Properties of quicklime(CaO)-activated Class F fly ash with the use of CaCl2

Cement and Concrete Research 111 (2018) 147–156

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Properties of quicklime(CaO)-activated Class F fly ash with the use of CaCl2 Dongho Jeon, Woo Sung Yum, Yeonung Jeong, Jae Eun Oh

⁎

T

School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology (UNIST), UNIST-gil 50, Ulju-gun, Ulsan 44919, Republic of Korea

A R T I C LE I N FO

A B S T R A C T

Keywords: CaO-activation Class F fly ash CaCl2 Calcium oxychloride Cementless binder

This study reports the significant effect of CaCl2 on strength improvement in CaO-activated Class F fly ash system. In these systems, the presence of CaCl2 promotes (1) a higher degree of fly ash dissolution, (2) increasing C-S-H formation, and (3) an overall reduction of pore size and volume, resulting in increased strength. However, while more C-S-H, which is responsible for strength increase, was produced with higher CaCl2 content, extreme doses of CaCl2 resulted in strength degradation; more importantly, the best quantity of CaCl2 for the greatest strength was dependent on the water-to-binder weight ratio (w/b). This dependency was closely related to the formation of calcium oxychloride, which generally causes severe expansive cracking and depends on w/b.

1. Introduction

likely obtained in a similar way. However, while most earlier cement studies explained that the reaction of CaCl2 with cement compounds (e.g., tricalcium silicate (C3S)) mainly caused the acceleration of the portland cement hydration [13–15], their binders contained no cement compounds. Thus, the accelerating effect of CaCl2 in the Ca(OH)2-activated fly ash system still requires further research. CaO activation has also been studied to develop high-strength cementless binders mainly using ground granulated blast furnace slag (GGBFS) [8,16,17]. Although CaO and Ca(OH)2 are chemically very similar, CaO activation proved to be more effective in developing a greater strength than Ca(OH)2 activation [8]. However, very few studies have applied CaO activation to Class F fly ash, probably because an earlier study [3] reported that the use of CaO for fly ash produced significantly low strengths at 28 days (at most ~9 MPa). Therefore, it is still necessary to find a new effective way to significantly increase the strength development of CaO-activated fly ash system. This paper reported the significantly improved strength of the CaOactivated Class F fly ash systems when CaCl2 was incorporated. This study also discussed several important aspects affecting the strength using various material characterization techniques, such as powder Xray diffraction (XRD), mercury intrusion porosimetry (MIP), thermogravimetry (TG), inductively coupled plasma optical emission spectrometry (ICP-OES), and a phase diagram analysis.

Coal-fired power plants generate a significant quantity of fly ash as an industrial by-product all over the world [1]. Although fly ash is being widely used in concrete production, a large portion is still disposed in landfills, resulting in environmental and social problems, such as soil contamination, or landfill costs. Thus, as finding suitable ways to recycle fly ash is an important issue, numerous studies have been conducted to use fly ash as construction materials [1–6]. One well-known way to recycle fly ash is alkali activation, which is a chemical process that transforms fly ash into a strong binder with comparable mechanical properties to those of portland cement [7]. However, this may create difficulties in commercializing fly ash due to the high pH toxicity and expensive cost of alkaline activators (e.g., NaOH, sodium silicates) [8]. Thus, other types of Ca-based activators have been suggested to be used for the fly ash activation [3,4,9,10]. In particular, Ca(OH)2 has been suggested for fly ash activation, which is relatively safe and inexpensive compared to the alkaline activators. However, given that the Ca(OH)2-activated fly ash binder generally has shown a lower strength than those of alkali-activated fly ash binder and portland cement [3,6], the use of additional chemical additives has been attempted to increase the strength in earlier studies [6,9]. Shi et al. [4,9,11] reported that a small addition of Na2SO4 or CaCl2·2H2O in Ca(OH)2-activated fly ash led to strength improvement due to the early ettringite formation and densification of the microstructure, respectively. Given that CaCl2 generally accelerates the hydration of portland cement with boosting early strength [12], one might simply conjecture that the strength enhancement in their studies was

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Corresponding author. E-mail address: [email protected] (J.E. Oh).

https://doi.org/10.1016/j.cemconres.2018.05.019 Received 21 August 2017; Received in revised form 25 May 2018; Accepted 28 May 2018

Available online 13 June 2018 0008-8846/ © 2018 Elsevier Ltd. All rights reserved.

2. Experiment The raw fly ash was collected from the Ha-dong power plant in South Korea. The chemical composition of the fly ash is given in Table 1, which was determined by an X-ray fluorescence spectrometer

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Table 1 Chemical compositions of raw fly ash. Formula

Oxide weight %

SiO2 Al2O3 Fe2O3 CaO K2O MgO TiO2 Na2O SO3 P2O5 BaO SrO ZrO2 MnO

56.8% 23.3% 7.4% 5.0% 1.7% 1.5% 1.4% 0.9% 0.9% 0.6% 0.1% 0.1% 0.1% 0.1%

Table 2 The contents of the crystalline and amorphous phases in fly ash.

