Waste Glass Powder as Cement Replacement in ConcreteHongjian Du, Kiang Hwee Tan

Journal of Advanced Concrete Technology, volume ( ), pp.12 2014 468-477

Effects of PFA and GGBS on Early-Ages Engineering Properties of Portland Cement SystemsXiangming Zhou , Joel R. Slater Stuart E. Wavell, , Olayinka OladiranJournal of Advanced Concrete Technology, volume ( ), pp.10 2012 74-85

Estimation of strength, permeability and hydraulic diffusivity of pozzolana blended concrete throughpore size distributionB. Kondraivendha , B. Sabet Divsholi Susanto Teng,Journal of Advanced Concrete Technology, volume ( ), pp.11 2013 230-237

Effect of nano-CaCO3 on compressive strength development of high volume fly ash mortars andconcretesSteve W. M. Supit, Faiz U. A.ShaikhJournal of Advanced Concrete Technology, volume ( ), pp.12 2014 178-186

Journal of Advanced Concrete Technology Vol. 12, 468-477, November 2014 / Copyright © 2014 Japan Concrete Institute 468

Scientific paper

Waste Glass Powder as Cement Replacement in Concrete Hongjian Du1* and Kiang Hwee Tan2

Received 25 August 2014, accepted 2 November 2014 doi:10.3151/jact.12.468

Abstract The pozzolanic reactivity of waste glass powder was experimentally studied at cement replacement levels of 0, 15, 30, 45 and 60% by weight. Results revealed that the concrete compressive strength was not decreased by the cement substi-tution after 28 days because of the pozzolanic reaction between glass powders and cement hydration products, if the replacement is below 30%. Also, the resistance to chloride ion and water penetration continuously increases with in-creasing glass powder content up to 60% cement replacement. At 60% replacement level, the electrical resistivity and water penetration depth were reduced by 95% and 80%, respectively, while the compressive strength was maintained as 85%. These improvements in durability properties are due to the refined microstructures, particularly at the interfacial transition zone. Pore size distribution was measured to confirm the refinement in the capillary pores, which partially block the pathways for water and chloride ions. This study also demonstrates that high performance concrete (improved strength and impermeability against chloride and water) could be achieved by using glass powder as 15% additive, which contributes to the pozzolanic reaction instead of being inert fines for compact packing.

1. Introduction

Due to the awareness on the need for environmental protection, turning solid wastes or by-products into in-gredients of concrete has drawn increasing attention. Apart from material and energy conservation, reuse of some solid wastes could result in better performances of concrete in several areas. For example, use of waste glass as fine aggregates provides concrete with higher resistance to chloride penetration (Ling et al. 2011; Tan and Du 2013; Du and Tan 2014a). Fine glass particles (smaller than 75 µm) can exhibit pozzolanic reactivity, thereby improving the microstructure of paste and the strength and durability of concrete in the long term (Wright et al. 2014; Du and Tan 2014a). The amorphous silica in the glass would dissolve in an alkaline envi-ronment such as due to OH- ions in the pore solution of cement paste. Thereafter, it could react with calcium hydroxide (CH) to form secondary calcium silicate hy-drate (C-S-H), a process known as pozzolanic reaction which can be expressed by CH + S + H → C-S-H. Pre-vious literature shows that the pozzolanic reactivity of glass particle increases due to the larger surface area available for the reaction (Shayan and Xu 2004; Du and Tan 2014b). At the same time, with finely ground glass powder (GP), no alkali-silica reaction has been observed (Jin et al. 2000). According to the authors’ previous work (Du and Tan 2013; Du and Tan 2014b), ASR ex-

pansion in mortar containing glass particle as fine ag-gregates would be negligible provided that the glass particles were smaller than 0.3 mm. This finding is con-sistent with the threshold particle size of 0.3 mm as re-ported by Shayan and Xu (2004) and Rajabipour et al. (2010).

Some studies have been conducted to characterize the pozzolanic activity of finely ground GP in concrete (Shao et al. 2000; Dyer and Dhir 2001; Shi et al. 2005; Neithalath 2008; Schwarz et al. 2008; Jain and Neitha-lath 2010; Matos and Sousa-Coutinho 2012; Khmiri et al. 2013; Carsana et al. 2014; Kim et al. 2014; Mirza-hosseiniand and Riding 2014; Du and Tan 2014c). De-pending on its fineness, GP would generally exhibit pozzolanic reaction at a slower rate compared with the cement hydration. Thus, the replacement of cement by GP might decrease strength at an early age but would increase it at a later age. Most of the previous research works emphasized on the pozzolanic reaction of GP on the mechanical characteristics of mortar/concrete with a fixed glass powder to the binder ratio. Shao et al. (2000) found that concrete with 30% glass powder finer than 38 µm did not exhibit higher compressive strength until after 90 days of curing. Shi et al. (2005) also observed that mortar with 20% glass powder could give a higher compressive strength from 28 days onwards, provided that glass powders are close to or smaller than Portland cement in size. A similar conclusion has been recently reported by Khmiri et al. (2013). Shayan and Xu (2004) reported that the use of glass powder (smaller than 10 µm) as cement replacement up to 40% can reduce the ASR expansion for concrete. However, all the concrete mixes with cement partially replaced by GP showed continuously reduced compressive strength. Also, the influence of GP as a supplementary cementitious mate-rial on the hydration and the microstructure of paste has

