Zeolite to improve strength-shrinkage performance of high-strength engineered cementitious composite

Construction and Building Materials 234 (2020) 117335

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Zeolite to improve strength-shrinkage performance of high-strength engineered cementitious composite Qing Wang a, Jun Zhang b, J.C.M. Ho a,⇑ a b

School of Civil Engineering, Guangzhou University, Guangzhou 510006, China Department of Civil Engineering, Tsinghua University, Beijing 100084, China

h i g h l i g h t s  Using natural zeolite (to replace partial sand) can decrease the 28-day autogenous shrinkage of ECC.  Using calcined zeolite can decrease both 28-day autogenous and drying shrinkage.  The reduction of 28-day total shrinkage of ECC containing zeolite is due to the increase of the internal relative humidity.  Using Zeolite can improve the simultaneous limits of strength-and-shrinkage achieved by ECC.

a r t i c l e

i n f o

Article history: Received 8 July 2019 Received in revised form 17 October 2019 Accepted 18 October 2019

Keywords: Zeolite Shrinkage Engineered cementitious composite High strength Internal curing

a b s t r a c t Engineered cementitious composite (ECC) is a class of high-performance material because it displays strain hardening by multiple crack formation under tension. This unique property makes ECC an ideal constructional binding mortar or repairing material particularly for concrete cracks in existing structures. However, a major issue of ECC is the high shrinkage, which creates differential shrinkage and extra tensile stress that adversely affects its durability. To mitigate shrinkage, a novel way of using zeolite as an internal curing agent without sacrificing the strength of ECC is herein advocated. Zeolite is structurally porous that can trap water and act as a water reservoir to increase internal relative humidity (IRH). In this paper, the shrinkage, IRH and compressive strength of ECC containing 15% (18%), 20% (24%), and 30% (36%) of zeolite replacing quartz sand by weight (and by volume) is studied experimentally. Test results indicate that the 28-day total shrinkage of ECC decreases with the zeolite replacement ratio. From the shrinkageto-strength ratio, it shows that ECC with 30% zeolite yields the lowest shrinkage per compressive strength, and hence the optimal ratio for quartz sand replacement in this study. With the beneficial effect observed, zeolite replacement ratio greater than 30% (36%) is recommended for future study on shrinkage reduction of ECC. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Engineered Cementitious Composites (ECC) is a class of fiber reinforced cementitious material with high performance that can improve the ductility and durability of concrete. It was initially designed by Li [1,2] using micromechanical approach that gives it a unique characteristic of strain hardening, which was not exhibited in traditional Fiber Reinforced Cement Composite (FRCC) under tension. It increases the tensile load-carrying capacity after the onset of tension crack. Previous studies showed that the strain hardening in ECC happens due to the formation of multiple parallel fine cracks, the ends of which were bridged by the fibers. In each ⇑ Corresponding author. E-mail address: [email protected] (J.C.M. Ho). https://doi.org/10.1016/j.conbuildmat.2019.117335 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

individual crack, the crack width increases steadily to around 40–100 lm under tension, beyond which the tensile capacity reaches maximum and starts to drop [3–6]. The initial increasing crack width and additional cracks contribute to further increase in strain as the bridging fibers’ stresses increases. At about 40– 100 mm crack width depending on the volume of fibers when the stress transfer capability of fibers reaches their limit, cracking propagation stops and crack localization occurs at a strain of 3– 5% with crack spacing of 3–6 mm and average crack width around 60 lm [7–10]. Beyond this point, tension softening would occur and the load capacity decreases as the deformation increases. With the above unique strain hardening characteristic, ECC is considered an ideal construction material in the application of ductility design of reinforced concrete (RC) structures, binding mortar as well as repairing material for existing defective or damaged

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Q. Wang et al. / Construction and Building Materials 234 (2020) 117335

