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EFFECTS OF TYPE OF ACTIVATOR ON FIBER-MATRIX INTERFACE PROPERTIES

AND TENSILE BEHAVIOR OF STRAIN-HARDENING GEOPOLYMER COMPOSITES

Conference Paper · October 2018

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EFFECTS OF TYPE OF ACTIVATOR ON FIBER-MATRIX INTERFACE PROPERTIES AND TENSILE BEHAVIOR OF STRAIN-HARDENING GEOPOLYMER COMPOSITES

Behzad Nematollahi1 and Jay Sanjayan2

1ARC DECRA Fellow, Center for Sustainable Infrastructure, Swinburne University of Technology 2Professor and Director, Center for Sustainable Infrastructure, Swinburne University of Technology

Abstract:

This study reports the quantitative influences of type of activator on the microscale fiber-matrix interface

properties, and their consequent effects on the tensile performance of a fiber-reinforced strain-hardening

geopolymer composite (SHGC). Single-fiber pullout tests were conducted to measure the fiber-matrix

interface properties of two fly ash-based SHGCs made by using one sodium (Na) and one potassium (K)

based activators. Effects of the experimentally measured interface properties on the crack bridging (σ-δ)

relation of the SHGCs were investigated using a micromechanics-based model to explain the

experimentally observed macroscopic tensile ductility of the SHGCs. The results indicated that the type

of activator had significant effects on the fiber-matrix interface properties. The chemical bond strength

of polyvinyl alcohol (PVA) fiber with the Na-based SHGC matrix was 83% lower than that of the K-

based SHGC matrix. In contrast, the frictional bond strength and slip hardening coefficient of the PVA

fiber with the Na-based SHGC matrix were about 2 times and more than 18 times, respectively higher

than those of the K-based SHGC matrix. The ultimate tensile strength and tensile strain capacity of the

Na-based SHGC were 139% and 50%, respectively higher than those of the K-based SHGC. The superior

tensile performance of the Na-based SHGC is attributed to its beneficial fiber-matrix interface properties,

and thereby its higher pseudo strain-hardening (PSH) performance indices, as compared to the K-based

SHGC.

Keywords: strain-hardening geopolymer composite (SHGC); type of activator, fiber-matrix

interface; tensile performance; PSH performance indices.

i. Introduction

Strain-hardening cementitious composite (SHCC) is a micromechanics-based design fiber-reinforced

brittle matrix composite which exhibits pseudo strain-hardening (PSH) behavior with extreme tensile

ductility in the range of 3-5% at a moderate fiber content of 2% or less by volume (Li, Wang & Wu

2001). Material sustainability, however, has not often been a concern in development of SHCCs. In

comparison to conventional concrete, large amount of cement is usually used in this composite resulting

in high material cost, autogenous shrinkage and heat of hydration (Wang & Li 2007). In addition, the

associated increase in the CO2 emissions and embodied energy arising from the production of ordinary

Portland cement (OPC) can compromise sustainability credentials of SHCCs. A sustainable approach to

significantly enhance the material sustainability performance of the composite is to incorporate a

geopolymer binder as ‘complete’ replacement of OPC in SHCC composition.

Geopolymer is an emerging OPC-less binder purported to provide an environmentally friendly

alternative to OPC. Previous studies reported that at least 80% less CO2 is emitted and about 60% less

energy is needed for manufacture of fly ash-based geopolymer in comparison to production of OPC (Li

& al. 2004; Duxson & al. 2007). Geopolymers can be manufactured by alkaline activation of fly ash

and/or slag, being industrial by-products coal power stations and iron manufacture, respectively which

contain high volumes of silica and alumina (Nematollahi, Sanjayan & Shaikh 2015a).

