Partial replacement of metakaolin with thermally treated rice husk ash in metakaolin-based geopolymer

Construction and Building Materials 221 (2019) 527–538

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Partial replacement of metakaolin with thermally treated rice husk ash in metakaolin-based geopolymer Huajun Zhu a,b,⇑, Guangwei Liang a,c, Zuhua Zhang d,⇑, Qisheng Wu a, Jianzhou Du a a

School of Materials Science and Engineering, Yancheng Institute of Technology, Yancheng, Jiangsu 224051, PR China Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province, Yancheng Institute of Technology, Yancheng 224051, PR China c School of Materials Science and Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, PR China d Centre of Future Materials, University of Southern Queensland, West Street, Toowoomba, Queensland 4350, Australia b

h i g h l i g h t s  The metakaolin-based geopolymers added with RHA were prepared.  The incorporation of RHA enhanced the strength and refined the pore structure.  The improved mechanism of RHA in geopolymer was revealed successfully.  The recycling of RHA in an efficient way was realized.

a r t i c l e

i n f o

Article history: Received 9 December 2018 Received in revised form 23 February 2019 Accepted 14 June 2019

Keywords: Partial replacement Metakaolin Rice husk ash Geopolymer Compressive strength Microstructure

a b s t r a c t The influences of rice husk ash (RHA) on the reaction kinetics, mechanical property, and microstructure of metakaolin-based geopolymer (MG) were investigated. Geopolymer pastes containing different contents of RHA were prepared by sodium silicate activation. Analysis results show that both metakaolin and rice husk ash are involved in the geopolymerization, contributing to the development of mechanical properties and formation of compact microstructure. The longer reaction duration of setting time in the group incorporated RHA was ascribed to the procedure of SiO2 dissolution. The geopolymers added with RHA have a compact pore structure, which show a relatively high compressive strength. The replacement of metakaolin by RHA did not cut the total quantity of gel phase formation, but changed the form of partial gels existence and an optimal mixed point was reached at the 20% replacement level. The abundant in gel along with the pore channel and secondary filling effects make a significant contribution to the development for refinement of pore and improvement of strength. This study can be used as an indicator for the application of RHA in geopolymer, which is of great value in urging RHA towards realization in recycling. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction The development of recycled and waste materials in the field of cementitious materials attracts great attention in recent years [1–3], which has become one of the significant motivations that urge the worldwide application of cementitious materials [4–6]. The utilization of supplementary cementitious material (SCM) such as glass powder, red brick powder etc. makes a great contribution to the improvement of rheology and microstructure of geopolymer ⇑ Corresponding authors at: School of Materials Science and Engineering, Yancheng Institute of Technology, Yancheng, Jiangsu 224051, PR China (H. Zhu), Centre of Future Materials, University of Southern Queensland, West Street, Toowoomba, Queensland 4350, Australia (Z. Zhang). E-mail addresses: [email protected] (H. Zhu), [email protected] (Z. Zhang). https://doi.org/10.1016/j.conbuildmat.2019.06.112 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

[7–9]. Insufficient diversification of supplementary cementitious materials is one of the main factors hindering its development, while the emergence of rice husk ash (RHA), in this urgent to be solved blank, has wrote a thick and heavy color successfully [10–12]. Geopolymers, as a novel building material, show a considerable advantage in the workability, energy consumption and greenhouse gas emissions in comparison with Portland cement [13–14]. Metakaolin-based geopolymers (MG) are synthesized by alkali activation under the conditions of ambient or slightly elevated temperature [15]. Considering their unparalleled workability and outstanding bonding capacity in emergency repair compared to slag-based and fly ash-based geopolymers, and easy to be available in many parts of the world, metakaolin-based gropolymers have gradually be regarded as one of the promising potential

