Construction and Building Materials 25 (2011) 2030–2035
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The study of using rice husk ash to produce ultra high performance concrete Nguyen Van Tuan a,⇑, Guang Ye a, Klaas van Breugel a, Alex L.A. Fraaij a, Bui Danh Dai b a b
Microlab, Faculty of Civil Engineering and Geosciences, Delft University of Technology, P.O. Box 5048, 2600 GA Delft, The Netherlands Department of Building Materials, National University of Civil Engineering, 55 Giai Phong Road, Hanoi, Viet Nam
a r t i c l e
i n f o
Article history: Received 17 April 2010 Received in revised form 16 September 2010 Accepted 13 November 2010 Available online 17 December 2010 Keywords: Compressive strength Fineness Rice husk ash Silica fume Ultra high performance concrete
a b s t r a c t The limited available resource and the high cost of silica fume (SF) in producing ultra high performance concrete (UHPC) give the motivation for searching for the substitution by other materials with similar functions, especially in developing countries. Rice husk ash (RHA), an agricultural waste, is classified as ‘‘a highly active pozzolan’’ because it possesses a very high amount of amorphous SiO2 and a large surface area. The possibility of using RHA to produce UHPC was investigated in this study. The result shows that the compressive strength of UHPC incorporating RHA, with the mean size between 3.6 lm and 9 lm, can be achieved in excess of 150 MPa with normal curing regime. The interesting point is that the effect of RHA on the development of compressive strength of UHPC is larger than that of SF. Besides, the sample incorporating the ternary blend of cement with 10% RHA and 10% SF showed better compressive strength than that of the control sample without RHA or SF. This blend proved to be the optimum combination for achieving maximum synergic effect. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Ultra high performance concrete (UHPC) refers to materials with a cement matrix and a characteristic compressive strength in excess of 150 MPa, possibly attaining 250 MPa, and containing steel fibers in order to achieve ductile behavior under tension and, if possible, to dispense with the need for passive (non-prestressed) reinforcement [1]. Because of these outstanding properties, UHPC offers new and sometimes exiting possibilities: lighter structures, larger structures, hybrid structures, new design and new products with a potential for a better economy and resource consumption than with traditional concrete, steel and other building materials [2–4]. In UHPC, silica fume (SF) with extreme fineness and high amorphous silica content plays a very important role with physical (filler, lubrication) and pozzolanic effects. However, the limited available resource and the high cost constrain its application in modern construction industry, especially in developing countries. For these reasons, it gives a motivation for searching for other materials to substitute SF with similar functions. One possibility is using rice husk ash (RHA), which is obtained by burning rice husk, an agricultural waste. Rice husk constitutes about one fifth of the 690 million metric tons of rice paddy produced annually in the world [5]. When completely incinerating the husk in controlled conditions, the residue, RHA, contains 90–96% silica in amorphous form. The average particle size of RHA ranges in general from 5 lm to 10 lm with a very high spe⇑ Corresponding author. Tel.: +31 15 2782307; fax: +31 15 2786383. E-mail address:
[email protected] (V. T. Nguyen). 0950-0618/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2010.11.046
cific surface area (even more than 250 m2/g) [6]. This high surface comes from the porous structure of RHA [7,8]. Meanwhile the specific surface area of SF is 18–20 m2/g, which mainly comes from the very fine particles. Both SF and RHA are considered as ‘‘highly active pozzolans’’ [9]. When incorporated in cement, both SF and RHA affect the rate and the extent of hydration [10,11]. The addition of RHA in concrete, similar to SF, can lead to reduced porosity and Ca(OH)2 content in the interfacial transition zone (ITZ) between the aggregate and the cement paste, as well as the ITZ’s width compared with the control sample [12]. Furthermore, RHA has been studied to replace SF for achieving high strength/performance concrete [13,14]. However, its effects still remain unclear for UHPC. In this study, the effect of RHA in combination with and without SF on the compressive strength of UHPC was experimentally evaluated. Besides, the effect of fineness of RHA was also considered.