concrete structures, e.g. remedial work for non-structural cracks [11,12] occurred in RC water retaining structures due to inappropriate concrete mix design, improper concreting procedure or curing method. Despite its versatile application in the industry, the practical use of ECC in construction is still quite limited [13,14]. One major reason is that ECC has much higher shrinkage than concrete [15–17]. It is because in order to allow the formation of fine parallel cracks in ECC, coarse aggregates are not used in ECC [18]. Fine sand (<200 mm) is allowed in very small amount just only to avoid large amount of cement hydration heat and to control the fracture toughness [19,20]. Assuming 65% volumetric ratio of aggregates in concrete, the shrinkage strain in ECC can be as high as triple (1/0.35) of that in concrete. This was also verified by previous reported studies [17,21,22], which showed that the 28-day total shrinkage in ECC could reach ~2000m. The extremely high shrinkage in ECC will make it impractical in construction application, particularly as binding mortar or repairing material, where it is normally applied to an older concrete with much lesser shrinkage. Under such a circumstance, differential shrinkage will create large tensile stress in ECC that offsets the extra tensile loadcarrying capacity contributed by the strain hardening of ECC. High-strength ECC (HSECC) that has higher strength-to-weight ratio with low moisture transport coefficient, high freezing resistance and low permeability has recently been developed by Zhang et al. [10,23]. This type of HSECC, whose compressive strength can reach over 80 MPa, consists of hybrid fibers of polyvinyl alcohol and steel that also exhibits strain hardening by multiple cracks formation. Hence, HSECC increases the concurrent strength and ductility performance of structural members that should have a very broad range of industrial application. From structural point of view, HSECC can potentially be used as ductile layer for steelconcrete composite bridge decks and high performance fillers for steel-tube columns[24–28], which require high-strength material to resist strong interior stress caused by coupled loads of temperature fluctuation, time-dependent deformation and mechanical loading, as well as adequate ductility to resist impact and blasting loads. From retrofitting’s point of view, HSECC is an ideal repairing mortar for beams and lower storey columns, where high strength and ductility but light self-weight are required, in momentresisting framed building after earthquake attack. In spite of the above merits, the shrinkage of HSECC is even worse than normalstrength ECC [10,29]. It is because firstly the lower water-tobinder (w/b) ratio will increase the self-desiccation due to higher capillary stress. Secondly, the water content is higher for having larger paste volume in order to restore workability. Then, moisture loss of HSECC increases and thus is the total shrinkage [30]. As stated before, large shrinkage of HSECC needs to be treated cautiously as it will trigger harmful differential shrinkage cracks, impair durability of structures, require more frequent and costly structural maintenance, and most importantly shortens the servicing period. When applying as repairing mortar to existing RC structures, it can cause delamination of ECC-concrete interface that adversely affect the structural strength and ductility. To mitigate the shrinkage in mortar/concrete, the two shrinkage mechanisms occur in mortar/concrete need to be understood and the effect minimized [31–33]. There are two major water migration processes leading to moisture loss inside mortar/concrete. The first type is due to the cement hydration when concrete is still a fluid mass, in which free water reacts with cement to form hydration product that has a smaller volume which is known as the chemical shrinkage. In high-strength mortar or concrete where the free water content is low and finer powders are used, it forms very fine partially saturated capillary pores in visco-elastic state of concrete that enables menisci to form and increase capillary stress to accelerate moisture loss (i.e. self-desiccation). Chemical shrinkage and self-desiccation occur almost simultaneously during the

early age of mortar/concrete and their associated total volumetric change is referred to as autogenous shrinkage [34–36]. The second type of moisture loss is due to environmental drying, which is a moisture transfer process as a result of the humidity difference between the ambient and that inside the capillary pores of mortar/concrete via evaporation. This causes a contraction in the total volume of the mortar/concrete which is known as drying shrinkage [37,38]. In order to mitigate the shrinkage in normal- and high-strength ECC, both the autogenous and drying shrinkage need to be controlled [17,39]. Internal curing has been increasingly adopted as an effective method to complement moisture loss and reduce shrinkage in cementitious composite. Some of these curing agents are pre-wetted lightweight aggregate [40,41], super absorbent polymer [42,43] and expanded perlite aggregate [44]. However, the drawback of using these agents is that it also decreases the strength of composite owing to the relatively large pores present in the mortar. Hence, in order to enhance the simultaneous strength-and-shrinkage performance of ECC, exploring a new type of internal curing agent that could decrease the shrinkage while keeping the compressive strength is essential. To this, the authors had in their previous research [10] proposed to use zeolite powder in expansive calcium sulfoaluminate cement (SAC) based HSECC. Technically, zeolite powder is porous and can act as a water reservoir in the composite to keep the composite in a high internal relative humidity (IRH) [45,46]. Moreover, it is inert (with water) and consequently will not affect the w/b ratio and hence strength of HSECC. Practically, zeolite is abundant in nature and cheaper compared with the other popular internal curing agents in China [47,48]. There were researches carried out previously on the use of zeolite aggregates to replace partial aggregates for providing internal curing of SAC-ECC [10] and proven successful. In this study, the authors take a step forward to produce HSECC by replacing SAC with Ordinary Portland Cement (OPC) that can further push up the maximum design limit of compressive strength over 80 MPa and decrease the cost of production, as OPC is cheaper and easier to produce than SAC. Nonetheless, the shrinkage of HSECC containing OPC (OPC-HSECC) is worse than that of SAC based HSECC because OPC is non-expansive [49,50]. To solve the problem, partial replacement of sand by zeolite powder is proposed herein to lower the shrinkage of OPC-HSECC. In this paper, the shrinkage and IRH of OPC-HSECC containing zeolite as internal curing agent were investigated experimentally. Various replacement ratios by weight (and by volume) of quartz sand by natural or calcined zeolite powder at 15% (18%), 20% (24%) and 30% (36%) were selected. The obtained test results showed that use of zeolite at the selected replacement ratios can successfully decrease the 28-day shrinkage of OPC-HSECC while maintaining similar level of compressive strength. Accordingly, zeolite powder can increase the simultaneous strength-and-shrinkage performance of OPC-HSECC, and is more cost effective for real industrial application.