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alternatives to the conventional Portland cement, with effective reduction in Carbon dioxide emission during the production process [16–20]. However, drawbacks that still existed owing to the high demands of water in procedure of metakaolin-based geopolymer pastes are now pretty much the agenda for it [21]. RHA is a common supplementary cementitious material, used in the Portland cement, for the improvement of mechanical property [22–26]. Previous studies have demonstrated that RHA can play a significant role in the synthesis of self-compacting concrete [27–29]. From the work of fracture method (WFM) results and the scanning electron microscopy (SEM) images available in the existing literature, the lower fracture energy in the self-compacting concrete containing RHA in comparison with it in the plain selfcompacting concrete, which is the result related to the presence of unhydrated RHA that act as flaws. Moreover, RHA is also used as an alternative source for silica fume in reactive powder concrete [30–33]. The porosity and chloride ion penetration show a significant reduction due to the replacement of SF with RHA. This phenomenon performs better in steam curing than that in normal curing, which is probably attributed to the reactivity of amorphous silica increases. However, just very limited researches about the separate application of RHA in the alkali-activated have been reported. Hence, a comprehensive research of the role of RHA as a partial alternative source material in metakaolin-based geopolymer is necessary. In this study, rice husk ash was selected as a precursor used in geopolymer synthesis and the alkali-activated metakaolin pastes with different contents of RHA are prepared. The effects of incorporation of RHA on the geopolymeration and the microstructures of products, particularly on the pore structure and hydration degree were revealed.

Fig. 1. Particle morphologies as determined by filed emission secondary electron imaging in an FESEM: (a) metakaolin; (b) rice husk ash.

2. Materials and experimental methods 2.1. Materials The metakaolin (MK) used in this study was obtained from Taojinfeng New Materials Co. Ltd. (China). RHA, which is prepared by burning agriculturalresidual rice husk, supplied by Debo Bioenergy Technology Co., Ltd. (Hefei, China). RHA was used as partial replacement of metakaolin to prepared geopolymer pastes. The chemical compositions of MK and RHA, determined by X-ray fluorescence (XRF), listed in Table 1. Blending the sodium silicate solution (Na2O = 8.16 wt%, SiO2 = 26.01 wt%, H2O = 65.83 wt%) with sodium hydroxide (96% purity) is to prepare alkali activator with a modulus of 1.5 (molar ratio of SiO2 to Na2O) and reach a concentration of 35 wt% by adding distilled water [34]. Prior to use, the activator needs to be equilibrated to room temperature.

MK

RHA

-Cristobalite -Calcium aluminum oxide

2.2. Raw materials characterization 2.2.1. Morphology Fig. 1 shows the morphologies of metakaolin and rice husk ash particles (specific testing conditions listed in Section 2.4.5). The metakaolin contains typical flaky shapes, while there are more particles that are analogous sphere and few irregular particles can be observed in rice husk ash. The RHA particles consisting of honeycomb-holes and loose layer are formed by the packing and gathering of nano-SiO2. This unique structure brings the huge specific surface area along with the ultrahigh pozzolanic activity. 2.2.2. Mineralogical compositions Investigations of the mineralogical compositions of metakaolin (MK) and rice husk ash (RHA) were determined by an X-ray diffractometer (RIGAKU, model D/max) with Cu Ka radiation, shown in Fig. 2. Broad humps features occurred at 16–26° 2h in MK, while 20-25° 2h in RHA, indicating that the difference of amorphous phases in each materials. Obviously, the SiO2 is abundant in RHA apart from

10

20

30

40

50

60

70

80

2 Theta ( ) Fig. 2. Cu Ka radiation XRD patterns of metakaolin (MK) and rice husk ash (RHA). Phase identified: Cristobalite, SiO2, PDF NO. 39-1425; Calcium aluminum oxide, CaAl2O4, PDF NO. 53-0191.

Table 1 Chemical compositions of metakaolin and rice husk ash. LOI is loss on ignition at 1000 °C, wt%.

Metakaolin Rice husk ash

SiO2

Al2O3

CaO

MgO

Fe2O3

K2O

TiO2

Na2O

LOI

55.87 93.1

42.25 0.3

0.04 1.5

0.04 0.6

0.38 0.2

0.31 2.3

0.20 0.03

0.26 0.06

0.61 0.8

H. Zhu et al. / Construction and Building Materials 221 (2019) 527–538

conducted to provide the hardened samples with high early age strength. RHA, possessing a huge specific surface area and an ultrahigh pozzolanic activity, was added as a partial replacement of MK to bring a ball-bearing effect and use as a supplementary cementitious material for supporting late age property. All the blends were formulated at a constant liquid/binder ratio of 0.7.