2. Experiments 2.1. Materials The materials used in this study were silica sand with a mean particle size of 225 lm, Portland cement (CEM I 52.5N) with a Blaine specific surface area of 4500 cm2/g, condensed silica fume, rice husk ash, and polycarboxylate based superplasticizer with 30% solid content by weight. The SF has an amorphous SiO2 content of 97.2% and its mean particle size is about 0.1–0.15 lm. The particle size distribution and the mean particle size of materials in this study were determined by laser diffraction. Rice husk, from Vietnam, was burnt in a drum incinerator developed by Pakistan Council of Scientific & Industrial Research [15] under uncontrolled combustion conditions. Details of the oven and rice husk combustion process were described
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Cumulative passing, by vol. (%)
V. T. Nguyen et al. / Construction and Building Materials 25 (2011) 2030–2035 2.2. Packing and composition
100 90 80
SF
RHA (5.6 µm)
70
Optimization of granular mixture is one of the keys in UHPC mix design. The optimization of granular mixtures in this study was achieved by using the packing model developed by de Larrard [16] with the compaction index of 12.5 [17]. Firstly, the percentage of SF was fixed at 20% by weight of binder. The combination of cement and SF hence was considered as one material. The packing of ternary system, sand– cement–SF, was only calculated for two materials, sand–binder. The optimized packing of this system was achieved with sand to binder ratio of 1. Secondly, water to binder (w/b) ratio of UHPC was fixed at 0.18. SF was replaced partially or fully by RHA. To deal with some aspects, i.e., the effect of RHA replacement, fineness of RHA and the synergic effect of RHA and SF, a set of 15 mixtures were prepared as shown in Table 1.
RHA (6.3 µm)
60 RHA (9.0 µm)
50 40
Silica sand
30 RHA (3.6 µm)
20
Cement (CEM I 52.5 N)
10 0 0.01
0.1
1
10
100
1000
2.3. Experimental methods
Particle Size (µm) Fig. 1. Particle size distribution of materials used in this study. elsewhere [6]. The obtained ash was ground in a vibrating ball mill for 90 min. The ash contains 87.96% amorphous SiO2, 3.81% loss on ignition and its mean particle size (dRHAmean) is 5.6 lm. The particle size distribution of these materials is shown in Fig. 1. The Scanning Electron Microscopic (SEM) images of typical RHA and SF particles are shown in Fig. 2. It can be seen that RHA particles are angular and still keep the porous structure after grinding (Fig. 2b and c). Other three different mean particle sizes of RHA, i.e., 9, 6.3 and 3.6 lm was used to evaluate the effect of fineness of RHA on compressive strength of UHPC.
All materials were prepared in a 20 l Hobart mixer. The volume of each batch was 3.5 l. Fig. 3 shows the mixing procedure. The workability of all mixtures was determined by means of flow table test. The flow measurements were controlled between 210 and 230 mm. For very low water to binder (w/b) ratio mixtures, i.e., w/b ratio of 0.15, the maximum flow measurement was achieved only from 170 to 185 mm. This may be caused by the effectiveness of superplasticizer. The same phenomenon was observed for the mixtures incorporating high amount of cement replacement, i.e., over 20%. Besides, the workability of some mixtures was determined by mini V-funnel test in order to check the viscosity of fresh UHPC mixtures. Mixtures were cast into the 40 40 40 mm3 cubes for compressive strength test. All mixtures were vibrated for 1 min using a vibrating table with a frequency of 2500 cycles/min.
(b)
(a)
20 µm
20 µm
(c)
(d)
5 µm
2 µm
Fig. 2. SEM images of RHA before grinding (a); RHA after grinding with some magnifications: 1250(b); 5000(c); and SF (d).
Table 1 UHPC compositions used in this study. Water to binder ratio (by weight)
Sand to binder ratio (by weight)
RHA (% by weight)
0.18 0.18 0.18 0.18 0.18 0.18 0.15–0.18–0.20–0.23
1 1 1 1 1 1 1
0–10–20 10–20–30 20 5 15 10
SF (% by weight)
The mean particle size of RHA (dRHAmean), lm 5.6
10–20–30 10 15 5 10
5.6 9.0–6.3–5.6–3.6 5.6 5.6 5.6
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Sand + Cement + RHA + SF
3 min. mixing speed 2
Powder + 70% Water
2 min. mixing speed 2
Scrape and stir
1 min. mixing speed 2
1-2 min. mixing speed 2
SP + 25% Water
Add 5% water
Scrape and Stir
2-5 min. mixing speed 3
End of mixing
Fig. 3. Mixing procedure for UHPC.