1Research Fellow, Department of Civil and Environ-mental Engineering, National University of Singapore, Singapore. *Corresponding author, E-mail: ceedh@nus.edu.sg 2Professor, Department of Civil and Environmental Engineering, National University of Singapore, Singapore.

H. Du and K. H. Tan / Journal of Advanced Concrete Technology Vol. 12, 468-477, 2014 469

rarely been reported (Shi et al. 2005). They found that curing temperature have a more obvious accelerating effect on the pozzolanic reactivity of glass powder than on that of fly ash. Both the hydration and pozzolanic reaction greatly govern the structure formation and hence all properties like strength development and du-rability.

As reviewed above, the properties of concrete with glass powder as cement replacement in higher propor-tion remains an unexplored area. Therefore, this re-search aims to study the pozzolanic reaction of GP up to 60% cement replacement and its influence on the micro-structure of cement paste, which has not been investi-gated before. The hydration rate and rheological proper-ties of paste were also characterized. The chemical composition and calcium hydroxide content of the hy-drated products were analyzed at different ages. Also, the microstructure development of cement paste was determined by measuring the pore size distribution. At the same time, the compressive strength and resistance to chloride and water penetration were investigated for concrete with the same cement replacement levels as the paste. The influence of pozzolanic reaction on the tran-sition zone between aggregate particles and cement paste is discussed. Based on the results, the optimum cement replacement content by glass powder is sug-gested.

2. Experimental program

2.1 Materials The crushing process of recycled waste glass can be found in the authors’ previous work (Tan and Du 2013; Du and Tan 2014a, 2014b). Waste bear bottles (soda lime glass) were collected from a local recycler in Sin-gapore. To finely grind the sand-sized particles, a ball miller was used. The size distributions of ground GP and cement used in this study are shown in Fig. 1. Both cement and GP show the same median particle size of around 10 µm. The chemical compositions of GP and OPC are displayed in Table 1. The specific gravities of cement and GP are 3.15 and 2.53, respectively. The sur-face appearance of GP and OPC are compared in Fig. 2.

GP is amorphous and its X-ray diffraction (XRD) pat-tern is compared with OPC in Fig. 3. GP has a negligi-ble water absorption capacity of 0.07%. Coarse aggre-gate with a maximum size of 10 mm was used in con-crete. The fineness modulus of natural sand was 2.80. 2.2 Specimens The microstructure of cement paste with various con-tents of GP was investigated. The water to cement (w/c) ratio for the paste is 0.485 by weight. The mix propor-tion for concrete is listed in Table 2. Cement was re-placed by GP at 0, 15, 30, 45 and 60% by weight. An additional mix (OPC+15GP) was cast in which the GP was used as an additive, instead of replacing cement at a content of 15%. Compared to the use of cement re-placement, the addition of GP in this mix leads to a lower water-to-binder (w/b) ratio of 0.42. This could help examine the utilization of GP as additive in high performance cementitious composites. For each paste mix, three 50 mm cube specimens were prepared. For each concrete mix, nine 100 mm cubes and five Φ100×200 mm cylinders were cast for compressive strength and durability tests, respectively. Paste and concrete were mixed in Hobart and pan mixers respec-tively. All the specimens were compacted on a vibration table and covered by plastic sheets to prevent moisture loss. After demolding on the next day, the paste speci-mens were cured in saturated lime water and concrete samples were cured in water until the test age. The cur-ing method for paste is following ASTM C 109 (2005) to prevent calcium leaching and curing method for con-crete specimens is following ASTM C 39 (2005).

Table 1 Chemical compositions of OPC and glass powder.

Composition, % OPC Glass powderSiO2 20.8 72.08 Al2O3 4.6 2.19 Fe2O3 2.8 0.22 CaO 65.4 10.45 MgO 1.3 0.72 SO3 2.2 —

Na2O 0.31 13.71 K2O 0.44 0.16 TiO2 — 0.1 Cr2O3 — 0.01

Table 2 Mix proportion of concrete with different glass powder replacement levels.

Content, kg/m3 Mix No.