100 RHA MK

Cumulative volume (%)

80

529

2.4. Experimental methods

60 2.4.1. Setting time and mechanical performance test The setting time of the fresh pastes, containing different contents of RHA, were determined by a standard Vicat needle according to ASTM C191 [42]. The compressive strengths of the cubic samples with dimensions of 30  30  30 mm after different curing ages were measured by A WHY-300 Auto Test Compression Machine capable of a maximum force value of 300 kN, in accordance with the Chinese standard GB/T 11837-2009 [39,43]. The samples were cured in steam curing environment (50 ± 2 °C) until 7 days and 28 days curing ages arrived. Six replicate samples of each set were used for testing and calculating the average value as well as standard deviation.

40

20

0 -1 10

10

0

10

1

10

2

10

3

2.4.2. Isothermal calorimetry (ICC) To observe the geopolymerization procedure, a 3114/3236 TAM 83 Air (Thermometric AB) isothermal calorimeter equipped with an internal mixing device was selected to evaluate the reaction kinetics. The pastes were formulated via the internal mixing procedure by 0.8 ml of alkali-activated solution while every 1.000 g of mixed raw materials at 20 °C. The mixed step needs to be carried out after 6 h balance due to the outside heat turbulence brought by the raw materials and ampoules.

Particle size ( m) Fig. 3. Particle size distribution of MK and RHA.

Table 2 Physical properties of MK and RHA. Materials

D50 (mm)

SSA (m2/g)

Particle density (g/cm3)

MK RHA

39.8 36.2

11.2 67.3

2.6 2.3

a trace of calcium aluminum oxide crystalline phases. This result is consistent with its chemical compositions in Table 1. Moreover, the SiO2 in RHA has already been proved to be existed in amorphous state and can be used efficiently [35–37]. 2.2.3. Physical properties Fig. 3 shows the results of particle size distribution (PSD) testing for MK and RHA, obtained by LS 13320 Laser Diffraction Particle Size Analyzer. The particle size of MK is comparable with that of RHA, albeit possessing disparate particle morphologies. In terms of PSD parameters, the D10, D50 and D90 of RHA are 7.4 mm, 36.2 mm and 101.1 mm respectively, while those for the MK are 5.6 mm, 39.8 mm and 161.1 mm. Table 2 lists the basic physical properties of the raw materials used in this experiment [38]. The median particle size (D50) of MK is 39.8 mm, which shows a slightly higher in comparison with that of RHA. The Brunauer-Emmett-Teller (BET) method via nitrogen sorption was determined to probe the internal and external surface area of particles on a Micromeritics TriStar II3020 instrument. The specific surface areas (SSA) of MK and RHA are 11.2 m2/g and 67.3 m2/g, although the particle sizes was comparable, which indicates that the RHA has the higher SSA than MK. It is further confirmed that the existence of porous microstructure significantly elevates the SSA of RHA. Particle density was obtained via the Archimedes method, using kerosene and a specific gravity flask. It needs to note that the particle density is the average value obtained by three replicate tests result. The density of RHA is lower than that of MK, which is due to the extraordinary microstructure (porous and loose) of RHA.