210
3.0
Superplastisizer (solid) (% by weight of binber)
Compressive strength (MPa)
SF + RHA (SF fixed at 10%)
2.5
SF (No RHA)
2.0 RHA (No SF)
1.5 1.0 0.5
Flow values between 210 and 230 mm
RHA (91 days)
190 170
SF (91 days) RHA (28 days)
RHA (7 days)
150
SF (28 days)
130 SF (7 days) RHA (3 days)
110
SF (3 days)
90
0.0 0
10
2
30
40
50
RHA; SF or (RHA and SF) (% by weight of total binder) Fig. 4. The amount of superplasticizer of UHPC mixtures vs. % RHA, % SF or % (RHA + SF), for achieving a constant flow value of mixtures between 210 and 230 mm; w/b ratio = 0.18, dRHAmean = 5.6 lm.
After casting, samples were cured in a fog room (20 ± 2 °C, RH > 95%) for one day. After demoulding, the samples were still stored in the fog room until the day of testing.
70 0
10
20
30
SF or RHA (% by weight in binder) Fig. 5. Compressive strength of UHPC samples vs. % SF (dotted line) or % RHA (solid line), w/b ratio = 0.18, dRHAmean = 5.6 lm.
Thus, the cement replacement by 30% RHA and 10% SF was considered as the ‘‘maximum’’ replacement amount in this study. 3.2. The effect of the percentage of cement replacement by RHA on compressive strength of UHPC
3. Results and discussion 3.1. The workability of UHPC mixtures The amount of superplasticizer of mixtures for achieving a constant flow value from 210 mm to 230 mm are shown in Fig. 4. It can be seen that cement replacements by SF between 10% and 20% have a positive effect on the workability of the mixtures. This may be resulted from the fine spherical particles of SF, which act as a lubricant. However, when more than 20% cement is replaced by SF, the amount of superplasticizer increases dramatically to 1.43% and 2.38% for 30% and 40% SF, respectively. The cement used in this study has the specific surface area of 0.45 m2/g. The surface area of SF is about 20 m2/g. Therefore, the total surface area of the blend of cement and SF depends on the SF replacement percentage. For example, when cement is replaced by 20% SF, the total surface area of 1 g of this blend is 4.36 m2. This value will be almost double, 8.27 m2/g with 40% cement replacement by SF. Thus, the ‘lubricant effect’ may not compensate the effect of its surface area when more cement is replaced by SF. On the contrary, the amount of superplasticizer of UHPC increases with an increase of RHA replacement. Fig. 4 also shows that the RHA replacement of cement can be increased to 40% when combined with 10% SF. If the RHA replacing percentage is higher than 40%, the maximum flow value is achieved less than 170 mm even when using a high amount of superplasticizer. This also was caused by the high specific surface area of RHA. The mixture was observed to be very difficult to be cast in this case.
Fig. 5 shows that compared to the control sample, the compressive strength of UHPC was improved significantly with 10% SF or 20% RHA. Moreover, the compressive strength enhancement levels of the samples with binary blends were comparable. For samples containing SF, the highest compressive strength of UHPC was achieved with 10% SF replacement of cement. The higher replacement level, especially beyond 20%, led to reduction in compressive strength. The use of RHA as a partial replacement of cement revealed the different behavior of compressive strength development. The compressive strength of UHPC was obtained highest by using 10% RHA at 3 and 7 days, but by using 20% RHA at 28 and 91 days. Based on this result, it is clear that RHA can be used to produce UHPC for a replacement level less than 30%. 3.3. The effect of combination of RHA and SF on compressive strength of UHPC Based on the workability result of UHPC, the possibility of using RHA rose when combined with 10% SF. Fig. 6 shows the effect of RHA replacement percentage on the compressive strength of UHPC when the percentage of cement replacement by SF was fixed at 10%. It is interesting to see that the compressive strength of UHPC with 0 and 10% RHA was higher than that of the control sample. It should be noted that the total cement replacement percentages were 10 and 20%, respectively in this case. The compressive strength of UHPC decreases when the amount of cement replacement by RHA is higher than 20%. It also can be seen that the 28 days compressive strength of all samples was in excess of 150 MPa.