Water Cement Glass Powder Sand Coarse

aggregateOPC 185 380 0 960 825 15GP 185 323 57 955 825 30GP 185 266 114 947 825 45GP 185 209 171 940 825 60GP 185 152 228 933 825

OPC+15GP 185 380 57 907 825

0.1 1 10 1000

20

40

60

80

100

Perc

enta

ge p

asse

d, %

Size, μm

OPC Glass powder

Fig. 1 Particle size distribution of OPC and glass powder.

H. Du and K. H. Tan / Journal of Advanced Concrete Technology Vol. 12, 468-477, 2014 470

2.3 Test methods The rheological properties of cement paste containing GP were determined by a coaxial-cylinder viscometer named RotoVisco 1. After mixing the cement paste in the Hobart mixer for about 3 minutes, a sample weigh-ing about 150 g was taken out and placed in the outer cylinder of the viscometer, followed by inserting the inner cylinder immediately. The sensor then started to rotate at different shear rate while the torque was re-corded. Isothermal calorimetry was performed on tripli-cate paste samples by using an eight-channel microcalo-rimeter (TAM AIR).

Paste samples were taken from the center of 50 mm cube specimens for the purpose of mercury intrusion porosimetry (MIP), XRD and thermogravimetric analy-sis (TGA). For MIP test, paste samples (around 1 cm in cubic shape) were dried in dessicator at 50 ˚C for 1 day

before testing, at the age of 7 and 91 days. Micromerit-ics AutoPore III with a maximum mercury pressure of 412.5 MPa was used to measure the pore size distribu-tion. For XRD and TGA analyses, paste samples were cured in 105 ˚C oven for 24 hours and then finely ground to be less than 75 µm. Shimadzu XRD-6000 diffractometer was employed to qualitatively determine the influence of GP on the chemical composition of the hydrated production. The XRD scan was between 10˚ and 60˚ with a speed of 0.5˚/min. TGA was carried out using LINSEIS L81-II. The powder sample was heated from ambient temperature up to 950˚C at a rate of 10˚C/min while the weight loss was recorded during the TGA test. The content of CH could be obtained from two different intervals in the weight loss curve, corre-sponding to the decomposition of Portlandite and calcite, respectively.

10 15 20 25 30 35 40 45 50

A,FAF, A

luA,BFP

A,B

AAA

A,B

FA,Alu

A,B

A,B,F

GB,G

A

A

A,BABAG

G

OPC

2θ

GP

A=alite, C3SB=belite, C2S Alu=aluminate, C3A F=ferrite, C4AFG=gypsum, CaSO4

P: Periclase, MgO

A

Fig. 3 XRD patterns for OPC and GP.

(a) OPC particles.

(b) Finely ground glass powder.

Fig. 2 Particle morphologies using SEM.

H. Du and K. H. Tan / Journal of Advanced Concrete Technology Vol. 12, 468-477, 2014 471

For concrete specimens, compressive strength and rapid chloride penetration test (RCPT) were carried out according to BS EN 12390-3 (2009) and ASTM C 1202 (2005), respectively. RCPT is commonly criticized for its drawbacks including temperature rise due to the ap-plied high voltage of 60 V (McGrath and Hooton 1999). Thus, a reduced voltage of 20 V was applied for 24 hours, to minimize the side effects of RCPT. At 28 days, water penetration depth into concrete was determined using a water pressure of about 0.75 MPa for 7 days, in accordance to BS EN 12390-8 (2009). The side and top surfaces were both coated with impermeable epoxy while the bottom surface was exposed to the water pres-sure. After removed from the test rig, the concrete cyl-inders were axially split and the average depth water front measured.

3. Test results and discussion

3.1 Rheological properties The shear stress (τ) was recorded with shear rate ( γ ) decreasing from 50 to 0.5 s-1 during the descending branch of the test loop. Test results are shown in Fig. 4(a). Yield stress (τ0) and plastic viscosity (µ) of fresh cement paste are linearly related using Bingham model (Mindess et al. 2003).

0τ τ μγ= + (1)

The values of τ0 and µ for pastes with various GP contents are displayed in Fig. 4(b). Both the yield stress (τ0) and viscosity (µ) decrease with higher cement re-placement level. Yield stress indicates the minimal strength to make the mixture flowable. The reduced yield stress implies that the inter force between cement and glass particles is less than that between cement and cement particles. With increasing GP contents, the parti-cle density of cement is diluted and hence lesser interac-tion between cement and water, leading to a smaller yield stress and plastic viscosity. This could also be due to the negligible water absorption and the smooth sur-face of GP (as shown in Fig. 2). Previous studies also indicate that the bond between cement paste and fine glass particles was decreased due to the surface smooth-ness of glass powder (Taha and Nounu 2009; Ali and Al-Tersawy 2012).