2.4.3. TG and FT-IR The geopolymer powders at the ages of 7 days and 28 days were characterized by Thermogravimetric analysis (TGA), using a TGA STA 4493C analyzer (NETZSCH) by heating samples at a heating rate 10 °C/min from the room temperature to 1000 °C with nitrogen atmosphere. The FTIR spectral analysis of 7 days and 28 days aged samples were determined by Fourier transform infrared spectrometer (American Nicolet STD-11202624D) in the range of 600–4000 cm 1 wavenumbers with ATR method. 2.4.4. Pore structure examination The pore structure of 28 days curing age samples were characterized by a TriStar II 3020 instrument (Micrometritics) according to the Barrett-Joyner-Halenda (BJH) method with nitrogen adsorption techniques. The hardened geopolymer pastes, which were crushed into small granular specimens with the size of around 2–3 mm, were stored in absolute alcohol to terminate hydration of pastes for at least 24 h and then dried at 60 °C in a vacuum state for 10 h. 2.4.5. XRD and SEM The mineral phases of geopolymer samples at each age were obtained by using an X-ray diffractometer (RIGAKU, model D/max) with Cu Ka radiation, scanning at a range of 5–80° and a rate of 6°/min. The hardened pastes were cracked into pieces and selected to store in absolute alcohol for terminating hydration. A field emission scanning electron microscope coupled with Oxford EDS (JEOL, model JSM-7001F) was used to examine the micro-morphology of geopolymer products at 10 kV accelerating voltage and 10 mm working distance.

3. Results and discussion 3.1. Effects of RHA on geopolymerization

2.3. Composition design and synthesis of pastes Six mix designs involving different dosages of MK and RHA were determined for this study, in combination with the previous studies [39–41]. MK, as the main constituent was changed at a manageable ranging between 50% and 100%, whereas RHA contents varied from 0% to 50% (by mass) of the overall binder composition, as illustrated in Table 3. The designed solid precursor blends of above 50% MK were

Table 3 Mix proportions of geopolymer pastes and the liquid to binder mass ratio. Sample ID

MK contents (%)

RHA contents (%)

L/B

Curing condition

M10R0 M9R1 M8R2 M7R3 M6R4 M5R5

100 90 80 70 60 50

0 10 20 30 40 50

0.7 0.7 0.7 0.7 0.7 0.7

50 °C 50 °C 50 °C 50 °C 50 °C 50 °C

steam steam steam steam steam steam

curing curing curing curing curing curing

The roles of rice husk ash on geopolymerization as determined by isothermal calorimetry are plotted in Fig. 4. To explore the influence of rice husk ash in the initial reaction stage, the heat evolution rate of six different mixed pastes collected from the beginning of mixing to the following 12 h are shown in Fig. 4a. The maximum rate of heat evolution among all the pastes occurred in the paste containing 20% rice husk ash, indicating that the bucking effect from duration of SiO2 dissolving to the reactive process was minimum, which creates an optimal reactivity in the whole system and achieve a deeper reaction extent. It could also be observed that the excessive RHA would cut the cumulative heat release (Fig. 4b), meaning that the reaction extent was weaken owing to the lack of Al source available for reaction. However, the appropriate incorporation of RHA accelerates the reactive rate and deepens the hydration to some extent from the perspective of holistic analysis.

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(a) 10

240 80 60 40 20 0 0.0

4

0.1

0.2

0.3

0.4

Initial to Final Initial

200

Setting time (min)

Heat evolution (mW/g)

6

Heat evolution (mW/g)

10% RHA 0% RHA 20% RHA 30% RHA 40% RHA 50% RHA

8

20% RHA 30% RHA 10% RHA 40% RHA 50% RHA 0% RHA

0.5

160

120

Reaction time (h)

80

2

40

0

0 0

2

4

6

8

10

MK

12

M9R1

M8R2

M7R3

M6R4

M5R5

Samples

Reaction time (h)

Fig. 5. The initial and final setting times of the pastes containing different contents of RHA.

(b) 120

initial setting time cumulative heat

90

Initial setting time (h)

60

10% RHA 20% RHA 30% RHA 0% RHA 40% RHA 50% RHA

60

30

3

50

2

Cumulative heat (J/g)

Cumulative heat (J/g)

4

40

0 0

2

4

6

8

10

12

Reaction time (h)

1 0

Fig. 4. Effects of different RHA substitution on (a) heat evolution rate and (b) cumulative heat release during geopolymerization.

10

20

30

40

50

RHA content (mass%) Fig. 6. Variation of cumulative heat released within initial setting period.