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230
250
4.0
170 150
28 days
130 7 days
110
7 days
90 3 days
(SF fixed at 10%)
70
200 3.0 165
2.5
150
2.0
115 100
75
1.5
55
1.0
50 Flow values between 210 and 230 mm
0.5
0
0.0
50 0
10
20
15.5
30
3.4. The effect of RHA fineness on compressive strength of UHPC One of the most important aspects using RHA to produce UHPC is particle size of RHA. This concerns the energy required for grinding rice husk to appropriate fineness. Grinding energy increases with fineness of RHA. If long periods of grinding are required either the production output will be reduced or larger milling capacity will be required for a given output, the latter obviously results in higher capital costs for plant, especially in the developing countries [8]. Because of the porous structure, RHA is easy to grind. For example, the mean size of RHA from 1–2 mm can be reduced to 9 lm only with 30 min ground by vibrating ball mill. These mean sizes of 6.3, 5.6 lm can be achieved with grinding times of 60 and 90 min, respectively. However, the efficiency of this mill does not increase with grinding time [18]. It was also found when the grinding time increases, the pore structure of RHA is gradually collapsed [19]. This collapsibility can affect the properties of concrete. Thus the effect of RHA fineness on properties of UHPC, especially, the compressive strength is studied thereafter. Four different particle size of RHA with the mean size smaller than that of cement, namely 9, 6.3, 5.6, and 3.6 lm were chosen. The mean size of cement, CEM I 52.5N, is 13.7 lm. The replacement percentage of cement by RHA was fixed at 20%. Fig. 7 shows that the superplasticizer dosage needed for a given workability is reduced proportionally with the decrease of RHA particle size. In this study, the mean particle size of RHA of 3.6 lm can be achieved by grinding in planetary ball mill after 3 h. The compressive strength of these UHPCs is shown in Figs. 8 and 9. It can be seen that the compressive strength decreases linearly when increasing the mean size of RHA particle. The 28 days compressive strength of 150 MPa can be achieved with the mean size of RHA particle of about 8 lm at 20 °C curing. It means that 1 h grinding time by vibrating ball mill can be considered ‘optimum’ in this study. However, the coarser the RHA, the higher the amount of the superplasticizer is required and the higher the viscosity is. Although the flow values were controlled between 210 mm and 230 mm, the viscosity measured by the mortar funnel is different. The flow funnel times of mixtures increase with an increase of the mean particle size of RHA (see Fig. 7). The mixture with flow time mortar funnel over 115 s was observed to be difficult to be cast.
7.8
6.3
5.6
3.6
Fig. 7. The amount of superplasticizer and V-funnel time of UHPC mixtures incorporating 20% RHA with different particle sizes from 15.5 lm to 3.6 lm; flow values between 210 mm and 230 mm.
220 200
Compressive strength (MPa)
This result shows the benefit of using RHA in combination with SF to produce UHPC. The amount of SF replacement is only 10% but the total cement replacement percentage can increase to 40% by weight.
9.0
The mean size of RHA particle (µm)
RHA (% by weight of binder) Fig. 6. Compressive strength of UHPC samples vs. % RHA (solid line); the control sample without RHA or SF (dotted line), the percentage of SF replacement was fixed at 10%, w/b ratio = 0.18, dRHAmean = 5.6 lm.
V-funnel time (sec.)
91 days
28 days
SF
190
3.5
REF
210
Superplasticizer dosage (solid) (% by weight of binder)
Compressive strength (MPa)
235
28 days 91 days
180 160 140 120 3 days
7 days
100 80 60
1 day
9.0
6.3
40 3.6
4.6
5.6
6.6
7.6
8.6
The mean size of RHA particle (µm) Fig. 8. Compressive strength of UHPC samples using 20% RHA vs. the mean particle size of RHA (solid line); the control sample without RHA or SF (dotted line), w/b ratio = 0.18.
