Influence of fly ash on the pore structure and shrinkage characteristics of metakaolin-based geopolymer pastes and mortars

Construction and Building Materials 153 (2017) 284–293

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Influence of fly ash on the pore structure and shrinkage characteristics of metakaolin-based geopolymer pastes and mortars Tao Yang a, Huajun Zhu a,⇑, Zuhua Zhang b,⇑ a b

School of Materials Engineering, Yancheng Institute of Technology, Yancheng, Jiangsu 224051, China Centre of Future Materials, University of Southern Queensland, West Street, Toowoomba, Queensland 4350, Australia

h i g h l i g h t s
The porosity of the geopolymer decreases with the level of fly ash replacement.
The fly ash substitution leads to higher autogenous shrinkage and lower drying shrinkage.
The geopolymer mortars provide lower total shrinkage than the pastes.

a r t i c l e

i n f o

Article history: Received 8 January 2017 Received in revised form 3 May 2017 Accepted 6 May 2017

Keywords: Geopolymer cement Metakaolin Fly ash Shrinkage Pore structure

a b s t r a c t This study investigates the pore structure and shrinkage behavior of metakaolin-based geopolymer pastes and mortars containing 0–30% fly ash. Fly ash substitution decreases average reactivity of the solid precursors, resulting in a lower reaction rate and accompanying longer reaction time. Composition of the sodium aluminosilicate (N-A-S-H) gel formed in the geopolymers has been changed, and the continued reaction after hardening of the pastes generates a more compact binding gel phase with lower Al/Si ratio. Refinement of the pore structure entails a higher capillary tension developed in the binders to increase the autogenous shrinkage, but also restricts the water evaporation from the pore networks, resulting in a decreased drying shrinkage. The geopolymer mortars provide higher compressive strength and lower total shrinkage when compared with the pastes. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Geopolymers, as a family of alkali activated aluminosilicate materials, has gained worldwide interests in the last two decades, due to the main driver of promising a sustainable alternative to Portland cement [1–3]. Precursors used in geopolymers manufacture include calcined clays and various Si- and Al-containing industrial byproducts [4–9], among which metakaolin has higher reactivity and chemical consistency than the others, and has potential to synthesis good thermal resistance [10] and low permeability geopolymers [11,12]. However, there are also drawbacks in the metakaolin-based geopolymers. High specific surface area of the platy metakaolin particles leads to excessive mixing water demand and high yield stress [13]. Moreover, the high water/binder ratio will have apparent deleterious effects on the pore structure, durability and efflorescence of the geopolymer products [11,13]. ⇑ Corresponding authors. E-mail addresses: [email protected] (H. Zhu), [email protected] (Z. Zhang). http://dx.doi.org/10.1016/j.conbuildmat.2017.05.067 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.

Fly ash is a solid waste, and has become one of the major materials in the production of geopolymers [14]. Its spherical particles could work as ‘‘ball bearing” to reduce viscosity of the paste, and as ‘‘micro-aggregate” to improve particle packing to refine pore structure of the binder [13,15]. Many research studies have been aimed at the reaction kinetics, binder chemistry and microstructures of both metakaolin and fly ash geopolymer systems [16–21]. The replacement of fly ash for metakaolin in designing geopolymer mixes usually provides economic potential and good engineering properties [16,22–24]. However, only limited data has been reported at shrinkage characteristics of the metakaolin-based geopolymers with fly ash as a secondary source precursor. Shrinkage of the alkali-activated materials has been regarded as a serious problem for practical application [25–27]. Collins and Sanjayan [25] reported that pore structure was an essential parameter in determining drying shrinkage of alkali-activated slag concrete. Refinement of the microstructure in the concrete with heat-cuing could restrict water loss during drying, resulting in a reduction in the drying shrinkage [27]. Thus, modifying the pore size distribution should be desirable for mitigating the shrinkage