2. Materials, testing samples, set-up and procedure 2.1. Materials OPC, fly ash, ground granulated blast furnace slag, silica fume and zeolite were used for mixing HSECC mortar in this experimental study. Chemical compositions of raw materials are presented in Table 1. OPC is popularly used in the construction industry in China, and it complies with Chinese standard GB175 [51]. Fly ash, ground granulated blast furnace slag and silica fume are essential supplementary cementitious materials for lowering cement content and producing high-strength mortar/concrete.

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Q. Wang et al. / Construction and Building Materials 234 (2020) 117335 Table 1 Chemical composition of raw materials used in tests (wt%). Type

SiO2

CaO

Al2O3

Fe2O3

MgO

K2O

Na2O

SO3

LOI

OPC Silica fume (SF) Fly ash (FA) Slag (SG) Zeolite

23.67 90.56 47.02 38.83 64.27

59.98 0.81 5.08 38.70 1.99

7.21 0.41 35.06 12.92 12.7

3.07 0.52 3.88 1.46 0.68

2.07 0.95 1.36 4.63 0.46

0.62 1.59 1.30 0.37 2.90

0.17 0.63 1.18 0.28 2.30

2.14 – 0.89 0.60 0.34

1.01 3.72 1.85 0.06 14.18

They comply with the Chinese Standards GB/T1596 [52], GB/ T18046 [53] and GB/T27690 [54] respectively. Quartz sand with particle size between 75 and 150 mm was added in the design mix of the mortar base. To manufacture HSECC, polyvinyl alcohol fiber (PVA) was added into the mortar as reinforcement, mechanical properties given in Table 2.

2.2. Test specimens of HSECC Five groups in total 10 specimens of OPC-HSECC were designed and tested in this study. Group 1 consisted of control specimens which are ordinary cured specimens (OC) with only quartz sand and no zeolite replacement. The OC specimens were produced by adding 2% volume of PVA into the mortar base, which was the same in all other groups of specimens. The following describes the mix design of the mortar base: Fig. 1. Typical slump flow of mixtures (270 ± 5 mm).

- w/b = 0.2 by weight; - quartz sand/binder = 0.833 by weight; - Binder consists of OPC, FA, GGBS and SF in ratio 7:1:1:1 by weight; The above mix design of mortar was shown in an authors’ previous study capable of reaching around 100 MPa under standard water curing condition [23]. Because of the low w/b ratio, about 0.9% of polycarboxylate based superplasticizer was added to the mortar for achieving a similar slump flow of 270 ± 5 mm (shown in Fig. 1). For the rest of the testing specimens, they all consisted of zeolite, which was to replace partial sand in the OC specimen: - Group 2 consisted of specimens containing 15% (18%) of quartz sand by weight (and by volume) replaced with natural zeolite. - Group 3: 15% (18%) of quartz sand replaced with calcined zeolite. - Group 4: 20% (24%) of quartz sand replaced with calcined zeolite. - Group 5: 30% (36%) of quartz sand replaced with calcined zeolite. The readers should note that there will be a slight change in the total absolute volume of aggregates and paste due to the replacement of quartz sand by an identical weight of zeolite rather than by an identical volume. Since the density of zeolite is smaller than sand, the volume of aggregates will increase whereas that of paste will decrease after replacement. Then, the weight of cement will change slightly, which is shown in Table 3. It can be observed that the change in cement’s weight is insignificant, being about 1% for 15% zeolite replacement. The reason of using replacement ratio

