1 INTRODUCTION
⌅Rice
husk is an agricultural by-product, in which 20% of the grain is husk,
and approximately 20% of the husk is converted to ash after combustion (
1
1.
Givi, A.N.; Rashid, S.A.; Aziz, F.N.; Salleh, M.A. (2010) Assessment of
the effects of rice husk ash particle size on strength, water
permeability and workability of binary blended concrete. Constr. Build.
Mater. 24 [11], 2145-2150.
https://doi.org/10.1016/j.conbuildmat.2010.04.045
.
).
In 2020, the rice production in China was estimated at 211.86 million
tons; hence, which can produce about 42.37 million tons of rice husk (
2
2.
National Bureau of Stastics. Announcement of the National Bureau of
statistics on grain production data in 2020. [online] Retrieved from
http://www.stats.gov.cn/tjsj/zxfb/202012/t20201210_1808377.html
. [Accessed: 10 Oct 2020].
). RHA burnt under 700 ºC possess large amounts of amorphous SiO2 (
3-6
3.
Deshmukh, P.; Bhatt, J.; Peshwe, D.; Pathak, S. (2012) Determination of
silica activity index and XRD, SEM and EDS studies of amorphous SiO2 extracted from rice husk ash. T. Indian I. Metals. 65 [1], 63-70.
https://doi.org/10.1007/s12666-011-0071-z
.
4.
Sarangi, M.; Nayak, P.; Tiwari, T.N. (2011) Effect of temperature on
nano-crystalline silica and carbon composites obtained from rice-husk
ash. Compos. Part B-Eng. 42 [7], 1994-1998.
https://doi.org/10.1016/j.compositesb.2011.05.026
.
5.
Kang, S.H.; Hong, S.G.; Moon, J. (2019) The use of rice husk ash as
reactive filler in ultra-high performance concrete. Cem. Concr. Res.
115, 389-400.
https://doi.org/10.1016/j.cemconres.2018.09.004
.
6.
Villaquiran-Caicedo, M.A.; de Gutierrez, R.M.; Gallego, N.C. (2017) A
novel MK-based geopolymer composite activated with rice husk ash and
KOH: Performance at high temperature. Mater. Construcc. 67 [326].
https://doi.org/10.3989/mc.2017.02316
.
), which shows high pozzolanic activity and a favorable filling effect. The high pozzolanic activity, that is, amorphous SiO2 in RHA can react with calcium hydroxide to form secondary hydrate
products; moreover, RHA acts as the filling nucleation point for
hydration products and refines the pore structure (
7
7.
Rodriguez de Sensale, G. (2010) Effect of rice-husk ash on durability
of cementitious materials. Cem. Concr. Comp. 32 [9], 718-725.
https://doi.org/10.1016/j.cemconcomp.2010.07.008
.
), which makes it an ideal supplementary cementitious material (SCM) to effetely enhance mechanical properties (
8-12
8.
Zhang, Z.G.; Yang, F.; Liu, J.C.; Wang, S.P. (2020) Eco-friendly high
strength, high ductility engineered cementitious composites (ECC) with
substitution of fly ash by rice husk ash. Cem. Concr. Res. 137, 106200.
https://doi.org/10.1016/j.cemconres.2020.106200
.
9.
Bheel, N.; Keerio, M.A.; Kumar, A.; Shahzaib, J.; Ali, Z.; Ali, M.;
Sohu, S. (2021) An investigation on fresh and hardened properties of
concrete blended with rice husk ash as cementitious ingredient and coal
bottom ash as sand replacement material. Silicon-Neth. 14 [2], 677-688.
https://doi.org/10.1007/s12633-020-00906-3
.
10.
Intaboot, N. (2020) Innovation of interlocking block mixing with
biomass for sound absorption and thermal conductivity in Thailand. J.
Adv. Concr. Technol. 18 [8], 473-480.
https://doi.org/10.3151/jact.18.473
.
11.
Pavía, S.; Aly, M. (2016) Influence of aggregate and supplementary
cementitious materials on the properties of hydrated lime (CL90s)
mortars. Mater. Construcc. 66 [324], e104.
https://doi.org/10.3989/mc.2016.01716
.
12.
Tashima, M.M.; Soriano, L.; Monzó, J.; Borrachero, M.V.; Akasaki, J.L.;
Payá, J. (2014) New method to assess the pozzolanic reactivity of
mineral admixtures by means of pH and electrical conductivity
measurements in lime:pozzolan suspensions. Mater. Construcc. 64 [316],
e032.
https://doi.org/10.3989/mc.2014.00914
.
) and durability (
13-15
13.
Peng, G.F.; Yang, J. (2016) Influence of rice husk ash on the
properties of concrete: a review. In: International forum on energy,
environment and sustainable development (IFEESD) Shenzhen Peoples R.
China. 143-149.
14. Pradhan, B. (2014) Corrosion behavior of steel
reinforcement in concrete exposed to composite chloride-sulfate
environment. Constr. Build. Mater. 72, 398-410.
https://doi.org/10.1016/j.conbuildmat.2014.09.026
.
15.
Antiohos, S.K.; Tapali, J.G.; Zervaki, M.; Sousa-Coutinho, J.; Tsimas,
S.; Papadakis, V.G. (2013) Low embodied energy cement containing
untreated RHA: A strength development and durability study. Constr.
Build. Mater. 49, 455-463.
https://doi.org/10.1016/j.conbuildmat.2013.08.046
.
) of concrete.
However,
the alkali content of RHA was slightly higher than that of fly ash and
granulated blast furnace slag, and significantly exceeded that of cement
and silica ash (
16
16.