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behavior and crack propagation of the geopolymers [26]. In this study, metakaolin-based geopolymer pastes and mortars containing 0–30% fly ash were prepared to investigate the pore structure and shrinkage behavior of the products. Reaction process, elemental compositions and microstructures were characterized through isothermal conduction calorimetry (ICC), X-ray diffractometry (XRD), mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS). Understanding of the shrinkage behavior of metakaolin-fly ash geopolymer pastes and mortars will provide useful information for future commercial-scale development. 2. Experimentals 2.1. Materials Metakaolin powder was supplied by Taojinfeng New Materials Co. Ltd. (China) and produced by heating kaolin clay powder at 700 °C for 1 h. Fly ash was supplied by Xuzhou Guohua Power Station (China). Their compositions as detected by X-ray fluorescence (XRF) are given in Table 1. The particle size distribution parameters D10, D50 and D90 of metakaolin as determined by laser diffraction are 1.4, 5.9 and 17.0 lm, while those for fly ash are 3.3, 14.5 and 82.7 lm. Alkaline activator was prepared by blending sodium silicate solution (Na2O = 12.8 wt%, SiO2 = 30.3 wt%, silicate modulus SiO2/Na2O = 2.45) with sodium hydroxide pellets (96 wt% purity) and distilled water to reach a combined modulus of 1.4 and concentration of 30 wt% (the mass content of SiO2 and Na2O in solution). This activator was allowed to equilibrate to room temperature prior to use. 2.2. Geopolymer preparation Table 2 shows mixing proportions of the precursors and activator. The precursors were mixed with the activator solution for 5 min. The constant liquid to binder ratio (L/B) of 0.62 gave a good workability during mixing. The pastes containing 0, 10, 20 and 30 wt% fly ash were denoted as P0, P10, P20 and P30. The labels ‘M’ and ‘S’ represented the geopolymer mortars prepared with sand to binder ratios (S/B) of 0.5 and 1.0, respectively. The specimens were cast into 30  30  30 mm for measuring compressive strength, and 20  20  80 mm for measuring shrinkage behavior and residual water. The specimens were cured at the ambient conditions, sealed, and demolded after 24 h. 2.3. Testing and measurement Shrinkage was reported by measuring six specimens to obtain an average value. The specimens were demolded at 24 h, and then cured at a constant temperature of 24 ± 2 °C and 45 ± 5% relative humidity (RH) during the measuring periods. Polyethylene film was used to wrap the specimens for the autogenous shrinkage tests with the purpose of preventing moisture egress during curing. The shrinkage strain was evaluated in accordance with the specifications of ASTM C 490, using a length comparator along the longitudinal axis at ages of 36 h to 50 days. Linear shrinkage was determined from Eq. (1), where Lin (mm) is the demoulded length,

Table 2 Mix proportions of the geopolymer pastes and mortars, and their compressive strengths after 50 days of the autogenous shrinkage experiment curing. Compressive strength is reported as mean and standard deviation among 6 replicate specimens. Mixtures

FA contents

S/B

Compressive strength (MPa)

P0 P10 P20 P30 M0 M10 M20 M30 S0 S10 S20 S30

0% 10% 20% 30% 0% 10% 20% 30% 0% 10% 20% 30%

0 0 0 0 0.5 0.5 0.5 0.5 1.0 1.0 1.0 1.0

15.2 ± 1.8 15.2 ± 1.4 17.6 ± 1.5 18.7 ± 1.2 13.6 ± 1.6 13.5 ± 1.5 16.5 ± 1.1 18.7 ± 1.1 15.5 ± 1.4 16.3 ± 1.3 15.8 ± 1.0 18.4 ± 1.0

Lfi (mm) is the measured length and 75 (mm) is the effective length of the specimens without two head nails.

Linear shrinkage ¼

Lin  Lfi  100% 75

ð1Þ

Mass change was also measured with the same curing conditions as the drying shrinkage test. The weight percentages (wt%) of residual water in the specimens were calculated via Eq. (2), where Wh (g) is the initial weight of water in the specimens, DW (g) is the total weight change and Ws (g) is the total weight of the specimens.

Residual water ¼

W h  DW Ws

ð2Þ

The geopolymerization process was analyzed using a 3114/3236 TAM 83 Air isothermal conduction calorimeter (Thermometric AB, Sweden) at 20 °C by an internal mixing procedure [28]. The solid precursors and geopolymer samples were tested by X-ray diffraction (XRD) using a Thermo ARL9900 machine with Co Ka radiation, with a scanning rate of 2.4°/min from 8 to 80° 2h, which needed 30 min to obtain a complete diffractogram. The morphologies of polished samples were analyzed using A ZEISS EVO MA18 scanning electron microscope (SEM) with back-scattered electron (BSE). An equipped energy dispersive spectroscopy (EDS) was used to conduct elemental composition analysis. Samples were coated with gold. Mercury intrusion porosimetry (MIP) analysis was conducted using a Poremaster GT-60. The specimens were crushed into granular samples of 1 mm and then dried at 60 ± 2 °C for 6 h, which is expected to have little effect on pore structure of the samples [29]. A WHY-200 Auto Test Compression Machine was used to test the compressive strengths of specimens under autogenous-curing conditions at age of 50-day. 3. Results and discussion 3.1. ICC analysis Fig. 1 shows the isothermal conduction calorimeter data of the geopolymerization. From Fig. 1a, the fly ash substitution decreases the maximum heat evolution rate in the first peak, which

Table 1 Compositions of metakaolin and fly ash by X-ray fluorescence analysis. LOI is loss on ignition at 1000 °C, wt%.