Besides, the grinding energy cost increases dramatically, from 3.0 kW h/kg to 7.5 kW h/kg when the mean size of RHA particle reduces from 5.6 lm to 3.6 lm. From this result, the reasonable mean size of RHA particle was proposed to be 5.6 lm. 3.5. The synergic effect of RHA and SF on compressive strength of UHPC Interestingly, in Fig. 6, the UHPC sample made by the blend of 10% RHA and 10% SF shows better compressive strength results than that of the control sample without RHA or SF and samples using other blends. There must be a synergic effect between RHA and SF on the compressive strength of UHPC. To investigate this effect, the total cement replacement percentage by blend of RHA and SF was kept at 20%. The ratio, RHA/(RHA + SF), varies from 0% to 100%. Fig. 10 shows the amount of superplasticizer of UHPC mixtures, for achieving a constant flow value. The more cement was replaced by RHA, the higher the amount of superplasticizer was needed. The compressive strength of UHPC samples vs. RHA/(RHA + SF) and vs. time are shown in Figs. 11 and 12, respectively. It is found that the highest compressive strength of UHPC samples from 3 days to 91 days was achieved using the blend of 10%
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220 91 days
SF
200
Compressive strength (MPa)
Compressive strength (MPa)
220
RHA RHA (5.6 m) (3.6 m)
180 160 REF
140 120 100 RHA (9.0 m)
80 60
RHA (6.3 m)
3
180 160
7
14
28
120 3 days
100 1 day
60 0.00
91
10
28 days
7 days
140
80
40 1
200
0.25
100
Time (days) Fig. 9. Compressive strength of UHPC samples using 20% RHA with different mean particle sizes from 3.6 lm to 9.0 lm vs. time, w/b ratio = 0.18.
1.3 1.20
1.2
Superplastisizer (solid) (% by weight of binber)
1.10
1.1 1.00
1.0 0.9 0.8
0.85 0.76
0.7 0.6 Flow values between 210 and 230 mm
0.5 0.4 0.00
0.25
0.50
0.75
1.00
RHA / (RHA + SF) Fig. 10. The amount of superplasticizer of UHPC mixtures vs. RHA/(RHA + SF), for achieving a constant flow value of mixtures from 210 mm to 230 mm, w/b ratio = 0.18, dRHAmean = 5.6 lm.
1.00
220 200
Compressive strength (MPa)
From the results presented above, the use of RHA improves the properties of UHPC. Firstly, the incorporation of the RHA in UHPC can gain higher compressive strength than that of the control sample. When more cement is replaced by RHA, i.e., 20% replacement, the compressive strength of the RHA sample even surpasses that of the SF sample. Secondly, the combination of RHA and SF improves the workability of UHPC mixtures and compressive strength of UHPCs. Especially, the combination of 10% RHA and 10% SF shows the synergic effect on the compressive strength of UHPC. This result can be explained firstly by the improvement of packing of granular mixtures. The result from the work [20] shows that the mixtures with water to cement ratio of 0.20, up to 50% of the cement will remain unhydrated. In this aspect, cement becomes ‘very expensive’ aggregates, and the substitution of cement by other materials is helpful. Moreover, the addition of finer materials can give better packing density, thus improves of the properties of concrete. As demonstrated earlier by Detwiler and Mehta [21], cement-blending can even be successful when an inert type of
0.75
Fig. 11. Compressive strength of UHPC samples vs. RHA/(RHA + SF) (solid line); the control sample without RHA or SF (dotted line), w/b ratio = 0.18, dRHAmean = 5.6 lm.