by weight rather than volume is to follow the industrial practice, where weight is much easier to measure. Natural and calcined zeolite particles were used in this study to improve the shrinkage performance. Natural zeolite is a kind of aluminosilicate mineral containing large number of micro-pores and crystal water [55]. Nevertheless, it was reported in previous studies [56] that the internal curing efficiency of natural zeolite is rather low. Therefore, natural zeolite was calcined in a 500 °C muffle furnace for 30 min in order to improve the water absorption, which was used in Groups 3 to 5 specimens. Furthermore, each group comprised 2 identical OPC-HSECC specimens, in which one of them was completely sealed (denoted by subscript ‘‘s”), while another was not sealed after 3-day curing (denoted by subscript ‘‘d”), for studying the autogenous and drying shrinkage. Fig. 2 shows the two identical specimens under shrinkage testing. The particle size distributions (PSD) of natural zeolite, calcined zeolite and quartz sand were shown in Fig. 3. It is evident that calcination would enlarge the size of zeolite particles ranging from 1 to 20 lm, whereas it decreased the size of zeolite particles ranging from 200 lm to 1 mm due to evaporation of crystallization water at 500 °C. For size from 150 to 200 mm, no effect on their particle size was observed. The specific surface areas of calcined zeolite, natural zeolite and quartz sand are 49 m2/g, 41 m2/g and 1.2 m2/ g, The pore content of calcined zeolite, natural zeolite and quartz sand was determined as 0.26 ml/g, 0.045 ml/g and 0.002 ml/g respectively the same as in previous research [10]. The pore size distribution of the three particles using nitrogen adsorption test were displayed in Fig. 4. The significantly higher pore volume in calcined zeolite was because of the evaporation of physically bonded water in natural zeolite under high temperature. On the

Table 2 Properties of PVA fiber. Density (g/cm3)

Tensile strength (MPa)

Elastic modulus (GPa)

Diameter (mm)

Length (mm)

1.2

1620

42.8

0.039

12

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Q. Wang et al. / Construction and Building Materials 234 (2020) 117335

Table 3 Mix Proportions of test series. Group

Mix.

Cementitious MaterialsTotal (Cement:SF:SG:FA)

Cement kg/m3

Sand

Natural Zeolite

Calcined Zeolite

Water

PVA fiber (volume,%)

Superplasticizer (%)

1 2 3 4 5

OC IC-1N IC-1C IC-2C IC-3C

1 1 1 1 1

787.8 779.2 779.2 776.5 770.8

0.833 0.708 0.708 0.666 0.583

0 0.125 0 0 0

0 0 0.125 0.167 0.250

0.20 0.20 0.20 0.20 0.20

2 2 2 2 2

0.88 0.90 0.88 0.90 0.92

(0.70:0.10:0.10:0.10) (0.70:0.10:0.10:0.10) (0.70:0.10:0.10:0.10) (0.70:0.10:0.10:0.10) 0.70:0.10:0.10:0.10)

180 160

Nitrogen Adsorption

Volume (cm3/g)

140

Calcined Zeolite Natural zeolite Quartz sand

120 100 80 60 40 20 0 0.0

0.2

0.4

0.6

0.8

1.0

P/P0 Fig. 4. Pore distribution of Calcined zeolite, Natural zeolite and quartz sand based on nitrogen adsorption tests.

2.3. Test set-up Fig. 2. Two identical HSECC specimens under plastic sealing (left) and drying (right) conditions.

(1) Two 20  100  400 mm plates were put on the inner surface of the longer side of the mould; (2) Four pieces of 2-mm thick plastic sheets were put on all inner surfaces of the mould; (3) A 1-mm thick vinyl sheet was put on the bottom surface of the mould.

10

Calcined zeolite Natural zeolite Quartz sand

Volume fraction (%)

8

6

4

2

0 0.01

0.1

1

10

The dimensions of shrinkage mould were 100  100  400 mm. Before placing the specimens, the moulds were set up as follows:

100

1000

10000

Particle size (µm)

After initial setting of OPC-HSECC in the mould, the inner plastic sheets were removed to create a free restraint condition as usual [58], whereas the two filler plates were removed to create an unsaturated condition for drying of specimens. Thus, the shrinkage measurement will show the effects of both autogenous and drying shrinkage. The IRH and temperature at the central position of the OPC-HSECC samples were measured by a digital integrated sensor following authors’ previous setup [10]. The deformation along the long side of specimen was measured by two linear variable differential transducers (LVDTs) fixed on specimen’s ends with maximum travelling range of 2 mm and a precision of 1 mm following Zhang et al. [59,60]. Fig. 5 shows the schematic diagram of the test set-up.