Wang, J.; Xiao, J.; Zhang, Z.; Han, K.; Hu, X.; Jiang, F. (2021) Action
mechanism of rice husk ash and the effect on main performances of
cement-based materials: A review. Constr. Build. Mater. 288, 123068.
https://doi.org/10.1016/j.conbuildmat.2021.123068
.
,
17
17.
Zhu, H.J.; Liang, G.W.; Xu, J.; Wu, Q.S.; Zhai, M.N. (2019) Influence
of rice husk ash on the waterproof properties of ultrafine fly ash based
geopolymer. Constr. Build. Mater. 208, 394-401.
https://doi.org/10.1016/j.conbuildmat.2019.03.035
.
) required by Chinese standard GB/T 50733-2011 (
18
18.
GB/T 50733-2011 (2011) Technical code for prevention of
allali-aggregate reaction in concrete. Standards Press of China.
Beijing, China.
), which may induce ASR in mortar or
concrete. To address this issue, additional acid treatment before
burning could remove the alkali impurities in RHA (
19
19.
Zhang, H.X.; Zhao, X.; Ding, X.F.; Lei, H.; Chen, X.; An, D.M.; Li,
Y.L.; Wang, Z.C. (2010) A study on the consecutive preparation of
D-xylose and pure superfine silica from rice husk. Bioresour. Technol.
101 [4], 1263-1267.
https://doi.org/10.1016/j.biortech.2009.09.045
.
,
20
20.
Ma, X.Y.; Zhou, B.; Gao, W.; Qu, Y.N.; Wang, L.L.; Wang, Z.C.; Zhu,
Y.C. (2012) A recyclable method for production of pure silica from rice
hull ash. Powder Technol. 217, 497-501.
https://doi.org/10.1016/j.powtec.2011.11.009
.
),
with the aim of avoiding the ASR of concrete. What needs to be pointed
out is that the acid pretreatment methods also increases the production
cost and brings secondary chemical pollution to the environment.
Therefore, it is essential to explore other measures to reduce the
potassium content in RHA.
Sugita S. (
21
21.
Sugita, S. (1996) Fundamental study on the effective utilization of
rice husk ash as concrete materials. Hokkaido: Hokkaido University.
)
firstly reported that two periods burning type, in which rice husk was
burned at a first lower temperature and then a higher temperature, could
be applied to produce RHA with a high silica content, while the
influence of carbonization and ashing time on the potassium content of
RHA has not be involved. Thereafter, Hongtao Li (
22
22.
Li, H.B.; Li, B.X.; Xu, Y.N.; Gao, X.Y. (2013) Effect of calcination
method on morphological characteristics and composition of rice husk
ash. Trans. Chin. Soc. Agric. Mach. 44 [4], 131-136.
https://doi.org/10.6041/j.issn.1000-1298.2013.04.024
. (in Chinese).
)
found that the potassium content of RHA produced at 275 ºC for 30 min
and then 815 ºC for 120 min presented a lower value than that of RHA
fabricated at 815 ºC for 150 min; and also revealed that the potassium
content of RHA showed two situations including a high activity at 275 ºC
and low activity at 815 ºC; however, amorphous SiO2 in RHA will transfer to crystalline silica at 815 ºC. Crucially, R.V. Krishnarao (
23
23.
Krishnarao, R.V.; Subrahmanyam, J.; Kumar, T.J. (2001) Studies on the
formation of black particles in rice husk silica ash. J. Eur. Ceram.
Soc. 21 [1], 99-104.
https://doi.org/10.1016/S0955-2219(00)00170-9
.
) reported that potassium oxide (K2O)
dissociates upon heating at 620 K (347 ºC) to form elemental potassium
whose melting point is 336.8 K; subsequently, the elemental potassium
reacts with carbon composition to form potassium complex compound that
is difficult to be released during the combustion process. Therefore,
whether prolonging the burning time at a temperature under 620 K (347
ºC) could prohibit the transformation from K2O to potassium
complex compound and reduce the potassium content or not needs to be
revealed. Based on these, it is necessary to explore the influence of
two periods combustion type on the potassium content of RHA, which could
provide proper parameters for manufacturing production.
On the
other hand, to achieve the high pozzolanic activity of RHA, RHA should
be ground to a fine particle size. Too many researchers only chose
several individual particle sizes to initiate comparative analyses. B.H.
Abu Bakar (
24
24.
Abu Bakar, B.H.; Azmi, M.J.M.; Ramadhansyah, P.J. (2011) Effect of rice
husk ash fineness on the chemical and physical properties of concrete.
Mag. Concr. Res. 63 [5], 313-320.
https://doi.org/10.1680/macr.10.00019
.
)
examined the influence of fineness levels of RHA varying from 6.65 μm
to 17.97 μm on the mechanical properties of concrete, and found that RHA
with a median particle size of 9.52 μm could enhance the increment of
compressive strength significantly. J. Alex (
25
25.
Alex, J.; Dhanalakshmi, J.; Ambedkar, B. (2016) Experimental
investigation on rice husk ash as cement replacement on concrete
production. Constr. Build. Mater. 127, 353-362.
https://doi.org/10.1016/j.conbuildmat.2016.09.150
.
)
also concluded that the concrete compressive strength increased with
RHA fineness modulus decreased; simultaneously, the interfacial
transition zone could be enhanced due to the pozzolanic and filling
effect of RHA. Moreover, A.P. Vieira (
26
26.
Vieira, A.P.; Toledo, R.D.; Tavares, L.M.; Cordeiro, C.C. (2020)
Cordeiro. Effect of particle size, porous structure and content of rice
husk ash on the hydration process and compressive strength evolution of
concrete. Constr. Build. Mater. 236, 117553.
https://doi.org/10.1016/j.conbuildmat.2019.117553
.