Metakaolin Fly ash

SiO2

Al2O3

CaO

MgO

Fe2O3

K2O

TiO2

Na2O

LOI

55.87 53.00

42.25 30.58

0.04 4.57

0.04 1.25

0.38 3.81

0.31 1.43

0.20 1.08

0.26 0.52

0.61 2.29

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corresponds to the dissolution of solid precursors. This is consistent with the modeling study of Provis et al. [30], who reported that the dissolution rate of metakaolin was markedly faster than that of fly ash at similar reaction temperature, because the platy metakaolin particles exhibited a larger contact surface area with alkaline solution than the spherical fly ash particles. The molecular structure of metakaolin particle also differs from the glass surface of fly ash particle. In addition, the maximum rate in P30 with 30% fly ash is higher than that in P20 with 20% fly ash. This is probably attributed to a dilution effect occurred in P30, where the dissolution of less-reactive 30% fly ash could not consume as much of the activator, resulting in a higher availability of the activator for reaction with the remained 70% metakaolin in P30. As reaction progresses, the heat evolution rate declines. The dissolved silicate and aluminate unites require time to reach a critical concentration for further condensation. After this induction period, polymerization between the dissolved species takes place and the second heat release peak appears. P0 of the pure metakaolin sample shows the highest maximum heat evolution rate in the second peaks, consistent with the more extensive dissolution of metakaolin. Fly ash substitution prolongs the induction period, and causes the second peak to be achieved later by extending the time required to reach the level of supersaturation for polymerization. Fig. 1b shows the cumulative heat released by the geopolymer systems within the first 72 h of reaction. The heat release of P0 is

(a) 2.0

16

P0 P10

Heat evolution (mW/g)

Heat evolution (mW/g)

12

1.5

1.0

P30 P20 8

4

0 0.0

P0

0.5

0.1

0.2

0.3

0.4

0.5

Reaction time (h)

P30

P20 P10

the highest in the samples. The glassy structure of fly ash is less energetically strained than the disrupted larger structure of metakaolin, so the fly ash substitution results in a reduction in the reactivity of solid precursors. However, the total heat release will not necessarily decline as the fly ash content increases. P30 shows more heat release (79.9 J/g) than P10 (74.0 J/g) and P20 (73.6 J/g), and is also approaching P0 (85.2 J/g) at the point of 72 h. This may be due to that the delayed development of a dense gel in the binder with less-reactive fly ash enables less-hindered mass transport to take place for a longer time, and the polymerization process is prolonged.

3.2. XRD analysis Fig. 2 shows the X-ray diffractograms of the unreacted metakaolin and fly ash. The unreacted metakaolin contains main crystalline phases of kaolinite (Al2Si2O5(OH)4, PDF # 01-0527) and a small quantity of quartz (SiO2, PDF # 85-0930). The unreacted fly ash contains crystalline phases of mullite (Al4.56Si1.44O9.72, PDF # 79-1458), quartz, magnetite (Fe3O4, PDF #39-1346) and hematite (Fe2O3, PDF # 85-2599). Due to the structural difference between the amorphous components in the two solid precursors, their aluminosilicate broad humps appear in different positions (metakaolin, between 22° and 30° 2h; fly ash, between 20° and 35° 2h). X-ray diffraction data collected from the 7 days autogenouscured geopolymer pastes are presented in Fig. 3a. The broad humps observed in metakaolin move to a slightly higher angel in the range between 22° and 40° 2h after geopolymerization, which corresponds to the formation of disordered aluminosilicates binder gel [31,32]. The unreacted metakaolin and fly ash make a contribution to the crystalline components of kaolinite, mullite and quartz, and there is not any new crystalline reaction product formed. Fig. 3b shows the X-ray diffractograms of the 50 days autogenous-cured geopolymer pastes. The diffraction patterns remain nearly constant as the curing age extends from 7 days to 50 days (Fig. 3a and b), indicating that the prolonged curing (after 7 days age) does not change the mineralogical phases. In addition, no significant crystallographic difference is observed between the diffraction patterns of the autogenous- and drying-cured samples, as shown in Fig. 3b and c.