The researchers at Swinburne University of Technology, Australia have recently conducted a series

of studies aimed on multiscale development and investigation of properties of strain-hardening

geopolymer composite (SHGC). SHGC is a sustainable strain-hardening composite where OPC is

completely replaced by a geopolymer binder. The quantitative influences of the geopolymer matrix-

related parameters such as type of alkaline activator, water to geopolymer solids ratio (W/GP solids),

sand size and sand content on the matrix and composite properties of fly ash-based SHGCs have been

previously investigated by the authors of this paper. The fly ash-based SHGC developed by the authors

exhibited comparable properties to typical SHCCs (Nematollahi, Sanjayan & Shaikh 2014a&b;

Nematollahi, Sanjayan & Shaikh 2015b&c&d; Nematollahi, Sanjayan & Shaikh 2016). At the same time,

it had 52% less carbon emissions and 17% less energy consumption as compared to SHCC mix 45 (M45)

(Nematollahi & al. 2017a).

However, the fiber-matrix interface properties, and their consequent effects on crack-bridging and

tensile performance of the developed fly ash-based SHGC have not yet been investigated. Understanding

the microscale fiber-matrix interaction properties and mechanisms is of primary importance in design of

SHGCs. Therefore, this study as a follow up investigation aims to provide an in-depth understanding of

the quantitative influences of type of activator on the microscale fiber-matrix interface properties, and

their consequent effects on tensile performance of the developed fly ash-based SHGC. This

understanding presents the rational basis for design of such OPC-less strain-hardening composites.

ii. Experimental Procedures

a. Materials and mix proportions.

The low calcium fly ash was provided by Gladstone power station in Queensland, Australia. Two

different sodium-based (Na-based) and potassium-based (K-based) activator combinations were used in

this study. The Na-based activator combination was made of 8.0 M sodium hydroxide (NaOH) solution

and D Grade sodium silicate (Na2SiO3) solution (29.4 wt.% SiO2 and 14.7 wt.% Na2O) with Na2SiO3 to

NaOH mass ratio of 2.5. The K-based activator combination was made of 8.0 M potassium hydroxide

(KOH) solution and KASIL 2236 Grade potassium silicate (K2SiO3) solution (24.8 wt.% SiO2 and 11.2

wt.% K2O) with K2SiO3 to KOH mass ratio of 2.5. The PVA fiber was supplied by Kuraray Co. Ltd.,

Japan with a surface oil coating of 1.2% by weight. The PVA fiber has a density of 1.3 g/cm3, length of

8 mm, diameter of 39 μm, elongation of 6% and elastic modulus of 41 GPa. The nominal tensile strength

of the PVA fiber is 1600 MPa. As shown in Table 1, the SHGC-Na and SHGC-K mixtures compare the

effect of type of activator on the basis of the same compressive strength of the SHGC matrix.

Table 1: Mix proportions of fly ash-based SHGCs.

Mix ID Fly ash (kg/m3)

Activator (kg/m3)

PVA fiber (kg/m3)

Target compressive strength of SHGC matrix (MPa)

SHGC-Na 1240.0 558.0 26 32

SHGC-K 1335.6 467.4 26 32

b. Mixing, curing and testing of specimens

To make the mixtures, alkaline solution was added to fly ash in a Hobart mixer and thoroughly mixed

for about 4 minutes. The PVA fibers were gradually added to ensure uniform fiber dispersion. Heat curing

(60°C for 24 hours) was adopted in this study. The procedure for the heat curing can be found in

Nematollahi, Sanjayan & Shaikh (2015b).

For each mixture, a minimum of three 50 mm cube matrix specimens and three 50 mm cube composite

specimens for compression tests, three 100×200 mm cylindrical matrix specimens for matrix elastic

modulus tests, four matrix prisms (without addition of the fibers) with the dimensions of 60 mm×60

mm×280 mm for matrix fracture toughness tests, and three rectangular coupon specimens with the

dimensions of 400 mm×75 mm×10 mm for uniaxial tension tests were prepared. All specimens were

tested 3 days after casting, as the strength of fly ash-based geopolymer does not change considerably

after the completion of the heat curing period and three days compressive strength of heat cured fly ash-

based geopolymer is comparable to a typical OPC strength development after 28-days (Nematollahi &

Sanjayan 2014).