Fig. 3. Particle size distribution of calcined zeolite, natural zeolite and quartz sand.

2.4. Test procedure other hand, the pore volumes in natural and calcined zeolite were higher than that in quartz sand, which enable them to act as water reservoirs in HSECC. Table 3 summarizes the details of the design mix of all OPC-HSECC samples based on mass design method in Chinese Standard JGJ 55 [57].

The mixing procedure of the OPC-HSECC samples is as follows: (1) The binder and quartz sand were mixed in a concrete mixer for 2 min.

Q. Wang et al. / Construction and Building Materials 234 (2020) 117335

5

Fig. 5. Schematic diagram of internal relative humidity and deformation measurement.

(2) Water + superplasticizer and/or pre-wetted zeolite slurry with designed water content were added to the mixture and subsequently mixed for 2 min. (3) The PVA fibers were gradually spread into the mixer by hand during low-speed mixing to ensure homogeneous distribution. The mixing was continued for another 2 min. (4) Fresh sample was placed in the mould in two layers and compacted. The casting surface was then covered by a plastic sheet to prevent moisture loss. (5) In an hour after casting, the humidity & temperature sensors and LVDTs were installed. The temperature, humidity and shrinkage measurement were continued until 28 days after casting. In addition to shrinkage specimen, three 100 mm cubic specimens were also cast and cured under sealed condition to determine the 28-day compressive strength. 3. Experimental results and discussion 3.1. Compressive strength at 28th day The average 28-day compressive strength of three 100 mm cubes of the OPC-HSECC tested specimens were summarized in Table 4. It is evident that the compressive strength of OC (104 MPa) is the highest, which shows that the replacement of quartz sand by natural/calcined zeolite will decrease the compressive strength. It is because of the relatively porous structure of zeolite compared with that of quartz sand, as confirmed by the results of nitrogen adsorption test presented before. Particularly in higher strength concrete/mortar, the failure plane will pass through the sand/aggregates. The use of porous zeolite will decrease the elastic modulus of the aggregates and the stress at a given strain. The 28-day compressive strength also decreased as the ratio of calcined zeolite increased. The reduction was gradual as the ratio of calcined zeolite increased from 0% to 20% (24%), with the compressive strength decreasing from 102 MPa to 100 MPa (~2% decrease). Nevertheless, the compressive strength dropped to 95 MPa (~5% decrease) when the ratio of calcined zeolite increased from 20% (24%) to 30% (36%). Consequently, it is apparent that the Table 4 28th day compressive strength of tested OPC-HSECC specimens. Group

Mix.

Compressive strength (MPa)

1 2 3 4 5

OC IC-1N IC-1C IC-2C IC-3C

104.53 102.18 101.05 100.24 95.36

preservation of water reservoir and maintenance of compressive strength of OPC-HSECC are contradicting to each other. In the event that the zeolite replacement of sand is too low, the shrinkage of OPC-HSECC will become large (as explained next in 3.2), whereas in the event that the zeolite replacement of sand is too much, the compressive strength of OPC-HSECC will be impaired. As a result, it is crucial to find an optimal ratio of calcined zeolite that could keep high water content to limit shrinkage, and at the same time not impairing significantly the compressive strength. Sufficient strength of ECC is the basis to guarantee structural application and adequate shrinkage for crack resistance, which is also critical considering the durability of structures. 3.2. Shrinkage of OPC-HSECC The effect of using OPC instead of SAC, as well as the use of natural and calcined zeolite, on the shrinkage of HSECC is discussed herein. In this regard, the respective measurement obtained in a previous test conducted by the authors [10] have been recited in Table 5 alongside with those measured in this study for the OPCHSECC specimens. Fig. 6 shows the variation of shrinkage against age of tested specimen up to 28 days after casting. Fig. 6(a) illustrates the variation of shrinkage in sealed condition, whereas Fig. 6(b) illustrates that in drying conditions. From Table 5 and Fig. 6, it is evident that: (1) The use of OPC to replace SAC increased significantly the 28day shrinkage from 679m to 1560m in sealing condition, and 726m to 2283m in drying condition. It therefore indicated that the use of OPC increased both the autogenous and total shrinkage at 28th day by 2–3 times. (2) The shrinkage platform e1, which is the turning point demarcating the abrupt decrease in the rate of increasing shrinkage, also increased significantly when compared with that of SAC-HSECC test specimens [34]. For the OC specimen tested in this study, e1  896m and 1980m in sealed and drying conditions as shown in Fig. 6(a) and (b) respectively. Nonetheless, the respective shrinkage value was only respectively 588m and 625m in SAC-HSECC [34]. (3) The average rate of shrinkage increases before reaching e1 of OPC-HSECC were about 896m5 = 179m/day and 1980  6 = 330m/day in sealed and drying condition respectively, which were considerably higher than those of SACHSECC, i.e. 588m5 = 118m/day and 650m4 = 162m/day. (4) The above observation can be explained by the expansive nature of SAC, which expands during the curing that offset the volumetric contraction of HSECC due to autogenous and drying shrinkage. Conversely, OPC is non-expansive and could not offset the contraction.