In this study, the cement has been replaced by weight instead of by volume. As the specific gravity of glass power is lower than cement, the solid-to-water ratio by volume is higher for GP blended paste compared with pure cement paste. However, this adverse effect at a higher solid-to-water volume ratio is less pronounced compared to the dilution of cement and smooth glass surface, as mentioned earlier. The continuously de-creased yield stress and plastic viscosity represent a better workability of paste mixture with higher content of GP. It is also noted that both the yield stress and vis-cosity increase substantially for the mix with 15% addi-tional GP, attributed to the increased amount of solid-to-water ratio.

3.2 Heat of hydration The heat evolution rate and cumulative amount of heat for pastes with varying contents of GP are shown in Fig. 5. It is clear that the maximum heat evolution rate and the total heat generated reduced continuously with higher OPC replacement level because of the dilution of cement and the slower rate of pozzolanic reaction of GP. The results in this study are consistent with previous findings (Dyer and Dhir 2001; Mirzahosseini and Rid-ing 2014). The benefit of lower hydration heat is helpful in preventing the temperature cracking, especially for structure members with large thickness and mass con-crete.

In the presence of GP, the time to reach the peak hy-dration rate is shortened, possibly because fine glass powders can accelerate the cement hydration via the adsorption of calcium ions from the liquid phase and play as nucleation and growth sites for C-S-H and other hydrates. At the same time, the high content of alkalis (Na2O) in GP may act as catalyst in the formation of calcium silica hydrate at an early age (Jawed et al. 1978; Khmiri et al. 2013). The time corresponding to the peak rate reduces from 418 minutes for OPC paste to 377 minutes for paste with 60% glass powder paste.

0 10 20 30 40 500

20

40

60

80

100

μ

OPC 15GP 30GP 45GP 60GP OPC+15GP

Shea

r stre

ss, τ

(Pa)

Rate of shear, γ (1/s)

1

(a) Relationship between shear stress and shear rate for cement paste with varying contents of glass powder.

0 15 30 45 60 750

5

10

15

20

25

30

μ

She

ar y

ield

stre

ss, τ

0 (Pa)

GP content, %15% Addition

τ0

0.0

0.3

0.6

0.9

1.2

1.5

Plas

tic v

isco

sity

, μ (P

a•s)

(b) Glass powder effect on the rheological parameters. Fig. 4 Test results of rheological properties of paste.

H. Du and K. H. Tan / Journal of Advanced Concrete Technology Vol. 12, 468-477, 2014 472

3.3 XRD The XRD patterns for paste with glass powder at age of 7, 28 and 91 days are shown in Fig. 6. The compositions of the hydration product for Portland cement paste are C-S-H and CH and their principal peaks do not change from 7 to 91 days. CH peaks tend to be weakened with increasing glass powder and longer curing time, espe-cially for 60% glass powder paste at 91 days in which the CH peaks almost disappear. The XRD results di-rectly reveal that CH is consumed to form the additional C-S-H in paste with glass powders.

At 91 days, peaks corresponding to CH can be obvi-ously seen for OPC and 30GP paste, which means that CH remains even after 91 days of pozzolanic reaction with glass powder. However, it is not true for paste with

60% glass powder in which the CH seems to be insuffi-cient for glass powder to be fully reacted. 3.4 Ca(OH)2 content The CH content was calculated from the TG and DTG curves, as illustrated in Fig. 7(a). Derivative curve was used to find the beginning and the end of the derivative peak and measure the weight loss correspondingly (Ko-caba 2009). In the temperature range of about 425 and 550 ˚C, the amount of CH can be calculated by deter-mining the water loss during the CH decomposition: Ca(OH)2→CaO+H2O. Also, the influence of carbona-tion on the CH content could be evaluated by the mass loss in the temperature range of 650 and 800 ˚C, where the calcite decomposes and release CO2: CaCO3→CaO+CO2. Hence, the content of CH was cal-culated taking into account molecular weight of each component and were calculated as percentage of the dry weight at 105 ˚C.

As shown in Fig. 7(b), the CH content in the refer-ence OPC paste remained at above 20% of the total weight after 28 days of curing, which is similar to pre-vious findings (Jawed et al. 1978). With higher cement substitution level, the CH content drops, especially at later age such as 91 days. At the beginning, the reduc-tion is caused by the dilution of Portland cement in the total cementitious material content. With longer curing and hydration time of 28 days, more CH is consumed in the pozzolanic reaction of glass powder. The initial per-centage drop in CH content was higher for 30% GP paste than in 60% GP paste. After 28 days, the reduction in CH content slowed down for 30% glass powder paste and stabilized at about 11%, indicating the pozzolanic reaction was almost completed. In contrast, the CH con-tent in 60% GP paste continued to drop and was less than 4% at 91 days. This depletion in CH reveals that 60% might be the maximum practical glass powder con-tent since the CH content is insufficient for the further pozzolanic reaction of glass powder. Dyer and Dhir (2001) has reported that the amount of CH continuously decreased with increasing GP replacement up to 40%, at the age of 28 days, but no information for higher GP replacement.