)
compared the mechanical properties of concrete blended RHA with D50 of
7, 14, and 20 μm, respectively, indicating that the compressive strength
of concrete increased as RHA particle sizes decreased due to the
increasing pozzolanic activity and filling effect. However, it is not
enough to determine the optimal particle size of RHA showing the best
pozzolanic activity only by comparing several group of RHA with
different average particle sizes, for RHA with a D50 of 6.65 μm presents
various RHA particle distributions from 0-100 μm, and it is hardly to
mill RHA of a D50 of 6.65 μm with the same particle distribution.
Therefore, a more effective and accurate method should be proposed to
reveal the optimum particle sizes on the mechanical properties of the
concrete or mortar.
This paper focuses on investigating the influence of two periods combustion style on the chemical properties of RHA, especially for the potassium content in RHA, through investigation on the chemical composition, morphology, and crystal pattern of RHA burnt at nine conditions. Meanwhile, grey incidence analysis, a new mathematical model, was conducted to reveal the optimum particle sizes of RHA on the mechanical properties of mortar, and the mechanical test of mortar blended RHA with four different particle sizes was conducted to analyze the influence of each granularity group of RHA on the mortar strength via grey incidence analysis. Additionally, the scanning electron microscope (SEM) and X-ray diffraction (XRD) tests were employed to research the microstructure and composition of hydration products of mortar mixtures and also to reveal the underlying mechanism of the substitution of cement by RHA on the increasing mechanical performance of mortar. Also, the results in this paper are expected to provide references for the industrial production of RHA.
2 MATERIALS AND METHODS
⌅2.1 Materials
⌅Ordinary Portland cement of P•O 42.5R with an average particle size of 13.83 μm was utilized to produce mortar mixes. The chemical oxides composition and loss on ignition (LOI) of cement are presented in Table 1 . Chinese ISO standard sand was employed to manufacture mortar, in which the particle size of fine, medium, and coarse sand are 0.08 mm-0.5 mm, 0.5 mm-1 mm, and 1 mm-2 mm, respectively. Raw rice husk was collected from a rice plant in Bengbu (Anhui province, China) and washed, screened, and dried for 24 hours prior to being used in a burning process.
Materials | SiO2 | Al2O3 | Fe2O3 | CaO | MgO | K2O | Na2O | SO3 | LOI | Total |
---|---|---|---|---|---|---|---|---|---|---|
Cement | 20.24 | 5.21 | 3.56 | 62.47 | 2.25 | - | - | 2.58 | 3.31 | 99.62 |
2.2 Experimental program
⌅2.2.1 Combustion experiment
⌅The first study evaluates morphological properties and chemical composition of RHA via two combustion types including straight high temperature (one period) and first lower temperature then higher temperature combustion (two periods), which aims to explore a new burning method to decrease the K2O content in RHA. The raw rice husk was burnt in a box furnace which could achieve the automatic function of controlling temperature and time. The combustion process contains two key factors, namely the burning temperature (300 ºC, 600 ºC) and residence time (30 min -180 min). In each combustion, rice husk with a mass of 1kg was embedded in the box furnace and then burned according to the set procedure. Nine different schemes were presented in this study as listed in Table 2 and Figure 1 .
Number | Different sustaining time (min) | |||||
---|---|---|---|---|---|---|
Style | Initial temperature | RT-300 ºC | 300 ºC | 300-600 ºC | 600 ºC | |
One period | L0 H60 | RT | h-28 | - | h-30 | 60 |
L0 H120 | RT | h-28 | - | h-30 | 120 | |
L0 H180 | RT | h-28 | - | h-30 | 180 | |
Two periods | L30 H30 | RT | h-28 | 30 | h-30 | 30 |
L30 H60 | RT | h-28 | 30 | h-30 | 60 | |
L30 H90 | RT | h-28 | 30 | h-30 | 90 | |
L60 H90 | RT | h-28 | 60 | h-30 | 90 | |
L90 H90 | RT | h-28 | 90 | h-30 | 90 | |
L60 H120 | RT | h-28 | 60 | h-30 | 120 |
Note: RT represents the room temperature. The character of h represents the heating process where the heating rate is 10 ºC/min. h-28 and h-30 illustrate that the heating process consumes 28 min and 30 min during RT-300 ºC and 300-600 ºC, respectively.
2.2.2 Mechanical study
⌅A second study
was developed to reveal the influence of RHA particle size on the
mechanical properties of mortar. RHA produced by L60H120 combustion type
was selected and ground for 5 - 30 min in a planetary ball mill at a
speed of 60 r/min. The cumulative distribution curve of RHA was measured
by laser granulometry (Mode JL-6000) equipment as presented in
Figure 2
.
Due to the chemical properties of RHA were significantly close to
silica fume, which also consists of non-crystalline silicon dioxide and
high specific surface area. Hence, substituting cement by RHA with four
mean particle sizes into mortar was conducted to investigate the
pozzolanic activity of RHA in accordance with GB/T 27690 (
27
27. GB/T 27690-2011 (2011) Silica fume for cement mortar and conctrete. Standardization Administration of China Beijing, China.
). The mixture proportions of mortar are shown in
Table 3
.
The sample names were derived from the mean particle size of RHA. For
instance, the sample of RHA35.5 implies that RHA substituted 10% of
cement (by mass) with a mean particle size of 35.5 μm. Whereafter, grey
correlation analysis was employed to analyze the influencing degree of
each group of RHA on the basis of the compressive and flexural strength
of mortar.