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Q. Wang et al. / Construction and Building Materials 234 (2020) 117335

Table 5 28th day total shrinkage strain of tested OPC-HSECC and SAC-HSECC (lm/m). Group

Mix.

Reference 1 2 3 4 5

SAC,s SAC,d OC,s OC,d IC-1N,s IC-1N,d IC-1C,s IC-1C,d IC-2C,s IC-2C,d IC-3C,s IC-3C,d

Shrinkage of platform

Total shrinkage, Seal

Total shrinkage, Dry

Drying Shrinkage

e1

es

ed

e dr

588 625 896 1980 816 845 733 812 656 798 428 586

679 – 1560 – 1142 – 1080 – 883 – 713 –

– 726 – 2283 – 1848 – 1667 – 1464 – 1074

47

Shrinkage strain (µm/m)

1600

1200

(2)

800 Sealed OC,s IC-1N,s IC-1C,s IC-2C,s IC-3C,s

400

(3)

0 0

7

14

21

28

Age(days)

(4)

(a) sealed 2400

(5)

Shrinkage strain (µm/m)

2000

1600

1200 Drying 800

OC,d IC-1N,d IC-1C,d IC-2C,d IC-3C,d

400

0 0

7

14

21

28

Age(days)

(b) drying Fig. 6. Variation of total shrinkage strain of tested OPC-HSECC specimens in sealed and drying conditions.

Then the effects of replacing partial quartz sand by natural zeolite, as well as its replacement ratio on the shrinkage of OPC-HSECC is discussed. From Table 5 and Fig. 6, the following is observed: (1) The replacement of quartz sand by natural zeolite of identical weight can decrease the 28-day total shrinkage. It is be

(6)

723 706 587 581 361

seen by comparing Specimens OC with IC-1N, the 28-day shrinkage with 15% (18%) zeolite replacement dropped from 1560m to 1142m in sealed condition (~27% reduction), and from 2283m to 1848m in drying condition (~19% reduction). The replacement of quartz sand by calcined zeolite of identical weight can further decrease the 28-day total shrinkage. It is be seen by comparing Specimens IC-1C with IC-1N, the 28-day shrinkage with 15% (18%) zeolite replacement dropped from 1142m to 1080m in sealed condition (~6% reduction), and from 1848m to 1667m in drying condition (~10% reduction). The 28-day total shrinkage decreased from 1080m to 713m as the percentage of calcined zeolite increased from 15% (18%) to 30% (36%) in seal condition (~34% reduction), as well as from 1667m to 1074m in drying condition (~36% reduction). Therefore, use of calcined zeolite can decrease the 28-day shrinkage of OPC-HSECC effectively. The shrinkage platform decreased as the ratio of calcined zeolite increases. It decreased from 816m (IC-1N) to 428m (IC-3C) in sealed condition, and from 890m to 451m in drying condition, which indicated 48% and 49% reduction respectively. The average rate of shrinkage increase was greater for calcined zeolite. Calcined zeolite was more effective than natural zeolite in decreasing the shrinkage at the same replacement ratio. For instance, by comparing Specimens IC-1C with IC-1N with 15% (18%) zeolite replacement, shrinkage dropped from 1142m and 1848m to 1080m and 1667m under sealed and drying condition, respectively. The main difference between calcined zeolite and natural one is that the pore content increased after calcination, which provides more space for free water reservation. However, it decreased as the ratio of calcined zeolite increased. The shrinkage rates increased abruptly from 163m/day to 733m/day when calcined zeolite replaced natural zeolite. It decreased gradually to 428m/day in sealed condition when the ratio of zeolite increased from 15% (18%) to 30% (36%). Similar trend was also observed in drying condition. A major reason of the increased shrinkage rate due to calcined zeolite was mainly because of the shorter duration of reaching shrinkage platform, which was within a day for all calcined specimens. Even though the shrinkage rate increased, it occurred within a shorter period of time which is beneficial for shortening the construction cycle and is easier to control. The above observation can be explained similarly by the role of water reservoir and the fact that the calcined zeolite is more porous than natural zeolite. It implies that calcined zeolite can store more water than natural zeolite and increase the water reservoir capacity in OPC-HSECC. The