0.0 0

12

24

36

48

60

72

Q

Reaction time (h)

(b)

100

M-Mullite Q-Quartz K-Kaolinite H-Hemitite G-Magnetite

M P30

Cumulative heat (J/g)

80

Q P0

60

M

MM

M

M

GM

M MQ

K

P10

H

M MQ

M

FA

K P20

40

Q K K

20

Q

K KK KK

K

K

K

KK K

0 0

12

24

36

48

60

MK

72

Reaction time (h)

5

10

15

20

25

30

35

40

45

50

55

60

65

2 Fig. 1. Effects of fly ash substitution on (a) heat evolution rate and (b) cumulative heat release during geopolymerization.

Fig. 2. Co Ka radiation XRD patterns of metakaolin and fly ash.

70

75

80

85

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T. Yang et al. / Construction and Building Materials 153 (2017) 284–293 K

(a) K

(a)

Curing scheme Autogenous-7 d M-Mullite Q-Quartz K-Kaolinite

Q

KQK

K KK K K

K

K

K

KK

Cracks

K

K

P0

MK

P10

Cracks

M M

M

5

10

15

20

P20

25

30

35

M

M

M M

M 40

45

50

55

60

65

P30

70

75

80

85

2

(b)

K

(b) K

Curing scheme Autogenous-50 d

KQK

K KK K K

K

MK

M-Mullite Q-Quartz K-Kaolinite

Q

K

K

FA shell

K

KK

K

P0 P10

M

P20 M

M M

M

M

M

M

P30 5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

Fig. 4. BSE images of the 50 days autogenous-cured geopolymer pastes: (a) P0 and (b) P30.

2 K K

Curing scheme Drying-50 d

K KK K K

K

P0 P30

M-Mullite Q-Quartz K-Kaolinite

Q

KQK

0.8

K

K

0.6

K

KK

K

P0 P10

M

5

10

15

20

P20 M

MM

M

M

M 25

30

35

40

45

50

55

60

65

Na/Al

(c)

0.4

0.2

M

70

P30 75

80

85

2 Fig. 3. Co Ka radiation XRD patterns of the geopolymer pastes with different curing schemes: (a) 7 days autogenous-curing, (b) 50 days autogenous-curing and (c) 50 days drying-curing.

3.3. SEM-EDS analysis BSE imaging of the 50 days-cured P0 sample (Fig. 4a) presents paste areas (darker gray regions) and embedded remnant meta-

0.0 0.0

0.2

0.4

0.6

0.8

1.0

Al/Si Fig. 5. Atomic Na/Al versus Al/Si ratios of the gels in the 50 days autogenous-cured P0 and P30.

kaolin particles (light gray regions as highlighted by the dash cycles). The incomplete reaction of metakaolin particles is also observed in other metakaolin-based geopolymers cured at room temperature [33]. There are micro-cracks (around 10 lm)

288

(c)

40

40

Curing scheme Drying-50 d

Curing scheme Autogenous-7 d Cumulative porosity (volume %)

Cumulative porosity (volume %)

(a)

T. Yang et al. / Construction and Building Materials 153 (2017) 284–293

P10

30

P30 P0 P20

20

10

0 1000

100

P10

30

P30 20

10

0 1000

10

100

(d)

40

P0

P10

P20 P30 20

10

0 1000

40

Curing scheme Autogenous-50 d Cumulative porosity (volume %)

Cumulative porosity (volume %)

Curing scheme Autogenous-50 d 30

10

Pore size (nm)

Pore size (nm)

(b)

P20

P0

100

10

30

S0

S10

20

S30

10

0 1000

100

Pore size (nm)

Cumulative porosity (volume %)

(e)

S20

10

Pore size (nm)

40

Curing scheme Drying-50 d 30

S0 S20

20

S10

S30

10

0 1000

100

10

Pore size (nm) Fig. 6. Pore size distribution and porosity of the (a) 7 days autogenous-cured pastes, (b) 50 days autogenous-cured pastes, (c) 50 days drying-cured pastes, (d) 50 days autogenous-cured mortars and (e) 50 days drying-cured mortars.

appearing inside and around the remnant metakaolin particles. This is probably attributed to that the weak platy metakaolin particles act as failure point when the contraction stress forms between the particles and developing gels during curing. In comparison, P30 (Fig. 4b) generates a more compact binding gel phase,

and non cracks is present in it. The binding gel phase adheres as closely to the surfaces of remnant metakaolin and fly ash particles, resulting from the prolongation of polymerization during hardening of the binder. These ongoing reaction and gel evolution are also observed in other studies of fly ash-based geopolymers [13,34].