0 12 24 36 48 60 720

1

2

3

4

5

6(a)

Rat

e of

hea

t evo

lutio

n, m

W/g

Time, h

OPC 15GP 30GP 45GP 60GP OPC+15GP

0 12 24 36 48 60 72

0

50

100

150

200

250

300

60GP

45GP

30GP15GP

OPC+15GP

Cum

ulat

ive

heat

, J/g

Time, h

OPC(b)

Fig. 5 (a) Rate of heat evolution, and (b) cumulative heat of cement paste with various replacement levels with glass powder.

10 15 20 25 30 35 40 45 50

CS

H

2θ

(a)91-day

28-day

7-dayCS

HC

H CHC

H

CH

10 15 20 25 30 35 40 45 50

CSH

(b)

2θ

CSHCH CHC

HCH

91-day

28-day

7-day

10 15 20 25 30 35 40 45 50

CSH

(c)

CSH

CHCH

CH

2θ

CH

91-day

28-day

7-day

Fig. 6 XRD patterns of cement paste with different con-tents of glass: (a) OPC, (b) 30GP and (c) 60GP.

H. Du and K. H. Tan / Journal of Advanced Concrete Technology Vol. 12, 468-477, 2014 473

3.5 Microstructure The pore size distribution of cement paste containing glass powder at 7 and 91 days are shown in Fig. 8, from which the change in the microstructure of paste can be seen clearly particularly for paste with 30 and 60% glass powder. The pore structures become more refined, as indicated by the reduced threshold and critical pore di-ameter, as well as the higher fraction of the micro-pores. For paste with 60% glass powder, the threshold and critical pore diameter was reduced from 488 and 140 nm at 7 days to 139 and 74 nm at 91 days, respectively. Moreover, the volume fraction of capillary pores (10 µm – 50 nm) in the total porosity (10 µm – 3.7 nm) was lowered from 44.6% at 7 days to 30.7% at 91 days. Ac-cording to Mindess et al. (2003), it is the capillary pores that govern the permeability and diffusivity of paste. Thus the MIP results reveal that the permeability of high volume glass powder paste is likely to be reduced. Poz-zolanic reaction between glass powder and CH is a slower chemical process compared with OPC hydration. Since glass powder is inert in the initial stage, the effec-tive w/c ratio is actually higher than that of the OPC paste. This results in a more porous microstructure for mix with glass powder as OPC replacement. However, at later age, most of the glass powders in 30GP and part of it in 60 GP mix have reacted with lime to form the secondary C-S-H. The pore structures are almost identi-cal for each paste mix, regardless of glass powder con-tent at 91 days. 3.6 Compressive strength of concrete The compressive strengths of concrete with varying contents of GP, at different ages, are shown in Fig. 9. At 7 days, the strength generally decreases with GP content more than 15%. However, at 28 and 91 days, no reduc-tion in strength was observed for concrete with 15 and 30% GP, due to the pozzolanic reaction between GP and cement hydration products. Instead, concrete with 15 and 30% GP possessed higher compressive strengths. This increase in later age strength was not observed for concrete in which more than 30% cement is replaced by GP. The pozzolanic reaction requires the hydration products, CH, whose amount is governed by the cement

content. Therefore, there is an upper limit for cement replacement level, beyond which no further pozzolanic reaction of GP can occur. In that case, GP can only play

200 300 400 500 600 700 800 90065

70

75

80

85

90

95

100TG

wei

ght l

oss,

%

Temperature, °C

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

(a)

DTG

, dm

/dT

(mg/

°C)

Portlandite

Calcite

0 28 56 840

5

10

15

20

25

(b)

91 91 91

60GP

30GP

% C

a(O

H)2

Curing time, days

OPC

7

Fig. 7 (a) TG and DTG curves for OPC paste at 28-day and (b) Portlandite content at different ages.

0

20

40

60

80

100

1E-3 0.01 0.1 1 10 1000

20

40

60

80

100

% o

f tot

al in

trusi

on v

olum

e

7-day

90-day

Pore size, μm

OPC 30GP 60GP OPC+15GP

Fig. 8 MIP curves for paste containing different contents of glass powder at 7 and 91 days.

0 15 30 45 60 750

10

20

30

40

50

60

70

91-day

28-day

15% Addition

Com

pres

sive

stre

ngth

, MPa

GP content, %

7-day

Fig. 9 Compressive strength of concrete with glass pow-ders at different ages.

H. Du and K. H. Tan / Journal of Advanced Concrete Technology Vol. 12, 468-477, 2014 474

the role of inert filler without being activated. The re-sults in this study indicate that GP will exhibit obvious pozzolanic reaction provided that the replacement level is lower than 30%. Mix of OPC+15GP exhibited much higher increase in compressive strength, that is 32%, 25% and 24% at 7, 28 and 91 days, respectively. It would be readily for the added fine GP to react as poz-zolans instead of being inert fillers.