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Q. Wang et al. / Construction and Building Materials 234 (2020) 117335

100

Internal Relative Humidity (%)

increase in the water capacity with calcined zeolite replacement can be verified by the increase in IRH measured, which is discussed next. (7) It is noted that by replacing part of the quartz sand by an identical volume of zeolite, the absolute content of cement will change. Nevertheless, it is shown in Table 3 that the change in cement’s weight is very small – being only 1% for 15% zeolite replacement – and since the water-tobinder ratio, which was kept constant in this study, is a more critical factor than the absolute cement content on the shrinkage, the change in shrinkage in the zeolite specimens is convinced to be primarily contributed by the replacement of quartz sand with zeolite.

3.4. Simultaneous strength-and-shrinkage performance of OPC-HSECC From Sections 3.2 and 3.3, it is clear that the use of natural or calcined zeolite can decrease the shrinkage of OPC-HSECC, however, it also decreases the strength simultaneously. Accordingly, it is not clear that if the use of zeolite can improve

80

70

Sealed OC,s IC-1N,s IC-1C,s IC-2C,s IC-3C,s

60

50

3.3. Internal relative humidity (IRH) of OPC-HSECC

Environmental RH

40 0

7

14

21

28

21

28

Age(days)

(a) sealed 100

Internal Relative Humidity (%)

Fig. 7 show the variation of IRH against age of the tested specimens up to 28 days after casting. Fig. 7(a) illustrates the respective variation in sealed condition, whereas Fig. 7(b) is in drying condition. The measured IRH at 28 days were also summarized in Table 6. It is seen that initially the IRH remained at 100% for a few days when the composite was a visco-elastic mass from the perspective of binding system. Then, the humidity declined at different rates, which can be revealed by the IRH measured at 28 days. It is evident that the IRH at 28 days was the lowest in sample OC, followed by IC-1 N and then IC-1C. The IRH also increased as the replacement ratio of calcined zeolite increased. As mentioned before, zeolite is porous and is able to store water within its structure. Therefore, the water content inside the composite increased as the ratio of calcined zeolite increased. The water content in HSECC with calcined zeolite is higher than that in natural zeolite because of the more porous structure as confirmed by the Nitrogen adsorption test. The IRH is the lowest in HSECC with no zeolite replacement because the quartz sand cannot store water that resulted in the lowest water content. On the other hand, the internal temperature variation of the specimens remained fairly constant at 24.3 ± 0.2 °C from the first to 28th day. The variation of shrinkage measured at 28 day can be explained by the measured IRH as shown in Table 6. The increase in IRH could create more saturated pores in the composite that resulted in less menisci formed in the capillary pores. Subsequently, the capillary stress was decreased and thus was the rate of self-desiccation and autogenous shrinkage. On the other hand, the use of calcined zeolite could decrease the drying shrinkage significantly whereas natural zeolite could not. The extent of drying shrinkage can be obtained by the difference between the shrinkage strains measured in drying and sealed conditions, which have been listed in the last column of Table 5 [29]. It can be observed that when there was no replacement of sand or when the replacement was by natural zeolite, the drying shrinkage strains were very similar at about 700m. Nevertheless, when calcined zeolite was used at 15% (18%) or 20% (24%), the drying shrinkage dropped to about 580m (~18% reduction). The drying shrinkage was further decreased to 360m (~50% reduction) when the ratio of calcined zeolite increased to 30% (36%), which implies a relatively humid condition that limited water loss from the capillary pores and decreased the drying shrinkage. The use of zeolite for partial replacement of quartz sand will also improve the ultimate tensile strength and strain of ECC [29].

90

90

80

70

Drying OC,d IC-1N,d IC-1C,d IC-2C,d IC-3C,d

60

50

Environmental RH

40 0

7

14

Age(days)

(b) drying Fig. 7. Variation of IRH of tested OPC-HSECC specimens in sealed and drying condition.