The similar results have been found in previous studies [40], implying that the utilization of rice husk ash in metakaolin based geopolymer has an optimal level. At this level, the side effect resulted from the lack of Al source was far less effective than the benefits brought by the deepening of the reaction extent of rice husk ash to geopolymerization. 3.2. Setting time and compressive strength Fig. 5 shows the setting times of the MG pastes as a function of the RHA content. It is clearly illustrated that the setting times were prolonged of various degrees by adding RHA. This phenomenon is probably owing to that the combined actions of the active SiO2 particles and loose layer structure in RHA. The SiO2 with high activity, existed in layered structures, accelerated the hydration process, while the dissolving procedure of SiO2 particles from structures retarded the setting times. Furthermore, the latter played a more significant role than the former during the whole setting process. This result is also consistent with the findings of calorimetry. It is worth noting that the relationships between the initial setting times and cumulative heat released within this period need to be identified (Fig. 6). For the reference M10R0 paste, the heat released within the initial setting time is 45.89 J/g, while the corresponding values of M9R1, M8R2 and M7R3 pastes are 60 J/g, 61.9 J/g and 59.3 J/g respectively, which shows a significant increase in

comparison with M10R0 paste. M6R4 and M5R5 pastes show comparable heat releases with the reference paste. It is probably attributed to the decrease of Al source in MK and increase of Si source in RHA brought by equivalent substitution of MK with RHA, which promotes more formation of gel phases and enriches the variety of gel. This further indicates that the first heat release peak was primarily determined by the gelation, which shows a closely related relationship between initial setting and the corresponding cumulative heat release during this period. The compressive strengths of geopolymer pastes at the ages of 7d and 28d are exhibited in Fig. 7. For M10R0 paste, the compressive strength at 7d is 30.08 MPa and it increases to 46.45 MPa at 28d. A sharp increase in compressive strength with the addition of RHA, the M9R1, M8R2, M7R3, M6R4 and M5R5 pastes show a rise of 46.3%, 62.2%, 65.0%, 63.4% and 65.0% respectively at 7d, while those for the 28d are 10%, 21.7%, 8.9%, 9.5% and 8.4%. All samples containing RHA gained strength appreciably compared with that containing no rice husk ash at 7d, and this situation remains more pronounced in M8R2 at 28d. The similar phenomenon between 7d and 28d could be due to the high reactivity of metakaolin and rice husk ash, resulting to the rapid reaction at early age. The increase in later strength was not obvious owing to the basic accomplishment of reaction at early age. Considering the outstanding optimization in this system, the compressive

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strength could be used as an evaluation of the effects of rice husk ash dosages on the reaction extent of geopolymerization. The changes in type or content of gel would like to happen parallel together with the change of microstructure in the geopolymer pastes when metakaolin was partial replaced by rice husk ash, which could be observed by the strength variation in a macroscopic way. What could not be ignored is that the influence of unreacted phases and the structure of the raw materials particles itself with respect to the integrity of geopolymer matrix, which will be further explored in the next few sections.

60

Compressive strength/MPa

7 days 28 days

50

40

30

20

3.3. Microstructure 10

0 M10R0

M9R1

M8R2

M7R3

M6R4

M5R5

Samples Fig. 7. Effects of RHA contents on compressive strengths of geopolymers at 7 d and 28 d, after curing at 50 °C.

15.0

0.16

0.6

14.5 0.14

14.0 0.12

0.5

0.4

Weight loss 400-600 ºC (%)

250 C 400-600 C 600-800 C

Weight loss 600-800 ºC (%)

Weight loss <250 ºC (%)

(a)

3.3.1. TG and FT-IR analysis Fig. 8 presents mass loss fractions of the geopolymer pastes at 7d and 28d, determined by TGA analysis, as a function of RHA content. The mass loss discussed includes three main temperature regions: below 250 °C, 400–600 °C and 600–800 °C. In the first temperature region, the mass loss attributed to the evaporation of free water and bond water, implying that the denser structure formed in the geopolymer containing over 20% RAH. The mass loss

13.5

M10R0

M9R1

M8R2

M7R3

M6R4

0.10

0.3

0.3

0.6

M5R5

Samples

(b)

16

0.2

14

0.1

13 M10R0

M9R1

M8R2

M7R3

M6R4

M5R5

Samples Fig. 8. Mass loss fractions at different temperature regions of geopolymers at (a) 7 d and (b) 28 d.