The pozzolanic reaction improves not only the pore structure in the bulk cement paste but also at the interfa-cial transition zone (ITZ) between coarse aggregates and cement paste. This ITZ governs the mechanical and transport properties of concrete since it is more porous compared to the bulk paste. CH content is also relatively higher at ITZ which favors the pozzolanic reaction of glass powder. The enhanced microstructure at ITZ has contributed to the compressive strength with up to 30% GP content.

3.7 RCPT result of concrete The total charge passing the concrete with different OPC replacement levels are shown in Fig. 10, at 7, 28 and 91 days. Due to the low voltage applied in this study, the temperature rise during the 24 hours test was less than 3 ˚C. The RCPT results show that the total charge passed decreased significantly with increasing glass powder content, regardless of the test age. RCPT is es-sentially an indication of the electrical conductivity, instead of a direct measurement for chloride permeabil-ity. Therefore, the result depends on not only the pore structure but also the chemistry of the pore solution, such as ions (Ca2+, Na+, K+, OH-, etc) concentrations. However, it is the pore structure and not the pore chem-istry that govern the transport of ions in cement com-posites. Therefore, it is also questionable to use RCPT to assess the permeability of concrete with supplemen-tary cementitious materials such as silica fume and fly ash (Shi et al. 1998).

At 7 days, the total charge passed reduced almost linearly with decreasing OPC content. The primary rea-son could be the dilution of OPC in the concrete, as a result of which there are fewer ions in the cement paste.

With longer curing time, all the concrete mixtures ex-hibited reduction in the total charge passed due to the further hydration of OPC. The reduction caused by glass powder became more distinct, especially for OPC re-placement between 15 and 45%. This further reduction is attributed to the pozzolanic reaction between GP and cement paste, resulting in a more refined microstructure of secondary C-S-H especially at the ITZ as well as a lower concentration of ions. Glass powder has a similar effect on the RCPT result whether it adds to or replaces OPC as cementitious material, particularly in the long term. At an early age, concrete mix with 15% GP addi-tion has a higher resistance possibly because of the hy-dration acceleration effect of GP, as discussed earlier.

3.8 Water penetration resistance of concrete The water penetration depth into concrete with different contents of GP is shown in Fig. 11. Consistent with the RCPT results, the depth of water penetration continu-ously decreases with higher glass powder content. Rela-tive to the plain concrete, the water penetration depth was reduced by 54, 65, 68 and 80% for cement re-placement level of 15, 30, 45 and 60%, respectively. Concrete with 15% GP additive showed much lower water penetration depth compared to the plain concrete. The refined pore structure, particular the ITZ is the main reason for this reduced permeability. The reduction in the total charge passed in the RCPT tests at 28 days was 60, 81, 89 and 95% for 15, 30, 45 and 60% of glass powder content, respectively. Contrary to RCPT results, the resistance to water penetration can accurately reflect the pore structure. Hence, both accelerated and non-accelerated tests demonstrate the better resistance against chloride ions and water ingress, for concrete containing glass powder as cement replacement and additive. 3.9 Interfacial transition zone in concrete The microstructures of paste at ITZ are compared for plain concrete and 30% GP concrete in Fig. 12. It is clear to find CH (plate crystals) and ettringite (needle shape) in the paste of the OPC concrete. Large pores

0 15 30 45 60 750

1000

2000

3000

4000

5000

6000

15% Addition

Tota

l cha

rge

pass

ed, C

oulo

mbs

GP content, %

7-day 28-day 91-day

Fig. 10 RCPT results of concrete with glass powders atdifferent ages.

0 15 30 45 60 750

10

20

30

15% Addition

Wat

er p

enet

ratio

n de

pth,

mm

GP content, %

Fig. 11 Water penetration depth into concrete with glass powder at 28 days.

H. Du and K. H. Tan / Journal of Advanced Concrete Technology Vol. 12, 468-477, 2014 475

exist between different hydration phases. In contrast, a dense and more homogenous structure is found for con-crete with 30% glass powder. The pozzolanic reaction over 91 days turned crystal CH into amorphous C-S-H. Large pores were much less noticeable. This change in the chemical compositions (more C-S-H formed) and pore size distribution (refined pore system) is the reason for the better mechanical and durability performances for concrete with glass powder as OPC substitution.

The pore size distribution of the microstructure was also investigated for concrete including the ITZ, as shown in Fig. 13. It is clear that with OPC partially re-placed by glass powder, the pores are more refined, in-dicated by the downwards shift of the curve. From the figure, it can be seen that the critical pore diameter be-

comes smaller for glass powder concrete. Therefore, the effect of glass powder pozzolanic reaction on the con-crete is distinct in comparison to that on paste. The rea-son is attributed to the existence of ITZ where the con-centration of CH and water was higher and it was there-fore easier to react with glass powder to form secondary C-S-H, as mentioned above.