The compressive strength of matrix and composite was measured in accordance with ASTM C109

(2007). The elastic modulus of the SHGC matrix (Em) was measured in accordance with AS1012.17

(1997). The matrix fracture toughness (Km) was measured in accordance with effective crack model

developed by Karihaloo & Nallathambi (1990) using three-point bending test on single edge notched

matrix prisms. The procedure of the matrix fracture toughness test can be found in Nematollahi, Sanjayan

& Shaikh (2015b). Tensile performance was evaluated using uniaxial tension test on rectangular coupon

specimens under displacement control at the rate of 0.25 mm/min over a gauge length of about 200 mm.

Details of the uniaxial tension test are given in Nematollahi, Sanjayan & Shaikh (2015b).

Single-fiber pullout tests were conducted to measure the fiber-matrix interface properties, including

chemical bond strength (Gd), frictional bond strength (τ0), and slip-hardening coefficient (β). For each

mixture, at least four single-fiber pullout specimens were tested under displacement control at the rate

of 0.03 mm/min. Details of the single-fiber pullout test are given in Nematollahi & al. (2017b&c).

iii. Results and Discussions

a. Compressive strength

The average compressive strength of each mixture is given in Table 2. While the compressive strength

of the SHGC matrix in SHGC-Na and SHGC-K was comparable, the compressive strength of SHGC-

Na was 41% higher than that of SHGC-K. This could be attributed to the effect of type of activator on

the fiber-matrix interface properties.

Table 2: Compressive strength results.

Mix ID Compressive strength; (MPa)

Matrix Composite

SHGC-Na 31.6±1.5 52.6±1.6

SHGC-K 32.3±1.4 37.3±1.3

b. Matrix fracture properties

The fracture properties of SHGC matrices (without addition of the fibers) are summarized in Table 3.

The elastic modulus of SHGC-Na and SHGC-K matrices was identical, which corresponds to their

comparable compressive strength as reported in Table 2. However, the Km and thereby the 𝐽𝑡𝑖𝑝 of SHGC-

Na matrix were 14% and 28%, respectively higher than those of the SHGC-K matrix. The different Km

of SHGC-Na and SHGC-K matrices could be attributed to their different geopolymer microstructures,

owing to their different type of activator (Fernández-Jiménez & Palomo 2005). According to Fernández-

Jiménez & Palomo (2005), in fly ash-based geopolymer the type of activator has a significant effect on

the microstructure of the aluminosilicate gel. It is thereby concluded that the K-based activator increases

the brittleness of the fly ash-based SHGC matrix.

Table 3: Matrix fracture properties.

Mix ID

Matrix elastic modulus, Em;

(GPa)

Matrix fracture toughness, Km;

(MPa.m1/2)

Crack tip toughness, Jtip;

(J/m2)

SHGC-Na matrix 7.5 0.269 9.6

SHGC-K matrix 7.5 0.237 7.5

c. Fiber-matrix interface properties

The fiber-matrix interface properties of each mix are summarized in Table 4. The Scanning Electron

Microscope (SEM) images of the typical pulled-out fiber of each mix are shown in Figs. 1 and 2. As can

be seen in Table 4, the type of activator had significant effects on the interface properties. The chemical

bond strength of the oil-coated fiber in SHGC-Na was 83% lower than that of SHGC-K. On the other

hand, the frictional bond and slip hardening coefficient of the fiber in SHGC-Na were about 2 times and

more than 18 times, respectively higher than those of the fiber in SHGC-K.

Table 4: Fiber-matrix interface properties using oil-coated PVA fibers.