Table 6 Internal relative humidity of tested OPC-HSECC specimens. Group

Mix.

IRH at 28 days, Seal (%)

IRH at 28 days, Dry (%)

1

OC,s OC,d IC-1 N,s IC-1 N,d IC-1C,s IC-1C,d IC-2C,s IC-2C,d IC-3C,s IC-3C,d

70.9 – 78.6 – 82.4 – 85.8 – 90.6 –

– 60.5 – 67.8 – 68.3 – 74.2 – 75.6

2 3 4 5

the simultaneous strength-and-shrinkage performance of OPCHSECC, i.e. lesser shrinkage at the same strength, or higher strength at the same shrinkage. It is then recommended in this study to use a normalized shrinkage strain (unit: m/MPa) to assess the overall performance of OPC-HSECC, and then the effectiveness of using zeolite to replace quartz sand can be assessed. The proposed normalized shrinkage strain is taken as the ratio of the measured

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Q. Wang et al. / Construction and Building Materials 234 (2020) 117335

Table 7 Normalized shrinkage strains of tested OPC-HSECC specimens. Group

Mix.

Normalized shrinkage strain, Seal e s,n(m/MPa)

Normalized shrinkage strain, Dry e d,n(m/MPa)

1

OC,s OC,d IC-1 N,s IC-1 N,d IC-1C,s IC-1C,d IC-2C,s IC-2C,d IC-3C,s IC-3C,d

14.92 – 11.18 – 10.69 – 8.81 – 7.48 –

– 21.84 – 18.09 – 16.50 – 14.60 – 11.26

2 3 4 5

30% (36%) to give the best simultaneous strength-andshrinkage performance of OPC-HSECC. Nevertheless, if a minimum 100 MPa strength is required, the optimal ratio of calcined zeolite is 20% (24%).

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements

28-day total shrinkage strain to the respective compressive strength in both sealed and drying conditions. The respective calculated values are denoted by es,n and ed,n and tabulated in Table 7. It is found from Table 7 that both of the normalized shrinkage strains decrease as zeolite is used to replace an identical weight of quartz sand in OPC-HSECC, and as the ratio of calcined zeolite increases. Hence, it can be confirmed that the use of zeolite up to 30% (36%) can actually improve the overall strength-shrinkage performance of OPC-HSECC and thus the objective of this study. 4. Conclusions The effect of using zeolite to replace partial quartz sand on the strength and shrinkage performance of high-strength engineered cementitious composite based on Ordinary Portland Cement are experimentally studied. Five groups of OPC-HSECC specimens which contained different sand components were tested: (1) 100% quartz sand; (2) 15% (18%) natural zeolite + 85% (82%) quartz sand by weight (and by volume); (3) 15% (18%) calcined zeolite + 85% (82%) quartz sand; (4) 20% (24%) calcined zeolite + 80% (76%) quartz sand; (5) 30% (36%) calcined zeolite + 70% (64%) quartz sand. Each group contained two identical specimens cured in sealed and drying conditions to study the autogenous and drying shrinkage respectively. The following conclusions are obtained: (1) The use of zeolite to replace partial quartz sand would decrease the compressive strength of OPC-HSECC because of the more porous structure of zeolite. The decrease was within 5% for zeolite replacement up to 20% (24%), but increased abruptly to 10% when the zeolite replacement was 30% (36%). (2) Using zeolite could decrease significantly the 28-day total shrinkage. Calcined zeolite was more effective than natural zeolite in decreasing the shrinkage at the same replacement ratio. The maximum 28-day total shrinkage reduction was 55% for 30% (36%) calcined zeolite replacement. (3) Using natural zeolite could decrease the 28-day autogenous shrinkage significantly. However, it was not effective in decreasing the 28-day drying shrinkage. (4) Using calcined zeolite could decrease both 28-day autogenous and drying shrinkage significantly. (5) The reduction of 28-day total shrinkage of OPC-HSECC containing zeolite was due to the increase of the internal relative humidity. It is because the zeolite particle is porous and could act as a water reservoir and keep the moisture content high within the capillary pores of the composite. (6) The overall effectiveness of using zeolite to replace partial sand to improve both strength and shrinkage can be revealed by a proposed normalized shrinkage strain (unit: m/MPa) which is defined as shrinkage strain per compressive strength of OPC-HSECC on the 28th day. It was found that the optimal ratio of calcined zeolite replacement of sand is

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