4. Conclusions

This study reported experimental results on the use of recycled glass powder as supplementary cementitious material in paste and concrete. Unlike previous work, this study examined the effects of a higher glass powder dosage of up to 60%. Based on the newly obtained re-sults, the following conclusions can be drawn: 1. The rate and total heat generated during hydration

consistently decreased with higher GP content due to the dilution of cement in the mix. Owning to its neg-ligible water absorption capability, higher glass pow-der content results in a smaller shear yield stress and plastic viscosity of the paste.

2. Calcium hydroxide content decreases with glass powder content and curing age, because of the re-duced cement content in the initial stage and its con-sumption by glass powder pozzolanic reaction at the later stage. Calcium hydroxide was almost depleted at 91 days when more than 30% cement was substi-tuted by glass powder.

3. An optimum cement replacement of 30% by glass powder was observed with respect to the develop-ment of compressive strength of concrete after 7 days.

C-S-H

CH

ettringite

(a) OPC concrete.

C-S-H

(b) 30GP concrete.

Fig. 12 SEM images of paste microstructures.

1E-3 0.01 0.1 1 10 1000.00

0.02

0.04

0.06

0.08

0.10

0.12

Cum

ulat

ive

pore

vol

ume,

mL/

g

Pore size, μm

OPC 30GP 60GP OPC+15GP

Fig. 13 MIP results for GP concrete at the age of 91 days.

H. Du and K. H. Tan / Journal of Advanced Concrete Technology Vol. 12, 468-477, 2014 476

However, with respect to the resistance to water and chloride penetration, 60% seems to be the optimum replacement. Compared to the reference concrete mix, the mix with 60% glass powder exhibited 75% strength at 7 days and 85% strength at 91 days.

4. Resistance to chloride ion and water penetration re-sistance were greatly improved by replacing cement with glass powder, due to the refined microstructure of paste, particular at the ITZ where calcium hydrox-ide concentration is higher and more readily available for pozzolanic reaction. With 60% cement replace-ment, the electrical resistance and water penetration depth of concrete decreases to 5% and 20% of the reference concrete.

5. When used as additional supplementary cementitious material at 15% level, glass powder can obviously reduce the porosity and the pore size distribution. Thus, large increases in compressive strength, resis-tance to water and chloride penetration were ob-served. This study only investigated finely ground glass

powder as cement replacement or additive in concrete based on soda lime glass, which is the most common type of glass and accounts for 90% of manufactured glass. It is a open question as to whether the results could apply to other types of glass like lead glass and borosilicate glass. The pozzolanic reactivity depends on the chemical composition and properties of the glass. To understand the pozzolanic performance of a specific glass powder, it is recommended to (1) carry out chemi-cal composition analysis to meet the minimum chemical requirement for pozzolans; (2) determine the strength activity index; and (3) investigate the different replace-ment levels on mechanical and durability performances on concrete.

Acknowledgements The technical assistance of Ms. Li Wei and Mr. Ang Beng Oon from the Structural Engineering and Material Laboratory, National University of Singapore, in the conduct of TGA and XRD tests, is gratefully acknowl-edged. References Ali, E. E. and Al-Tersawy, S. H., (2012). “Recycled

glass as a partial replacement for fine aggregate in self compacting concrete.” Construction and Building Materials, 35, 785-791.

ASTM C 39/C 39M, (2005). “Standard test method for compressive strength of cylindrical concrete speci-mens.” Philadelphia: American Society of Testing and Materials.

ASTM C 109/C 109M, (2005). “Standard test method for compressive strength of hydraulic cement mortars (using 2-in. or [50-mm] cube specimens).” Philadel-phia: American Society of Testing and Materials.

ASTM C 1202, (2005). “Standard test method for electrical indication of concrete’s ability to resist

chloride ion penetration.” Philadelphia: American Society of Testing and Materials.

BS EN 12390-8, (2009). “Testing hardened concrete part 8: depth of penetration of water under pressure.” London: British Standards Institution, London.

BS EN 12390-3, (2009). “Testing hardened concrete Part 3: compressive strength of test specimens.” London: British Standard Institution.

Carsana, M., Frassoni, M. and Bertolini, L., (2014). “Comparison of ground waste glass with other supplementary cementitious materials.” Cement and Concrete Composites, 45, 39-45.

Du, H. and Tan, K. H., (2013). “Use of waste glass as sand in mortar: Part II - alkali-silica reaction and mitigation methods.” Cement and Concrete Compo-sites, 35(1), 118-126.

Du, H. and Tan, K. H., (2014a). “Concrete with recycled glass as fine aggregates.” ACI Materials Journal, 111(1), 47-58.

Du, H. and Tan, K. H., (2014b). “Effect of particle size on alkali-silica reaction in recycled glass mortars.” Construction and Building Materials, 66, 275-285.