Mix ID Chemical bond

strength, Gd; (J/m2) Frictional bond

strength, τ0; (MPa) Slip hardening coefficient, β

SHGC-Na 0.59±0.67 1.73±0.51 0.463±0.200

SHGC-K 3.41±1.71 0.87±0.44 0.025±0.015

Fig. 1: SEM images of SHGC-Na using oil-coated PVA fiber.

Fig. 2: SEM images of SHGC-K using oil-coated PVA fiber.

It can be said that the K-based activator significantly enhances the chemical bond between the PVA

fiber and the SHGC matrix. As can be seen in Figs. 1 and 2, surface morphology of the pulled-out fibers

in SHGC-Na and SHGC-K is different. In SHGC-Na (Fig. 1), the pulled-out fiber tip is covered with

matrix debris and its diameter is relatively un-changed as compared to the original fiber diameter (40μm).

On the other hand, in SHGC-K (Fig. 2), the pulled-out fiber tip was sharpened and the fiber surface was

severely scratched. This explains the significantly higher chemical bond strength of the oil-coated fiber

in SHGC-K than SHGC-Na. This may be attributed to greater attack of the K-based activator to the oil-

coating agent on the surface of the PVA fiber. It has been reported that KOH solution can penetrate in oil

molecules faster than NaOH solution due its higher alkalinity and solubility in water (Diphare &

Muzenda 2013). As a result, it is believed that KOH breaks the oil coating agent on the fiber surface

quicker (Diphare & Muzenda 2013), which causes the increase of chemical bond. Without the protection

from the surface oil coating, oil-coated PVA fiber in the K-based SHGC matrix may also be weakened

due to strong alkalinity of the matrix, and delamination damage of the fiber during debonding may occur,

which results in the decrease of interface frictional bond, as shown in Table 4. As can be seen in Fig. 2,

the delamination surface of the oil-coated fiber in SHGC-K is relatively smooth as compared to the

surface of the oil-coated fiber in SHGC-Na (Fig. 1). This also explains the low interface frictional bond

of the oil-coated fiber in SHGC-K than SHGC-Na.

d. Uniaxial tensile performance

The uniaxial tensile stress–strain curves of each mixture are presented in Fig. 3. As can be seen, both fly

ash-based SHGCs, irrespective of the type of activator demonstrated PSH behavior. Table 5 presents the

uniaxial tension test results of each mixture. The first-crack strength of each mixture is estimated from

the tensile stress-strain curves following the method proposed by Kanda & Li (2006).

Fig. 3: Tensile stress-strain responses of fly ash-based SHGCs.

Table 5: Uniaxial tension test results.

Mix ID First-crack strength, σfc;

(MPa) Ultimate tensile strength,

σcu ; (MPa) Tensile strain

capacity, εcu; (%)

SHGC-Na 2.8 ± 0.40 4.3 ± 0.45 3.0 ± 0.19

SHGC-K 1.4 ± 0.062 1.8 ± 0.21 2.0 ± 0.26

As can be seen in Table 5, the type of activator had a significant effect on the uniaxial tensile behavior

of fly ash-based SHGC. The first-crack strength, ultimate tensile strength and tensile strain capacity of

SHGC-K were 52%, 58% and 33%, respectively lower than those of SHGC-Na. The lower first-crack

strength of SHGC-K corresponds to the lower fracture toughness of the K-based SHGC matrix, as

reported in Table 3 (Li, Wang & Wu 2001). The lower ultimate tensile strength of SHGC-K is due to the

significantly lower τ0 and β of the PVA fiber with the K-based SHGC matrix, as reported in Table 4,

which led to its inferior fiber-bridging strength (Nematollahi, Sanjayan & Shaikh 2015b; Nematollahi,

Sanjayan & Shaikh 2016).