Sample | Grinding time/min | Content / g | Mean particle size (μm) | |||
---|---|---|---|---|---|---|
RHA | Cement | ISO sand | Water | |||
Ref | - | 0 | 450 | 1350 | 225 | - |
RHA35.5 | 5 | 45 | 405 | 1350 | 225 | 35.5 |
RHA28.2 | 10 | 45 | 405 | 1350 | 225 | 28.2 |
RHA9.9 | 15 | 45 | 405 | 1350 | 225 | 9.9 |
RHA5.9 | 30 | 45 | 405 | 1350 | 225 | 5.9 |
2.2.3 Grey correlation analysis
⌅In this paper,
the interval grey correlation analysis based on the microscopic or
macroscopic geometric proximity of the factor sequence was adopted to
assess the correlation degree among subsequences and further find an
optimum subsequence for impacting the parent sequence (
28
28.
Feng, Y.X.; Hong, Z.X.; Cheng, J.; Jia, L.K.; Tan, J.R. (2017) Low
carbon-oriented optimal reliability design with interval product failure
analysis and grey correlation analysis. Sustainability-Basel. 9 [3],
369-383.
https://doi.org/10.3390/su9030369
.
). In the grey correlation theory, the index column is expressed as
, where i is the natural number sets, and m is related to the influencing factors.
It should be noted that there are i assessment plans in the analysis, i.e.,
. Meanwhile, parent sequences and subsequences are represented as
, respectively (
29
29.
Li, H.H.; Chen, D.Y.; Arzaghi, E.; Abbassi, R.; Xu, B.B.; Patelli, E.;
Tolo, S. (2018) Safety assessment of hydro -generating units using
experiments and grey-entropy correlation analysis. Energy. 165, 222-234.
https://doi.org/10.1016/j.energy.2018.09.079
.
). Therefore, the correlation coefficient,
, between subsequence and parent sequence with respect to jth factor could be obtained by Equation [
1
] (
30
30.
Wang, W.P.; Yang, Z.M.; Lu, Y.; Sin, Y.L.; Zhang, B. (2015) The
optimization degree of provincial industrial ecosystem and EKC of
china-based on the grey correlation analysis. In: Proceedings of 2015
(IEEE) International Conference on Grey Systems and Intelligent Services
(GSIS), Leicester, United Kingdom. 179-186.
):
Where
is equal to
ρ is the resolution coefficient, which is to enlarge the
difference between the various coefficients within the interval [0, 1];
generally, it is fixed at 0.5 (
31
31.
Chiang, K.T.; Chang, F.P. (2006) Optimization of the WEDM process of
particle-reinforced material with multiple performance characteristics
using grey relational analysis. J. Mater. Process. Tech. 180 [1-3],
96-101.
https://doi.org/10.1016/j.jmatprotec.2006.05.008
.
).
and
denote the minimum and maximum difference in the first level using Equation [
2
]. Meanwhile,
and
are the maximum and minimum difference in the second level, respectively using Equation [
3
] (
29
29.
Li, H.H.; Chen, D.Y.; Arzaghi, E.; Abbassi, R.; Xu, B.B.; Patelli, E.;
Tolo, S. (2018) Safety assessment of hydro -generating units using
experiments and grey-entropy correlation analysis. Energy. 165, 222-234.
https://doi.org/10.1016/j.energy.2018.09.079
.
).
Subsequently, the grey correlation degree, ri, could be obtained from Equation [ 4 ] by manipulating the grey correlation coefficients, which reflect the correlation degree of each subsequence in the same parent sequence. It means that we can find the optimal subsequence by comparing the correlation degree of all subsequences.
Where is the correlation coefficient from Equation [ 1 ], and j is within the interval [1, n].
On the other hand, using
calculates the correlation coefficient and correlation polarity
including the positive and negative correlation among the subsequence
could not be estimated accurately. Hence, in this paper, the judging
method of the correlation polarity was defined by a new mathematical way
listed in Equation [
5
], which confirms the correlation including positive and negative correlation from Equation [
6
] and Equation [
7
]. The expressions for each of these formulas are shown as follows (
32
32.
Deng, J. (1990) A course in grey systems theory. Press of Huazhong
University of Science and Technology, Wuhan, China, 1990. (in Chinese).
):
Where both of and are the new defining value, is the natural number set, and m represents the influencing parent sequence in the analysis. In Equation [ 6 ], the value of overs than zero, while k is within the interval [1, n]. It was noted that subsequence has a positive correlation on enhancing parent sequence when satisfies the requirement of Equation [ 6 ]. Otherwise, a negative correlation that subsequence plays a weakening role on the parent sequence is emerged according to Equation [ 7 ].
2.3 Experimental methods
⌅2.3.1 Scanning electron microscope
⌅Scanning
electron microscopy (SEM, Helios G4 UC, Japan) with energy-dispersive
X-ray (EDX) provided by the analytical and testing center of Sichuan
University was used to obtain the microstructure of RHA. Moreover, rice
husk ash was compacted to a flake layer; thereafter, the surface scan of
energy-dispersive X-ray was employed to obtain the accurate chemical
elements of RHA samples, as utilized in some previous studies (
33
33.
Li, H.; Li, B.; Xu, Y.; Gao, X. (2013) Effect of calcination method on
morphological characteristics and composition of rice husk ash. Trans.
Chin. Soc. Agric. Mach. 44 [4], 131-136.
https://doi.org/10.6041/j.issn.1000-1298.2013.04.024
.
,
34
34.