Du, H. and Tan, K. H., (2014c). “Transport properties of concrete with glass powder as supplementary cementitious material.” ACI Materials Journal, ac-cepted for publication.

Dyer, T. D. and Dhir, R. K. (2001). “Chemical reactions of glass cullet used as cement component.” Journal of Materials in Civil Engineering, 13(6), 412-417.

Helmuth, R., (1987). “Fly ash in cement and concrete.” Illinois: Portland Cement Association.

Jain, J. A. and Neithalath, N., (2010). “Chloride transport in fly ash and glass powder modified concretes-influence of test methods on micro-structure.” Cement and Concrete Composites, 32(2), 148-156.

Jawed, I. and Skalny, J., (1978). “Alkalis in cement: a review II. Effects of alkalis on hydration and performance of Portland cement.” Cement and Concrete Research, 8, 37-51.

Jin, W., Meyer, C. and Baxter, S., (2000) “Glascrete-concrete with glass aggregate.” ACI Materials Journal, 97(2), 208-213.

Khmiri, A., Chaabouni, M. and Samet B., (2013). “Chemical behavior of ground waste glass when used as partial cement replacement in mortars.” Construc-tion and Building Materials, 44, 74-80.

Kim, J., Moon, J. H., Shim, J.W., Sim, J., Lee, H.G. and Zi, G., (2014). “Durability properties of a concrete with waste glass sludge exposed to freeze-and-thaw condition and de-icing salt.” Construction and Building Materials, 66, 398-402.

Kocaba, V., (2009). “Development and evaluation of methods to follow microstructural development of cementitious systems including slags.” PhD thesis. Ecole Polytechnique Federale de Lausanne.

Ling, T. C., Poon, C. S. and Kou, S. C., (2011). “Feasibility of using recycled glass in architectural

H. Du and K. H. Tan / Journal of Advanced Concrete Technology Vol. 12, 468-477, 2014 477

cement mortars.” Cement and Concrete Composites, 33(8), 848-854.

Matos, A. M. and Sousa-Coutinho, J., (2012). “Durability of mortar using waste glass powder as cement replacement.” Construction and Building Materials, 36, 205-215.

McGrath, P. F. and Hooton, R. D., (1999). “Re-evaluation of the AASHTO T259 90-day salt ponding test.” Cement and Concrete Research, 29(8), 1239-1248.

Mindess, S., Young, J. F. and Darwin, D., (2003). “Concrete.” 2nd ed. New Jersey: Prentice Hall.

Mirzahosseini, M. and Riding, K. A., (2014). “Effect of curing temperature and glass type on the pozzolanic reactivity of glass powder.” Cement and Concrete Research, 58, 103-111.

Neithalath, N., (2008) “Quantifying the effects of hydration enhancement and dilution in cement pastes containing coarse glass powder.” Advanced Concrete Technology, 6(3), 397-408.

Rajabipour, F., Maraghechi, H. and Fischer, G., (2010). “Investigating the alkali-silica reaction of recycled glass aggregates in concrete materials.” Journal of Materials in Civil Engineering, 22(12), 1201-1208.

Shayan, A. and Xu, A., (2004). “Value-added utilization of waste glass in concrete.” Cement and Concrete Research, 34(1), 81-89.

Schwarz, N., Cam, H. and Neithalath, N., (2008). “Influence of a fine glass powder on the durability

characteristics of concrete and its comparison to fly ash.” Cement and Concrete Composites, 30(6), 486-496.

Shao, Y., Lefort, T., Moras, S. and Rodriguez, D., (2000). “Studies on concrete containing ground waste glass.” Cement and Concrete Research, 30(1), 91-100.

Shi, C., Wu, Y., Riefler, C. and Wang, H., (2005). “Characteristics and pozzolanic reactivity of glass powders.” Cement and Concrete Research, 35(5), 987-993.

Shi, C., Stegemann, J. A. and Caldwell, R. J., (1998). “Effect of supplementary cementing materials on the specific conductivity of pore solution and its implications on the rapid chloride permeability test (AASHTO T277 and ASTM C1202) results.” ACI Materials Journal, 95(4), 389-393.

Taha, B. and Nounu, G., (2009). “Utilizing waste recycled glass as sand/cement replacement in concrete.” Journal of Materials in Civil Engineering, 21(12): 709-721.

Tan, K. H. and Du, H., (2013). “Use of waste glass as sand in mortar: Part I - fresh, mechanical and dura-bility properties.” Cement and Concrete Composites, 35(1), 109-117.

Wright, J. R., Cartwright, C., Fura, D. and Rajabipour, F., (2014). “Fresh and hardened properties of concrete incorporating recycled glass as 100% sand replacement.” Journal of Materials in Civil Engineering, 26(10), 04014073.