experiment. References [1] V.N. Dwivedia, N.P. Singh, S.S. Das, N.B. Singh, A new pozzolanic material for cement industry: bamboo leaf ash, Int. J. Phys. Sci. 1 (2006) 106–111. [2] C.S. Poon, S.C. Kou, L. Lam, Compressive strength, chloride diffusivity and pore structure of High Performance metakaolin and silica fume concrete, Constr. Build. Mater. 20 (2006) 858–865. [3] A. Muthadhi, S. Kothandaraman, Optimum production conditions for reactive rice husk ash, Mater. Struct. 43 (2010) 1303–1315. [4] S.M. Agus, Utilization of uncontrolled burnt rice husk ash in soil improvement, Dimens. Tek. Sipil. 4 (2002) 100–105. [5] D.J. Cook, Using rice husk for making cement-like materials, Appropr. Technol. 6 (1980) 9–11. [6] S.D. Nagrale, H. Hajare, P.R. Modak, Utilization of rice husk ash, Int. J. Eng. Res. 2 (2012) 1–5. [7] A. Dass, R. Mohau, Prospects and problems in the production of cementitious materials from rice husk, in: Proc. UNIDO/ESCAPIRCTT Work. Rice-Husk Ash Cem, 1979: pp. 49–56. [8] V.M. Malhotra, P.K. Mehta, Pozzolanic and Cementitious Materials, London Taylor Fr. 2004. [9] M.T. Mauro, C.A.R. Da Silva, L.A. Jorge, M.B. Barbosa, The possibility of adding rice husk ash to the concrete, 2007. [10] I. Wada, T. Kawano, N. Mokotomaeda, Strength properties of concrete incorporating highly reactive rice-husk ash, Trans. Japan Concr. Inst. 21 (2000) 57–62. [11] K. Ganesan, K. Rajagopal, K. Thangavel, Rice husk ash blended cement: assessment of optimal level of replacement for strength and permeability properties of concrete, Constr. Build. Mater. 22 (2008) 1675–1683. [12] F. Giannotti da Silva, J.B.L. Liborio, P. Helene, Improvement of physical and chemical properties of concrete with Brazilian silica rice husk (SRH), Rev. Ing. Construcción 23 (2008) 18–25. [13] G.R. De Sensale, Strength development of concrete with rice-husk ash, Cem. Concr. Compos. 28 (2006) 158–160.
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