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EDS results for the atomic Na/Al ratio against Al/Si ratio of the binder gels in P0 and P30 are shown in Fig. 5. The Na/Al ratios of P0 are over the range of 0.14–0.39, with average value of 0.24 (0.10 as standard deviation of 10 spots), and those of P30 are over the range of 0.21–0.60, with average value of 0.40 (0.14). The relatively large standard deviation indicates that total Na+ cations as the negative charge compensators are clearly less correlative with the total [AlO4]- number in the gels, because Al has various polymerization status [35]. The Al/Si ratios of the gels in P0 and P30 distribute relatively concentrated. The measured Al/Si ratios of P0 are over the range of 0.62–0.79, with average value of 0.72 (0.06), consistent with the range in other sodium silicate activated metakaolin systems [16]. The replacement of 30% fly ash for metakaolin slightly reduces the Al/Si ratios in the gels of P30 to the range of 0.52–0.69, with average value of 0.61 (0.05). This is mainly due to that metakaolin contains more reactive Al2O3 content than fly ash [28]. The kinetics of geopolymerization is highly influenced by the availability of reactive Al and Si in solid precursors [35]. During the dissolution stage, Al is more rapidly dissolved in the alkaline medium to form the Al-rich gel phase at short reaction times [35]. Subsequently, the Al-rich gel provides a rapid coverage of the remnant precursor particles during the induction period, which restricts the dissolution of Al and Si monomers for the following polymerization stage, and then produces a substantial slowdown of the reaction kinetics [35]. This is consistent with the ICC results, where the fly ash substitution leads to the prolongation of the polymerization process, suggesting the formation of a more compact microstructure with higher Al/Si ratios. 3.4. Pore structure and compressive strength analysis The pores in geopolymers often consists of three classes: gel pores, formed in interstices of the aluminosilicate gel phase (2 nm  r  50 nm); capillary pores, the remains of originally water-filled spaces (10 nm  r  1000 nm); the air voids and hollow shell of fly ash particles (1 lm  r) [17,23]. The removal of water from the capillary pores determines shrinkage behavior of the geopolymers [25]. Thus, the porosity with pore sizes ranging from 10 to 1000 nm is defined as ‘‘effective porosity” in this study. Fig. 6 presents the measured pore size distribution of the geopolymer pastes and mortars with different curing schemes.

The pores in all of the systems distribute similarly in the range from 10 to 200 nm in diameter, which corresponds to the gel pores and the capillary pores. Table 3 lists the effective porosity, distribution-maximal pores (DMP) and the pore volumes in different size ranges in each sample. For the autogenous-cured geopolymer pastes (Fig. 6a and b), extending the curing time from 7 days to 50 days remarkably decreases the pore sizes and effective porosity, due to the gel filling effect as the geopolymerization proceeds [17,23]. There is a slight increase in the effective porosity of the drying-cured pastes when compared with the autogenous-cured pastes (Fig. 6b and c). Exposure to 45 ± 5% RH without sealing promotes the water loss and shrinkage of the specimens, and then widens the capillary pores. The geopolymer mortars formulated with S/B ratio of 1.0 (Fig. 6d and e) report the notably lower effective porosities when compared with the pastes, up to around 10.0% porosity difference. Sand addition as reinforcements in the paste contributes to a supportive network with a decreased void volume [36]. The pore structure of the geopolymers is also largely dependent on the fly ash contents. For the 50 days-cured pastes, the fly ash substitution reduces the effective porosity slightly, and 30% fly ash content causes significant decreases of DMP and the volume fraction of large capillary pores in 200–1000 nm. The gel phase with lower Al/Si ratio formed in P30 enables a more compact microstructure, which reduces the permeability resulting from water-filled spaces without gel filling, and leaves more small capillary pores distributed in the framework. For the geopolymer mortars, their effective porosities are much lower than those of the pastes, because the total volume of the detected gel pores and capillary pores formed in the gel phases is reduced when the geopolymer binder is replaced by the incorporated sand. Additionally, it is interesting to note that 10% fly ash content also reduces the effective porosity of the sample. The mechanism behind this trend is unclear, which is probably due to the influence of reinforcements with sand addition. The compressive strengths of the 50 days autogenous-cured geopolymer pastes and mortars increase with the fly ash content, as shown in Table 2. This is consistent with the SEM and MIP results, where the fly ash substitution leads to a more compact microstructure. For the geopolymer mortars formulated with S/B ratio of 1.0, S10 also possesses higher compressive strength than