0.5

0.4

0.3

0.2

Weight loss 400-600 ºC (%)

15

Weight loss 600-800 ºC (%)

Weight loss <250 ºC (%)

< 250 C 400-600 C 600-800 C

532

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in the 400–600 °C regions is probably assigned to the loss of structural water in gel phases, the change regulation exhibited in this region indicating that the existence of more contents and types of binder gel phase in the pastes containing RHA. All the samples showed distinct mass loss but not big otherness owing to the decarbonation event of CaCO3 at around 700 °C. The unburnt carbon contents brought by the introduction of RHA changed significantly when RHA dosages increased, which contributed to a higher mass loss of samples than that of the reference paste in this region, under the circumstance of identical activator dosage and curing temperature. To better identify the roles of rice husk ash in metakaolin-based geopolymers, the geopolymer samples at 7d and 28d were collected and analyzed via FTIR spectroscopy. Fig. 9 shows that the geopolymer products formed at different curing ages are consistent in major. The broad bands could be observed at around 900–1200 cm 1 obviously, which is ascribed to the stretching vibrations of TAOASi bands (where T = tetrahedral Si or Al). In line

with the previous studies [44–46], it is probable to present the CA(A)ASAH and NAAASAH gel that is centered at around 980 and 1000 cm 1 wavenumbers, respectively. Another interesting observation was that the shift to a higher wavenumber of the Si (Al)AO band with the addition of RHA, suggesting that there were more formation in the gel content and nature. This further indicates that the rice husk ash incorporated to geopolymer contributes more to the formation of gel in comparison with those without the addition of RHA. 3.3.2. Pore structure Fig. 10 plots the pore structure of geopolymer for 28-days cured samples incorporated with different contents of RHA. Most pores of samples were registered in the size below 10 nm in diameter (Fig. 10a), which were observed more noticeable with the increase in RHA. Combined with the volume fraction of pores registering in three size ranges (<10 nm, 20–40 nm, >40 nm) and the distribution-average pores (DAP) of each geopolymer were listed

(a) MK 1001.44

Transmission

M9R1 M8R2

992.59

M7R3

996.82

M6R4

992.95

M5R5

993.27

1004.22

3600

3200

2800

2400

2000

1600

1200

800 1200 1100 1000 900

-1

Wavenumbers (cm )

(b)

Transmission

MK M9R1

994.67

M8R2

991.49

M7R3

992.68

M6R4

997.25

M5R5

995.46

1003.04

3600

3200

2800

2400

2000

1600

1200

800 1200 1100 1000 900

-1

Wavenumbers (cm ) Fig. 9. FTIR spectra of geopolymers after 7 d and 28 d cured age.

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(a)

0.25

3

dV/d(LogD)(cm /g)

0.20

dV/d(LogD)(cm3/g)

0.08

MK M9R1 M8R2 M7R3 M6R4 M5R5

0.04

0.15 0.00 1

2

3

4

5

6

7

8

9

10

Pore diameter D (nm)

0.10

0.05

0.00 10

0

10

1

10

2

Pore diameter D (nm)

Cumulative pore volume (cm3/g)

(b)

0.20 MK M9R1 M8R2 M7R3 M6R4 M5R5

0.16

0.12

0.08

0.04 0

10

1

10

10

2

Pore diameter D (nm) Fig. 10. Effects of RHA contents on the (a) pore size and (b) pore volume distribution of geopolymers as determined by BJH absorption.