Bie, R.S.; Song, X.F.; Liu, Q.Q.; Ji, X.Y.; Chen, P. (2015) Studies on
effects of burning conditions and rice husk ash (RHA) blending amount on
the mechanical behavior of cement. Cem. Concr. Compos. 55, 162-168.
https://doi.org/10.1016/j.cemconcomp.2014.09.008
.
).
2.3.2 Mechanical properties
⌅All of the
mortar mixtures were bathed into JJ-5 cement and mortar mixer with a 5L
capacity. Initially, water was poured into the JJ-5 mixer, then cement
and RHA were blended for 120 s throughout the mixing procedure. Then,
sand was incorporated slowly and thoroughly mixed for 60 s. Finally, the
fresh paste was shaped in the molds and stored with plastic films at
the air temperature. After 24 hours, all mortar samples were demolded.
The flexural and compressive test complying with GB/T 27690 (
27
27. GB/T 27690-2011 (2011) Silica fume for cement mortar and conctrete. Standardization Administration of China Beijing, China.
)
was conducted on three samples for each mortar mixture. Paste bars
having the dimension of 40 mm×40 mm×160 mm were cast. After curing at 65
± 2 ºC for six days and then 21 days in a standard room, all specimens
were subject to the compressive and flexural test. Prior to the test, a
piece of an unforced segment was cut from the specimens by a saw bar
with a thickness of about 0.6 mm and polished by sanding paper, whiling
the cement hydration process of stores pieces was ceased by isopropyl
alcohol and dried in an oven at 60 ºC for two hours.
2.3.3 X-ray diffraction
⌅The crystal structure of RHA was measured using an X-ray diffractometer (XRD, EMPYREAN, Holland) operating at an accelerating voltage of 60 kV and 60 mA. Under continuous scanning pattern, Cu Kα radiation worked as the source of X-rays, and crystalline phases were scanned from 5◦ to 80◦ with a 2θ increment of 0.02◦/step. Although the scanning was performed up to 85° (2θ), the range of 10 - 50° was analyzed. This is because essential peaks were satisfactorily detected in this range, and meaningful data could not be obtained due to the overlap of peaks at a high angle of > 50° (2θ).
To obtain the crystal structure of paste samples blended RHA, the hydration of paste samples cured for 28 days was stopped by isopropyl alcohol and dried in an oven at 60 ºC for two hours, thereafter, X-ray diffraction was applied after grinding the pieces and passing the sieve of 240 meshes.
3. RESULTS AND DISCUSSION
⌅3.1 Influence of combustion type on morphology and chemical properties of RHA
⌅3.1.1 Morphology
⌅Figure 3 displays the macro-morphology and SEM micrographs of RHA using one period burning method. It can be seen that, as the burning time increases at 600 ºC, the black color of RHA tends to change to gray-white. As shown in the SEM micrographs, the scattered particles on the outer surface of RHA present a decreasing development with the increment of high temperature at 600 ºC. This phenomenon demonstrates that rice husk biomass such as lignin, cellulose, and spent carbon polymer can be burnt with the increase in the total burning time, which in turn presents a whiter color.
Figure 4 shows the macro-morphology and SEM micrographs of RHA by two periods burning methods. Similarly, the white color of RHA tends to enhance as the total burning time increases; however, RHAs produced in L60H90, L90H90, and L60H120 appear remarkably similar gray-white color, which is possible due to the reducing biomass which could be wholly oxidized and then burnt when the burning time was up to 90 min at 600 ºC. This perspective could be certified by the low carbon content in Table 5 . From the SEM micrographs, massive granular impurities are attached on the outer surface of RHA in L30H30 due to the insufficient combustion time. The scattered particles on the outer surface of RHA present a rapidly decreasing tendency as the total burning time increases.
3.1.2 X-ray diffraction analysis
⌅The XRD patterns of RHA by nine burning methods are presented in
Figure 5
.
It could be seen that all samples emerge no noticeable sharp peak from
the diffraction curve. Besides, a hump at 2θ=near 15°-25° could be
observed, which confirms the presence of amorphous silica in terms of
the qualitative assessment of RHA. Previous researchers, who explored
the varied temperature to the influence on the crystallinity of silica
in RHA from XRD patterns, also found that amorphous silica could be
obtained and untransformed to crystalline silica such as cristobalite
and tridymite using temperature lower than 700 ºC, which was is
responsible for the critical pozzolanic activity of RHA (
35
35.
Rao, K.D.; Pranav, P.; Anusha. (2011) Stabilization of expansive soil
with rice husk ash, lime and gypsum-an experimental study. Int. J. Eng.
Sci. Tech. 3 [11], 8076-8085.
,
36
36.
Cizer, Ö.; Balen, K.V.; Elsen, J.U.; Gemert, D.V. (2006) Carbonation
and hydration of calcium hydroxide and calcium silicate binders with
rice husk ash. In: 2nd International RILEM Symposium, Rilem Publications
SARL, France. 611.
).