Table 3 Pore size distribution and porosities of the geopolymer pastes and mortars as determined by MIP. Samples

Curing scheme

Pore size distribution by volume ratio (%) 10–50 nm

P0 P10 P20 P30 P0 P10 P20 P30 P0 P10 P20 P30 S0 S10 S20 S30 S0 S10 S20 S30

Autogenous  7 d Autogenous  7 d Autogenous  7 d Autogenous  7 d Autogenous  50 d Autogenous  50 d Autogenous  50 d Autogenous  50 d Drying  50 d Drying  50 d Drying  50 d Drying  50 d Autogenous  50 d Autogenous  50 d Autogenous  50 d Autogenous  50 d Drying  50 d Drying  50 d Drying  50 d Drying  50 d

11.5 9.3 11.6 10.0 9.5 10.2 11.9 16.1 10.9 10.3 11.6 15.2 12.1 9.9 11.7 15.1 12.3 10.4 11.4 13.3

50–100 nm 12.0 9.6 14.6 9.3 9.0 8.5 11.8 20.3 10.3 9.2 12.7 20.6 10.7 7.7 10.1 14.3 11.2 9.9 8.9 14.6

100–200 nm

200–1000 nm

38.9 37.6 52.4 35.6 37.2 27.0 40.0 62.2 59.1 33.4 40.0 63.3 64.6 34.8 41.8 49.6 62.7 48.0 27.0 51.7

37.6 43.5 21.4 45.1 44.2 54.3 36.4 1.4 19.8 47.1 35.7 0.9 12.6 47.5 36.3 21.0 13.8 31.6 52.8 20.5

Effective porosity (%)

DMP (nm)

38.9 38.6 37.5 37.0 36.1 35.4 35.3 34.3 36.2 35.7 35.7 35.3 28.1 25.1 27.2 25.0 28.5 25.1 27.8 25.1

199 201 182 206 203 220 198 128 191 204 196 127 161 206 196 172 169 187 216 177

T. Yang et al. / Construction and Building Materials 153 (2017) 284–293

(a) 600

-6

Autogenous shrinkage ( 10 )

400

200

0

-200

P0 P10 P20 P30

-400

-600 0

10

20

30

40

50

Time (days)

(b)

600

400 -6

Autogenous shrinkage of the geopolymers is a reduction in volume due to self-desiccation and chemical shrinkage [37]. Selfdesiccation means that the continuing geopolymerization removes pore water from the capillary pores to form the water-air menisci, resulting in a capillary pressure of the pore fluid in hardened binder [37,38]. Chemical shrinkage is the volume change resulting from the polymerization and reorganization of the aluminosilicate paste in the fresh state [28,39]. In this study, the autogenous shrinkage of the geopolymers was measured after 24 h of casting, so the chemical shrinkage mainly occurred in the fresh state will not be discussed in the following part. Fig. 7 shows the autogenous shrinkage of the geopolymer pastes and mortars. In Fig. 7a, the autogenous shrinkage of the geopolymer pastes increases with the fly ash content, from 123  106 mm/mm to 480  106 mm/mm. According to the MIP results, a pore structure refinement of the sample with fly ash substitution leads to an increase in pore fluid surface tension. The higher capillary stress entails a higher autogenous shrinkage strain, which is related to the internal RH drop during selfdesiccation of the geopolymers [40]. In comparison with the autogenous shrinkage of 60 days-cured Portland cement pastes (approx. 450–2300  106 mm/mm), the data of the geopolymer pastes is much lower [40]. It is should be noted that the autogenous shrinkage curves of the pastes evolve in two distinct stages: the expansion behavior during the initial curing age, and the following progressive increase of the shrinkage strain, resulting in the final shrinkage behavior. Melo Neto et al. [37] also observed the expansion behavior in alkali-activated slag mortars. They concluded that the initial geopolymerization drove pore fluid to the sites where extensive self-desiccation happened, and this transportation increased the internal RH of partial regions to diminish the capillary stress that caused shrinkage [37]. The autogenous shrinkage decreases from the range of 123– 480  106 mm/mm for the pastes to the range of 87– 294  106 mm/mm for the mortars, as shown in Fig. 7b and c. The non-shrinkage sands act as inside filler in the pastes to decrease contraction forces in the samples. The autogenous shrinkage of the mortars increases with the fly ash content, while the autogenous shrinkage of S10 is higher than that of S20. This is probably due to that S10 possesses a more refined pore structure. Fig. 8 shows the drying shrinkage of the geopolymer pastes and mortars. This shrinkage behavior is due to the evaporation of internal water from pore network of the binder to the external environment with relatively lower humidity level. During the drying process, a capillary stress is formed in the capillary water with menisci, resulting in the shrinkage strain. Most of the drying shrinkage takes place within the first day, because of the rapid internal RH loss from the freshly formed surface of the specimens after demoulding. Table 4 lists the autogenous shrinkage-to-drying shrinkage ratios of the geopolymers, over the range of 2.0–15.2%, which means that the volume change is mainly due to the drying shrinkage rather than the autogenous shrinkage. This is in contrast to the results from the slag-based systems reported by Melo Neto et al. [37], where the autogenous shrinkage represented above 30% of the drying shrinkage. It is probably attributed to that the autogenous shrinkage (87–480  106 mm/mm) of metakaolin-based