in Table 4. It is worthwhile to note that not only would rice husk ash played roles on the refinement of pore distribution but also it could reduce the cumulative pore volume of pastes from 0.1837 cm3/g to 0.1038 cm3/g as the RHA dosages increased (Fig. 10b). The reduction of pore size may be one of the main supports for the improvement of strength in Section 3.2. To better understand the relationship between the pore structure and rice husk ash, the models of pore channel and secondary filling effect were built. As indicated in Fig. 11, the SiO2 particles were dissolved from the layered pore channels in RHA at the alkali activator environment, resulting to more SiO2 micro-particles and empty pore channels emerged. The pore channels provided a space for the occupation and hydration of fine metakaolin particles and the dissolved SiO2 micro-particles acted as filler for the whole pastes while participating in the reaction. The mutualpenetration effect sustained under the combined action of pore channel and secondary filling effect was the assignable cause for the refinement of pore structure and enhancement of compressive strength.

3.3.3. Mineral compositions and morphology The XRD analysis of hardened geopolymers pastes (Fig. 12) after 7d and 28d curing shows the presence of major amorphous components, as reflected by the broad hump from 20 to 40° 2h, including feldspar, gel phases, andalusite and quartz (initially exist in the RHA). On a separate note, there is no distinguishing difference of 7day cured samples with 28-day cured samples. The type and content of gel phases were enriched by the utilization of RHA, which implied an adequate reaction level. Moreover, the unreacted dissolved SiO2 particles have a potential filling effect on the geopolymer system. The compact structure formed in the pastes was not merely original from the filling of unreacted SiO2 but also the existence of feldspar and andalusite. This structure was the primary reason to the refinement of pore in Section 3.3.2. The SEM micrographs and the EDS results of M10R0, M9R1, M8R2, M7R3, M6R3 and M5R5 samples are shown in Fig. 13. Compared to the reference sample, there are noticeably reduced cracks

Table 4 Pore size distribution of the28 days aged geopolymer pastes as determined by BET. Samples

M10R0 M9R1 M8R2 M7R3 M6R4 M5R5

Pore size distribution by volume ratio (%)

DAP (nm)

<10 nm

10–40 nm

>40 nm

86.0 86.4 87.3 86.7 86.4 86.8

12.1 11.8 11.3 11.8 12.2 11.8

1.9 1.8 1.4 1.5 1.5 1.4

Fig. 11. A sketch of dissolution and secondary filling effects from RHA in the reactive procedure.

65.2 69.7 63.2 60.4 58.5 53.7

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(a)

F Q Q-Quartz F-Feldspar C-(C-A-S-H) N-(N-A-S-H) A-Andalusite C N

A M5R5 M6R4 M7R3 M8R2 M9R1 MK

10

20

30

40

50

60

70

80

2 Theta ( ) F Q

(b)

Q-Quartz F-Feldspar C-(C-A-S-H)

N-(N-A-S-H) S-Calcium silicate A-Andalusite

SC N A M5R5 M6R4 M7R3 M8R2 M9R1 MK

10

20

30

40

50

60

70

80

2 Theta ( ) Fig. 12. Cu Ka radiation XRD patterns of the samples with different curing ages: (a) 7d and (b) 28d. Phase identified: Quartz, SiO2, PDF NO. 39-1425; Feldspar, (Ca,Na) (Al,Si)2Si2O8, PDF NO. 20-0528, (Na,Ca)(Si,Al)4O8, PDF NO. 09-0456, NaAlSi3O8, PDF NO. 19-1184, CaAl2Si2O8, PDF NO. 05-0528; Calcium aluminum oxide, CaSi2O5, PDF NO. 15-0130, Andalusite, Al2(SiO4)O, PDF NO. 39-0376, C-A-S-H gel phase, Ca3Al2Si3O12(OH), 34-1417, NAAASAH gel phase, NaAlSiO4
xH2O, 12-247.

and almost no pores could be observed with the addition of RHA. The gels package the particles (especially unreacted SiO2) and link them thickly, making a significant contribution to the development of compact structure. The mixture of gels and particles acts as a potential filler for the cracks in the pastes, which has a more visible effect in M8R2 sample. It is interesting to note that the silica grains uncoated with gels fill into cracks to refine it (Fig. 13e and f), con-

sistent with the assumption of SiO2 particles could fill the pore or crack of pastes in Section 3.3.2. Meanwhile, it is not hard to find that the more gels formed with the increasing of RHA by comparing the EDS testing results of M10R0 sample (Fig. 12a) with that of M8R2 sample (Fig. 12c). Fig. 14 presents the average values of Na/Si and Al/Si calculated by three spots of gel phase of each geopolymer sample. The atomic

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Fig. 13. SEM images and partial EDS patterns of samples after 28-days curing aged.