3.1.3 Chemical composition
⌅In this study, a semi-quantitative method, energy-dispersive X-ray, was used to determine the chemical composition of RHA, as listed in Table 4 . From Table 4 , in one period, the carbon content of RHA could be noticeably reduced when the total burning time at 600 ºC increased from 60 min to 120 min; meanwhile, the silica content of RHA also could noticeably increase. In two periods, the carbon content of RHA in L30H60 could be noticeably decreased by 86.5% compared with that of RHA in L30H30. Moreover, the silica content of RHA increased rapidly by prolonging the high temperature stage in L30H30, L30H60, and L30H90. However, the silica content of RHA by extending the low temperature stage in L30H90, L60H90, and L90H90 has an insignificant variation, indicating that it was hard to increase when the burning time at 600 ºC was up to 90 min.
styles | Number | C | O | Na | Mg | Al | Si | P | S | Cl | Ca | Fe | K | LOI* |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
One period | L0H60 | 28.09 | 41.67 | 0.07 | 0.14 | 0.04 | 26.39 | 0.13 | 0.11 | 0.08 | 0.36 | 0.35 | 2.56 | 24.5 |
L0H120 | 4.53 | 49.85 | 0.03 | 0.00 | 0.10 | 41.77 | 0.17 | 0.00 | 0.13 | 0.89 | 0.03 | 2.49 | 10.4 | |
L0H180 | 5.60 | 45.26 | 0.00 | 0.09 | 0.00 | 45.63 | 0.64 | 0.42 | 1.86 | 0.38 | 0.13 | 1.86 | 7.2 | |
Two periods | L30H30 | 39.56 | 37.85 | 0.06 | 0.12 | 0.04 | 20.90 | 0.07 | 0.04 | 0.00 | 0.30 | 0.00 | 1.07 | 27.2 |
L30H60 | 5.34 | 53.19 | 0.05 | 0.07 | 0.17 | 38.91 | 0.00 | 0.06 | 0.08 | - | 0.27 | 1.86 | 11.4 | |
L30H90 | 5.52 | 49.62 | 0.02 | 0.23 | 0.09 | 41.52 | 0.17 | 0.30 | 0.13 | 0.88 | 0.03 | 1.47 | 5.31 | |
L60H90 | 4.85 | 53.17 | 0.04 | 0.19 | 0.09 | 40.10 | 0.17 | 0.11 | 0.09 | 0.25 | 0.04 | 0.93 | 4.93 | |
L90H90 | 3.34 | 52.58 | 0.07 | 0.09 | 0.13 | 41.36 | 0.08 | 0.07 | 0.08 | 0.41 | 0.18 | 1.61 | 4.28 | |
L60H120 | 2.66 | 48.26 | 0.05 | 0.12 | 0.06 | 46.03 | 0.15 | 0.06 | 0.18 | 0.54 | 0.24 | 1.65 | 4.5 |
*LOI: loss on ignition
Figure 6
shows the potassium and silica content in RHA with different combustion types. It is observed in
Figure 6
that the potassium content in RHA generated by one period decreased by
58.2%, 40.9%, and 12.3% (average value), respectively, compared to the
potassium of RHA produced by two periods that the total burning time was
60 min, 120 min, and 180 min, respectively. This phenomenon indicates
that increasing combustion time at a low temperature of 300 ºC could
reduce potassium content when total combustion time was constant. Two
patterns of potassium were presented in RHA: potassium oxide and
potassium complex compound. Potassium oxide in RHA could be dissociated
to potassium when the temperature exceeds 620 K (347ºC) (
23
23.
Krishnarao, R.V.; Subrahmanyam, J.; Kumar, T.J. (2001) Studies on the
formation of black particles in rice husk silica ash. J. Eur. Ceram.
Soc. 21 [1], 99-104.
https://doi.org/10.1016/S0955-2219(00)00170-9
.
),
thereafter the potassium reacts with other elements to form a potassium
complex compound that could fix carbon to form black particles in RHA,
thus resulting in a high potassium content in RHA. Hence, prolonging the
low temperature at 300 ºC under 620 K could reduce the formation of
black particles in RHA; meanwhile, potassium whose melting point is
336.8 K could fully evaporate accompanied by carbon in RHA as the
combustion time increases at 300 ºC. On the other hand, RHA straightly
burnt from room temperature to 600 ºC contributes to a longer combustion
time above 620 K (347 ºC), thus forming a more potassium complex
compound and maintaining a high potassium content in RHA. However, the
decreasing potassium content of RHA in L90H90 and L60H120 was not
noticeable compared to that of RHA in L0H180. Moreover, the potassium
content of RHA in L30H90, L60H90, and L90H90 was 1.47%, 0.93%, and
1.61%, respectively, which implied that a longer time at 300 ºC than 60
min could not always motivate the evaporation of potassium accompanied
by carbon due to the restricted oxygen in the furnace which was supplied
from the outside air in this study. Hence, a reasonable low-temperature
stage should be applied to effectively reduce the potassium content in
RHA.
On
the other hand, considering the amorphous silica and potassium content
as well as the energy consumption among nine burning types, L60H90
presents a lower potassium content and electric energy consumption than
L90H90 and L60H120. However, L60H120 has a greater silica content;
meanwhile, the equivalent alkalis content of RHA in L60H120 was 1.33% by
transforming the potassium to corresponding oxides, which was lower
than 2.5% of fly ash and 1.5% of silica ash per GB/T 50733-2011 (
16
16.
Wang, J.; Xiao, J.; Zhang, Z.; Han, K.; Hu, X.; Jiang, F. (2021) Action
mechanism of rice husk ash and the effect on main performances of
cement-based materials: A review. Constr. Build. Mater. 288, 123068.
https://doi.org/10.1016/j.conbuildmat.2021.123068
.
). To ensure the high pozzolanic activity of RHA, L60H120 with a superior silica content was selected in this paper.
3.2 Influence of RHA particle size on mechanical properties of mortar
⌅3.2.1 Pozzolanic activity
⌅The pozzolanic activity of RHA produced by L60H120 was investigated according to GB/T 27690 (
27
27. GB/T 27690-2011 (2011) Silica fume for cement mortar and conctrete. Standardization Administration of China Beijing, China.
).
The flexural and compressive strength of mortar containing RHA with
four varying particle sizes at 7 and 28 days are plotted in
Figure 7
and
Figure 8
, respectively. Also, the pozzolanic activity index of RHA is presented in
Figure 8
.