Autogenous shrinkage ( 10 )

3.5. Autogenous and drying shrinkage analysis

geopolymers is much lower than that (approx. 500– 2500  106 mm/mm) of slag-based geopolymers [37,38]. The total drying shrinkage of the geopolymers decreases as the fly ash content increases. This suggests that the refinement of pore structure should have restricted the evaporation of internal water during drying. As shown in Fig. 8c, the drying shrinkage of S10 is lower than that of S20, as the pore structure of S10 is more refined. This fact also supports the enhanced relationship between the pore structure and drying shrinkage of the geopolymers. In addition, adding non-shrinkage sand particles also reduces the drying

200

0

-200

M0 M10 M20 M30

-400

-600 0

10

20

30

40

50

Time (days)

(c) 600 400 -6

S0 and S20, due to the lower effective porosity. Quite unexpected, the geopolymer pastes and mortars develop relatively similar compressive strengths. Although adding the sand particles decreases the effective porosity, the weaker matrix also dominates strength development of the specimens.

Autogenous shrinkage ( 10 )

290

200

0

-200

S0 S10 S20 S30

-400

-600 0

10

20

30

40

50

Time (days) Fig. 7. Autogenous shrinkage of the geopolymer pastes and mortars formulated with different S/B ratios: (a) 0, (b) 0.5 and (c) 1.0.

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(a) 7000

Table 4 Comparison of the autogenous and drying shrinkage of the 50 days-cured geopolymer pastes and mortars (%).

-6

Drying shrinkage ( 10 )

6000

5000

4000

3000

P0 P10 P20 P30

2000

1000

Specimens

Autogenous (106)

Drying (106)

A/D

P0 P10 P20 P30 M0 M10 M20 M30 S0 S10 S20 S30

123 207 293 480 107 164 187 294 87 137 173 260

6113 5713 5370 4130 4804 3907 3689 2638 2460 2087 2263 1713

2.0 3.6 5.5 11.6 2.2 4.2 5.1 11.1 3.5 6.6 7.6 15.2

0 0

10

20

30

40

50

Time (days)

6000

-6

Drying shrinkage ( 10 )

13.87

M0 M10 M20 M30

5000

13.68

14

13.67

13.66

10

10.37

10.34

4000

3000

8.46

8.25

2000

10.04

8.32

6 4

8.23

2

0 Mortars S/B =

1.0

0 0

10

20

30

40

50

Mortars S/B

=0.5

5000

FA

FA %

FA

Pastes

0%

6000

10

S0 S10 S20 S30

%

30

%

(c) 7000

20

FA

Time (days)

-6

8

10.01

1000

Drying shrinkage ( 10 )

12

)

7000

Water Contents (%

(b)

Fig. 9. Calculated water contents remaining in the 50 days drying-cured geopolymer pastes and mortars. Calculated water content is reported as mean and standard deviation among 6 replicate specimens.