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Fig. 13 (continued)

Na/Si and Al/Si ratios, varied 0.23–0.6 and 0.24–0.71 respectively, could be used as one of the indicators to evaluate the development of gel phases. The Na/Si ratio increases, while the Al/Si ratio decreases with the increasing RHA content. It indicates that the lack of Al source resulted from the replacement of metakaolin by RHA does not decrease the total quantity of gel phase formation, but appear in another form of gel. The gel phases, abundant not only in contents, but also in type, created a balance to achieve a best comprehensive performance of geopolymer.

4. Conclusions This study aimed at the influence of rice husk ash on the metakaolin-based geopolymer, the roles of RHA played on the reaction kinetics, mechanical property, and microstructure of geoploymer pastes were investigated at a constant liquid/binder ratio. The effective contact surface area between solid particles and alkali activator and the overall reactivity of raw materials were increased by the partial replacement of metakaolin with RHA in the synthesis

H. Zhu et al. / Construction and Building Materials 221 (2019) 527–538 0.7

0.9

Na/Si Al/Si

0.8

0.6

0.6

Al/Si

Na/Si

0.7 0.5

0.5

0.4

0.4 0.3 0.3 0.2

0.2 0

10

20

30

40

50

RHA content (mass%) Fig. 14. The atomic Na/Si and Al/Si ratios of the gels of 28d aged geopolymers.

procedure, which contributed to an excellent mechanical property and a denser structure in geopolymer. The longer reaction duration of initial setting time and completion of the reaction were attributed to the dissolution of SiO2 micro-particles from the layers of RHA. The optimum substitution level of metakaolin by RHA was 20%, where the reaction extent of geopolymerization was deepened accompanied by a beneficial development of compressive strength and refinement of pore structure. At this level, the compressive strengths gain 62.2% and 21.7% increasing at 7-day and 28-day curing age respectively. The content and nature of gel phase were enriched with the incorporation of RHA and the microstructure was refined via the mutual-penetration action supported by pore channel and secondary filling effect. The findings of this study suggest that rice husk ash can appropriately be used a supplementary cementitious material in metakaolin-based geopolymer. Declaration of Competing Interest None. Acknowledgements Acknowledged financial supports include the National Natural Science Foundation of China (51502259, 51572234), the ‘‘Six Top Talents” program of Jiangsu Province (2016-XCL-070), Postgraduate Practical Innovation Project of Jiangsu Province (SJCX18_0885), the Science and technology project from Ministry of Housing and Urban-Rural Development of the People’s Republic of China (2015-K4-007), the General Program of Natural Science Foundation of the Higher Education Institutions of Jiangsu Province of China (No. 16KJB430030) and Joint Open Fund of Jiangsu Collaborative Innovation Center for Ecological Building Material and Environmental Protection Equipments and Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province (JH201827). References [1] P. Duxson, J.L. Provis, Designing precursors for geopolymer cements, J. Am. Ceram. Soc. 91 (2008) 3864–3869. [2] B. Zhang, H. Tan, W. Shen, G. Xu, B. Ma, X. Ji, Nano-silica and silica fume modified cement mortar used as Surface Protection Material to enhance the impermeability, Cem. Concr. Compos. 92 (2018) 7–17. [3] S. Zhang, A. Keulen, K. Arbi, G. Ye, Waste glass as partial mineral precursor in alkali-activated slag/fly ash system, Cem. Concr. Res. 102 (2017) 29–40. [4] C. Shi, A. Fernández-Jiménez, A. Palomo, New cements for the 21st century: The pursuit of an alternative to Portland cement, Cem. Concr. Res. 41 (2011) 750–763.

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