As
can be seen, as RHA particle size decreases, substituting cement by RHA
noticeably increases the compressive strength of mortar, while the
flexural strength of mortar was slightly improved. Furthermore, the
pozzolanic activity index of RHA increases as its particle size
decreases since finer RHA opposes higher specific surface area, which
can provide more nucleation sites for cement hydration, thus
facilitating the pozzolanic reaction of RHA to form more C-S-H having a
low Ca/Si and fill the pores of mortar (
37
37.
Nguyen, V.T.; Ye, G.A.; van Breugel, K.; Fraaij, A.L.A.; Bui, D.D.
(2011) The study of using rice husk ash to produce ultra high
performance concrete. Constr. Build. Mater. 25 [4], 2030-2035.
https://doi.org/10.1016/j.conbuildmat.2010.11.046
.
,
38
38.
Ahsan, M.B.; Hossain, Z. (2018) Supplemental use of rice husk ash (RHA)
as a cementitious material in concrete industry. Constr. Build. Mater.
178, 1-9.
https://doi.org/10.1016/j.conbuildmat.2018.05.101
.
).
Figure 8
illustrates that the pozzolanic activity index of RHA milled from 5 min
to 30 min are 85%, 95%, 99%, and 101% at 7 days, respectively. The
curing days have a significant influence on the hydration degree of
cement, which found that the pozzolanic activity indexes of RHA are up
to 92%, 98%, 107%, and 114% at 28 days, respectively. It can be obtained
that all pozzolanic activity indexes of RHA at 7 days are lower than
105%, which does not conform to the prescriptive value of silica fume in
accordance with GB/T 27690 (
27
27. GB/T 27690-2011 (2011) Silica fume for cement mortar and conctrete. Standardization Administration of China Beijing, China.
).
However, the activity index of RHA at 28 days meets the requirement
that the activity index of silica fume should be greater than 85%
complying with GB/T 51003 (
39
39.
GB/T 51003-2014 (2014) Technical code for application of mineral
admixture. Standardization Administration of China, Beijing, China.
).
Therefore, it can be inferred that the particle size of RHA should
reasonably vary from 15 min to 30 min to meet its requirement of
pozzolanic activity, which contributes the particle packing effect and
pozzolanic effect to enhance the mortar strength.
3.2.2 Correlation analysis between RHA particle size and mortar strength
⌅As listed in Table 5 , the flexural and compressive strength of mortar at 7 and 28 days are presumed as parent sequences by , respectively. Moreover, the particle size system of RHA is cut apart into six subsequences including <5 μmˌ5-10 μmˌ10-15 μmˌ15-20 μmˌ20-45 μm and >45 μm, which were defined by , respectively (as listed in Table 6 ).
Sample | Flexural strength 7d/MPa | Compressive strength 7d/MPa | Flexural strength 28d/MPa | Compressive strength 28d/MPa |
---|---|---|---|---|
RHA35.5 | 7.22 | 22.85 | 9.07 | 31.26 |
RHA28.2 | 8.58 | 24.61 | 9.78 | 33.42 |
RHA9.9 | 8.64 | 25.61 | 10.02 | 36.49 |
RHA5.9 | 8.98 | 26.31 | 10.08 | 38.86 |
Note: all of the and belong to the parent sequences of .
Sample | (<5 μm) | (5-10 μm) | (10-15 μm) | (15-20 μm) | (20-45 μm) | (>45 μm) |
---|---|---|---|---|---|---|
A1 | 8.43 | 9.25 | 8.73 | 5.45 | 30.47 | 37.68 |
A2 | 14.39 | 11.94 | 9.95 | 5.59 | 24.68 | 33.45 |
A3 | 39.13 | 10.99 | 13.08 | 5.13 | 25.46 | 6.26 |
A4 | 47.96 | 11.75 | 13.45 | 5.03 | 18.52 | 3.31 |
Note: all of the belong to the parent sequences of
The correlation degree and polarity between the
subsequences and parent sequences were analyzed by grey correlation
analysis as summarized in
Table 7
.
It can be observed that, overall, during the six subsequences, the
correlation degree of four subsequences emerges as a consistent
developing rule that contains the same arranging order. Meanwhile, the
particle size of 15 μm is a critical factor in determining the
correlation polarity in different subsequences. From
Table 7
,
in terms of correlation polarity, the particle size below 15 μm in RHA
can have a positive influence on promoting the strength development of
mortar. According to the correlation degree of six subsequences, the
particle size of 5-10 μm in RHA contributes mostly to the strength
development, followed by 10-15 μm and <5 μm. Analyzed by the above
value of correlation degree, a smaller particle size of < 5 μm is
inconsistent with the conclusion that the finer particle of RHA
contributes to the higher pozzolanic reaction. That is possible because
of the increase of grinding degree, which destroys some pore structures
of RHA particles, thus inducing the porous microstructure to collapse
and partially break into finer ones (
40
40.
Nguyen, V. (2011) Rice husk ash as a mineral admixture for ultra-high
performance concrete. PhD, Delft University of Technology, Netherlands,
2011.
).
Particle size | <5 μm | 5-10 μm | 10-15 μm | 15-20 μm | 20-45 μm | >45 μm |
---|---|---|---|---|---|---|
7d Flexural strength | (+) 0.512 | (+) 0.957 | (+) 0.827 | (-) 0.863 | (-) 0.794 | (-) 0.405 |
7d Compressive strength | (+) 0.504 | (+) 0.917 | (+) 0.811 | (-) 0.868 | (-) 0.816 | (-) 0.402 |
28d Flexural strength | (+) 0.492 | (+) 0.903 | (+) 0.789 | (-) 0.881 | (-) 0.817 | (-) 0.399 |
28d Compressive strength | (+) 0.541 | (+) 0.921 | (+) 0.875 | (-) 0.851 | (-) 0.804 | (-) 0.412 |
Note: the sign of “+” and “-” present the positive and negative effects on the strength of mortar.