The underlying reasons for the lower tensile strain capacity of SHGC-K could be elucidated based on

the PSH strength and PSH energy performance indices proposed by Kanda & Li (2006). The two PSH

performance indices should be determined from the crack bridging relation of the composite. In this

study, the micromechanics-based model developed by Yang & al. (2008) was used to compute the fiber-

bridging constitutive law σ(δ) of fly ash-based SHGCs. The applicability of this micromechanics-based

model for evaluating the tensile performance of fly ash-based SHGCs was verified in a recent study by

the authors of this paper (Nematollahi & al. 2017d). The resulting PSH performance indices are plotted

in Fig. 4. As can be seen, in all fly ash-based SHGCs both PSH performance indices are greater than one.

Thereby, it can be concluded that the necessary strength-based and energy-based conditions of steady-

state flat crack propagation which result in sequential multiple cracking of geopolymer matrix are

satisfied. Therefore, both fly ash-based SHGCs investigated in this study exhibited PSH behavior. In

addition, as can be seen in Fig. 4, the PSH strength and energy indices of SHGC-K were considerably

lower than those of the Na-based SHGC. As a result, it is not surprising that SHGC-K with lower PSH

performance indices exhibited inferior tensile ductility than that of SHGC-Na.

Fig. 4: PSH performance indices of fly ash-based SHGCs.

Fig. 5 presents the clear pattern of multiple cracking in each mixture. The multiple cracking pattern

was obtained by marking all visible cracks on an unloaded specimen after the uniaxial tension test using

a permanent marker. The crack spacing was determined based on the number of visible cracks counted

manually from an unloaded specimen, and the gauge length. As can be seen in Fig. 5, enormous number

of cracks with almost equal crack spacing ranging from 2.5 mm to 3.5 mm was observed in the SHGC-

Na specimen. The saturated multiple cracking behavior observed in the SHGC-Na specimen corresponds

to its superior tensile strain capacity, as reported in Table 5. In contrast, the lowest number of cracks

with un-even spacing ranging from 4-11 mm was observed in the K-based SHGC specimen. The

unsaturated multiple cracking behavior observed in the K-based SHGC specimen corresponds to its

inferior tensile strain capacity, as reported in Table 5.

Fig. 5: Typical multiple cracking pattern of fly ash-based SHGCs.

iv. Conclusions

The quantitative influences of type of activator on the microscale fiber-matrix interface properties, and

their consequent effects on the tensile performance of a fly ash-based SHGC were investigated in this

study. The following conclusions are drawn:

1) The fiber surface oil-coating does not provide much benefit when used with the K-based activator

compared to the Na-based activator. The chemical bond of the oil-coated fiber in the K-based SHGC was

considerably higher than that of the Na-based SHGC. It can be said that the K-based activator

significantly enhances the chemical bond between the PVA fiber and the geopolymer matrix, which may

result in delamination damage of the fiber during the debonding stage. On the other hand, the frictional

bond and slip hardening coefficient of the oil-coated fiber in the K-based SHGC were considerably lower

than those of the Na-based SHGC, due to the relatively smooth delamination surface of the fiber.

2) Uniaxial tension test results indicated that the Na-based SHGC exhibited superior tensile

performance to the K-based SHGC, in spite of the higher fracture toughness of the Na-based SHGC

matrix. This is due to the significantly higher complementary energy of the Na-based SHGC, resulted

from its beneficial fiber-matrix interface properties.

3) The experimentally observed macroscopic composite tensile ductility corresponded well with the

two PSH performance indices of the composite calculated based on the computed crack bridging relation

of the fly ash-based SHGCs. It was found that the two PSH performance indices of the Na-based SHGC

were considerably higher than those of the K-based SHGC, thus providing a rational basis for the

observed superior tensile performance of the Na-based SHGC.

Acknowledgment The authors gratefully acknowledge Assistant Professor En-Hua Yang and Dr. Jishen Qiu from Nanyang

Technological University, Singapore for conducting the single-fiber pullout tests and micromechanics

modelling of the developed SHGC.

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