4000

3000

2000

1000

0 0

10

20

30

40

50

Time (days) Fig. 8. Drying shrinkage of the geopolymer pastes and mortars formulated with different S/B ratios: (a) 0, (b) 0.5 and (c) 1.0.

shrinkage of the mortars. In comparison, the data of the geopolymer mortars (1713–4804  106 mm/mm) is higher than that of 28 days cured-Portland cement mortars with S/B of 2.0 (approx. 500  106 mm/mm) [38], meaning that a better volume stability under drying conditions is found in the Portland cement. 3.6. Water loss analysis Fig. 9 shows the water contents remaining in the geopolymer pastes and mortars dried for 50 days. Sand addition reduces the

residual water contents, due to the decrease of initial mixed water in the mixture with constant L/B of 0.62. The fly ash substitution restricts the internal water loss of the geopolymers on account of the lower effective porosity. This is in agreement with the speculation about the drying shrinkage stain presented in Section 3.5. The autogenous and drying shrinkage strains of the 50 dayscured geopolymer pastes are plotted versus the effective porosity respectively in Fig. 10. A direct correlation between the shrinkage behavior and the pore structure is observed in the geopolymers. A lower effective porosity entails a higher capillary stress in the pore network of the binder to increase the autogenous shrinkage strain. Ma and Ye [41] reported similar results for sodium silicateactivated fly ash materials, where the pastes activated with the higher sodium and silica contents presented a finer pore structure and a higher autogenous shrinkage. However, they concluded that the continuous reorganization and polymerization of the N-A-S-H gel phase induced the autogenous shrinkage, instead of the selfdesiccation, because an increase and the final maintenance of a high stable internal RH were observed in the geopolymer binder [41]. In order to clarify the above arguments about the autogenous shrinkage mechanism, further research should be focus on the

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7000

600

-6

6000 400

300

Autogenous shrinkage Drying shrinkage

5000

200

-6

4000

Drying shrinkage ( 10 )

Autogenous shrinkage ( 10 )

500

100

0 33.5

34.0

34.5

35.0

35.5

36.0

3000 36.5

Effective porosity (%) Fig. 10. Correlations between autogenous shrinkage, drying shrinkage and effective porosity of the 50 days-cured geopolymer pastes.

changes of the internal RH and the pore fluid capillary stress of the geopolymers at early ages. Unlike the Portland cement paste, where the mixing water is chemically bound in the C-S-H gel phase through hydration, a large amount of free water is physically incorporated into the N-A-S-H gel phase formed in geopolymers [42]. It means that the excessive water is easy to evaporate from the binder to cause extensive drying shrinkage under low RH conditions. This structural stability problem in the geopolymer products is well documented [27,37,38,42], especially in both metakaolin and fly ash systems with coarser pore structures allowing for more water absorption during saturation and more water evaporation during drying [27,42]. In this work, the volume change of the metakaolin-based geopolymers is also mainly due to the drying shrinkage rather than the autogenous shrinkage. A more refined pore structure developed in the geopolymers with fly ash substitution restricts the water evaporation from the hardened paste, and reduces the susceptibility of the binder to drying shrinkage. Therefore, the replacement of fly ash for metakaolin in designing geopolymer mixes shows a possibility of decreasing the drying shrinkage strain and resulting structural stability problem of the geopolymer products. 4. Conclusions Metakaolin-based geopolymer pastes and mortars containing 0–30% fly ash were prepared to investigate the pore structure and shrinkage behavior of the products. The partial replacement of fly ash for metakaolin decreases the average reactivity of the solid precursors, resulting in a prolongation of the polymerization stage. This promotes densification of the binding gels prior to hardening, and then a more compact N-A-S-H gel phase with lower Al/ Si ratio is formed. The decrease of the effective porosity entails a higher capillary tension developed in the capillary pores to increase the autogenous shrinkage, but also restricts the water evaporation from the pore networks, which leads to the decreased drying shrinkage strain. The volume change of the metakaolinbased geopolymer products is mainly due to the drying shrinkage rather than the autogenous shrinkage. The replacement of fly ash for metakaolin in designing geopolymer mixes shows a possibility of decreasing the drying shrinkage strain and resulting structural stability problem of the geopolymer products.

Acknowledgements Acknowledged financial supports include the National Natural Science Foundation of China (51502259), the Australian Research Council linkage Project (LP130101016), the joint research fund between Collaborative Innovation Center for Ecological Building Materials and Environmental Protection Equipments and Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province (GX2015305 and CP201506), and the ‘‘Six Top Talents” Program of Jiangsu Province (2016-XCL-070).

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