RHA and 10% SF. The SF shows the strong effect at a very early age, i.e., at 1 day. However, the effect of RHA is more pronounced in the later ages, i.e., after 3 days (see Fig. 11). 3.6. Discussions
0.50
RHA / (RHA+SF)
180 10% SF+10% RHA
160 REF
140 20% SF
120 100 20% RHA
80
3
60 1
7
14
10
28
91
100
Time (days) Fig. 12. Compressive strength of UHPC samples using different combinations of RHA and SF vs. time, w/b ratio = 0.18, dRHAmean = 5.6 lm.
mineral admixture is used, provided the size ranges of the Portland cement and the mineral admixture are properly designed. The gap between cement size (10–15 lm) and silica fume size (0.1–1 lm) is very large, about 100–150 times. In this case, the amount of cement was replaced by SF was 20%, which may be not enough to fill all the empty spaces in the system, as proposed by Richard and Cheyrezy [3]. On the other hand, the size of RHA is not small enough to fill all the small spaces. Therefore, the combination of RHA and SF can make the packing of UHPC denser. Besides, the combination of RHA and SF can reduce the amount of superplasticizer of mixture when the same workability is kept (see Fig. 4). It means the better packing may also improve the workability of the mixture, also suggested from Popovis [22]. Secondly, the positive effect of RHA on cement hydration can improve the compressive strength of UHPC. The previous result [19] shows that the effect of RHA on cement hydration, determined by isothermal calorimetry, is more pronounced than that of SF with the low w/b ratio, especially in the later ages. Therefore when RHA is used in UHPC, it can stimulate the cement hydration. Moreover, it can also be seen in this work that the behavior of the blended cement hydration is different with the addition of RHA and SF. The hydration degree of cement blended with RHA is low at the early period and high at the later period, but vice versa in case of cement blended with SF. It means that the combination of RHA and SF can
V. T. Nguyen et al. / Construction and Building Materials 25 (2011) 2030–2035
give a synergic effect in the whole period of cement hydration. The increased hydration of cement blended with RHA at the later period was suggested to be due to the internal curing of RHA in the cement paste. Because RHA has the porous structure, the pores in RHA particles may act as ‘‘water wells’’. RHA may also absorb certain amount of free water into its pores during mixing. This water is released from these pores when the relative humidity in the paste decreases with progress of cement hydration process, and therefore increases the hydration degree of blended cement. This effect is larger when the w/b ratio of mixtures is lower. This mechanism is similar to that proposed by Weber and Reinhardt [23] and van Breugel et al. [24] when using the water saturated lightweight aggregates or proposed by Jensen and Hansen [25,26] when using superabsorbent polymer (SAP) particles for internal curing of concrete. Besides, the porous structure of RHA may allow water to store in its pores. Therefore, the w/b ratios of RHA mixtures are lower than those of SF mixtures although the workability of all mixtures was kept constant with the same w/b ratio. As consequence, a higher compressive strength can be obtained. 4. Conclusion Nowadays, the environmental pollution is one of the most important world wide issues. The agricultural waste, such as rice husk, has been affecting on the environment. The use of rice husk ash in construction is one of the solutions to reduce the environmental pollution. Besides, the limited availability and the high cost constrain the application of SF, especially in developing countries. This study showed the potential of using RHA to produce UHPC. From this study, some conclusions can be drawn: – RHA can be considered as a supplementary cementitious material using for producing UHPC. – The addition of RHA does not significantly decrease the compressive strength of UHPC compared to that of SF, when less than 30% RHA is added. – Compared to SF, the fineness of RHA has a favorable effect on compressive strength when cured in the normal condition. The optimum mean RHA particle size for producing UHPC was found to be 5.6 lm. The finer RHA can improve significantly the compressive strength of UHPC. The compressive strength of UHPC using the finest RHA with the mean particle size of 3.6 lm can reach to 180 MPa and 210 MPa at ages of 28 and 91 days. – The combination of SF and RHA can increase the total cement replacement percentage up to 40% to produce UHPC. – There is a synergic effect between SF and RHA on the compressive strength. The sample made by ternary blend of cement with 10% RHA and 10% SF showed better compressive strength than that of the control sample without RHA and SF. The combination of 10% RHA and 10% SF proved to be optimum for achieving maximum synergic effect. Acknowledgments The principal author would like to express gratefulness for the PhD scholarship sponsored by The Vietnamese Government. The supply of materials from ELKEM, ENCI, FILCOM and BASF compa-
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