On the other hand, using RHA with a particle size exceeding 15 μm could result in a negative correlation on the strength of mortar complying with correlation polarity analysis. The largest negative effect of particle sizes will be reflected when the particle size of RHA is greater than 45 μm, followed by 0-45 μm and 15 μm-20 μm in accordance with the correlation degree of six subsequences. Hereby, the grinding time of raw RHA should be reasonably controlled in 15-30 min to improve the particle content under 15 μm in RHA and significantly reduce the particle content above 45 μm.
3.2.3 Microstructure
⌅The
microstructure of Ref and RHA5.9 samples analyzed by scanning electron
microscope (SEM) and energy-dispersive X-ray (EDX) are illustrated in
Figure 9
and
Figure 10
. The chemical composition of hydrates products is listed in
Table 8
.
The atom ratio of Ca/Si in hydrates products decreased from 5.88 to
1.03 when cement was replaced by RHA with a mean particle size of 5.9
μm, indicating that using RHA substitute cement contributed to the
pozzolanic reaction of RHA, thus forming a new C-S-H gel that its Ca/Si
is lower than 1.5 and presents a fibrous gel of fine C-S-H (Ⅰ) (
41
41. Yu, Q.J.; Sawayama, K.; Sugita, S.; Shoya, M.; Isojima, Y. (1999) The reaction between rice husk ash and Ca(OH)2 solution and the nature of its product. Cement Concrete Res. 29 [1], 37-43.
https://doi.org/10.1016/S0008-8846(98)00172-0
.
). Furthermore, from
Figure 10
.
It could be observed that the peak intensity of portlandite (2θ=near
18°) in Ref was higher than that of portlandite in the RHA5.9,
indicating that RHA with a mean particle size of 5.9 μm presented a
superior pozzolanic activity. Additionally, incorporating RHA weakened
the peak intensity of ettringite (2θ=near 9°), which also indicates that
RHA accurate the hydration degree of cement.
Mixtures | Element | O | Mg | Al | Si | K | Ca | Ca/Si |
---|---|---|---|---|---|---|---|---|
Ref | At% | 58.14 | 1.01 | 2.02 | 5.60 | 0.34 | 32.90 | 5.88 |
RHA5.9 | 58.94 | 1.19 | 3.80 | 17.41 | 0.82 | 17.84 | 1.03 |
4 DISCUSSION
⌅In
this paper, using two periods burning condition could effectively
decrease the potassium content of RHA to a certain extent compared to
one period burning process, which is a new finding. The design idea was
original from R.V. Krishnarao (
23
23.
Krishnarao, R.V.; Subrahmanyam, J.; Kumar, T.J. (2001) Studies on the
formation of black particles in rice husk silica ash. J. Eur. Ceram.
Soc. 21 [1], 99-104.
https://doi.org/10.1016/S0955-2219(00)00170-9
.
),
which reported that potassium oxide in RHA could be dissociated with
potassium when the temperature exceeds 620 K (347ºC). Thereafter, the
dissociated potassium react with other substance to form potassium
complex compound which is hard to be removed at a high temperature.
Therefore, prolonging the low temperature under 347 ºC could reduce the
transferred potassium complex compound and contribute to the release of K2O.
However, the decreasing degree of potassium content in RHA was
gradually weakened as the burning time increased. This is possible due
to the fact that more time than 60 min at 300 ºC could not motivate the
evaporation of potassium accompanied by carbon due to the restricted
oxygen in the furnace which was supplied from the outside air in this
study. Hence, a firing device should be developed to contribute to the
effective combustion process in the future. Overall, two periods
combustion type of rice hush is promising to reduce the potassium
content in RHA.
In general, finer RHA possesses higher pozzolanic activity. The precise optimal particle size of RHA contributed to the gaining strength has not been reported in previous studies. Actually, RHA with an average particle size of 5.9-35.5 μm was composited by various particles ranging from 0-100 μm where fine particles show high pozzolanic activity, and coarse particles only show the filling effect and low pozzolanic activity. In this paper, correlation analysis could judge the influence of various particles in RHA on its pozzolanic activity. The particle size less than 15 μm in RHA positively promotes the mortar strength development, especially in the optimal particle group (5-10 μm).
5 CONCLUSIONS
⌅In this paper, nine combustion types were employed to investigate the influence on the morphology, chemical composition, and crystallinity of RHA due to different combustion modes. Moreover, the influence of particle size distribution of RHA on the strength of mortar was also explored by using grey correlation analysis and strength testing. The special conclusions can be drawn as follows:
-
An optimum processing technic of RHA as SCM was proposed, including combustion type L60H120 and particle size zone (5-10 μm).
-
The amorphous silica content of RHA can be up to 91% and carbon content of declines sharply when using L60H120.
-
The pozzolanic activity index of RHA critically depends on RHA particle size, and RHA containing a median particle size of 5.9 μm presents much better pozzolanic activity index of 101 % and 114 % than the other three RHAs at 7 and 28 curing days, respectively.
-
The improving mechanism of mortar by adding RHA includes both combined packing effect and pozzolanic reaction of RHA, and the substitution of cement by RHA contributes to the pozzolanic reaction to form C-S-H having a low Ca/Si, which results in the significant gaining strength of mortar.