(S8 Tiger; Bruker, Germany). This fly ash was Class F type, as it contained 87.5% of SiO2 + Al2O3 + Fe2O3 according to the ASTM C 618 [18]. Powder X-ray diffraction (XRD) (D8 Advance; Bruker AXS, Germany) was conducted on the fly ash with an incident beam of Cu-Kα radiation (λ = 1.5418 Å) for a 2θ scanning range of 8–60°. The X'pert High-Score Plus program [19] and an International Center for Diffraction Data (ICDD) PDF-2 database [20] were used for the XRD analysis. Corundum (NIST RMS 676a, crystalline alumina 99.02% ± 1.11%) was added as an internal standard [21]. The results showed that the raw fly ash contained crystalline quartz, mullite, and magnetite, as shown in Fig. 1. The weight percentages of amorphous and crystalline phases of fly ash are shown in Table 2; the raw fly ash had a relatively low weight content of the amorphous phase compared to general fly ashes in South Korea [22]. The amorphous content of Class F fly ash can vary largely from 60% to 90% as it depends on source of coal or burning procedure in South Korea [22,23]. The particle size distribution of the fly ash was estimated using a laser diffraction particle size analyzer (HELOS, Sympatec, Germany)

Phase

Content (%)

Quartz Mullite Magnetite Amorphous Total sum

19.0 21.2 1.6 58.2 100.0

Fig. 2. Particle size distribution of the raw fly ash. Table 3 Mixture proportions of paste samples. Group

CC0

CC5

CC10

CC15

Label

CC0-0.4 CC0-0.5 CC0-0.6 CC0-0.7 CC0-0.8 CC0-0.9 CC5-0.4 CC5-0.5 CC5-0.6 CC5-0.7 CC5-0.8 CC5-0.9 CC10-0.4 CC10-0.5 CC10-0.6 CC10-0.7 CC10-0.8 CC10-0.9 CC15-0.4 CC15-0.5 CC15-0.6 CC15-0.7 CC15-0.8 CC15-0.9

Binder (wt%)

w/b

Fly ash

CaO

CaCl2

Total

80

20

0

100

75

20

5

100

70

20

10

100

65

20

15

100

0.4 0.5 0.6 0.7 0.8 0.9 0.4 0.5 0.6 0.7 0.8 0.9 0.4 0.5 0.6 0.7 0.8 0.9 0.4 0.5 0.6 0.7 0.8 0.9

with a RODOS dispersing unit (see Fig. 2). The fly ash consisted of > 10% of large particles over 100 μm and median particle size of the fly ash was 23.27 μm. The mixture proportions are shown in Table 3. The fly ash was replaced with CaCl2 at 0, 5, 10, and 15 wt% in the total weight of binder to find the optimal CaCl2 content for strength improvement, but the CaO content was fixed at 20 wt%. CaCl2 was not an additive chemical in this study, but rather a replacement unlike the role of CaCl2 for the portland cement because we developed a new cementless binder having fly ash, CaO, and CaCl2 as main material components. In addition,

Fig. 1. The XRD patterns of raw fly ash. 148

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50 Compressive strength (MPa)

previous studies have reported that the mixture of CaCl2, Ca(OH)2, and H2O may generate calcium oxychloride phase (3Ca(OH)2·CaCl2·12H2O) and the formation of calcium oxychloride is largely affected by the weight ratios among CaCl2, H2O, and Ca(OH)2 [24,25]. Thus, in order to investigate the formation of calcium oxychloride with varying CaCl2 content, we fixed the weight ratio of H2O and CaO in the samples with an identical w/b. The water-to-binder weight ratio (w/b) of samples varied from 0.4 to 0.9. The fresh CaCl2-CaO-fly ash paste retained a suitable consistency without segregation in the entire range of the w/b in this study. The fly ash and CaO powders were fully dry-mixed with CaCl2 pellets and then mixed with de-ionized water for 5 min [6]. Note that we used newly purchased CaO and CaCl2 (extra pure, 98%) immediately after unpacking, and the relative humidity in the laboratory was quite low as we use a dehumidifier in the laboratory; thus, potential contact of the chemicals with moisture was likely very low. Three identical samples for each mixture proportion were cast in brass cubic (50 × 50 × 50 mm) molds to measure compressive strength. The samples were cured under 60 °C with 99% relative humidity for 3 and 28 days. Compressive strength testing was performed on the triplicate samples of each mixture. After testing, fractured specimens were collected to prepare powder samples for the XRD and TG analyses. The finely ground powder samples were solvent-exchanged using acetone for 14 days to stop further hydration. All samples were completely dried out in an ~60 cm Hg vacuum desiccator for 3 days to remove any residual water and solvent [26]. Previous studies have noted that the calcium oxychloride phase (3Ca(OH)2·CaCl2·12H2O) is easily decomposed in a drying condition [24,25]. Monosi et al. [25] reported that the calcium oxychloride phase disappeared in the ground sample, washed with methyl alcohol. Thus, to identify whether calcium oxychloride formed in this study, additional powder samples were prepared without any drying procedure (i.e., neither the solvent-exchange nor the vacuum-drying method). The XRD was carried out for the powder samples using the same instrument and conditions used to characterize the fly ash, and the measured patterns were analyzed using the same software and database. In particular, for the samples to detect the calcium oxychloride phase, the other model of XRD (D/MAX 2500V/PC; Rigaku, Japan) equipment, which is directly accessible to the author, was conducted upon grounding the samples because this process should be immediately performed before the instant decomposition of calcium oxychloride. The TG was conducted on powdered, hardened pastes at 28 days using the TG instrument (TG/DSC1; Mettler Toledo, USA) with a heating rate of 20 °C/min in a nitrogen atmosphere from 25 °C to 1000 °C. Pore volumes and size distributions in the hardened samples at 3 and 28 days were estimated with a mercury intrusion porosimeter (Autopore IV, Micrometrics, USA). The samples for the MIP testing were made in 5-mm cubic pieces. To halt further hydration, these cubic specimens were immersed in isopropyl alcohol for 14 days and then stored in the vacuum desiccator for 2 days [26]. Note that all the XRD, TG, and MIP analyses were carried out only for the samples at a w/ b = 0.4 as representatives because they produced the greatest strengths. The ICP-OES was performed using a spectrometer (700-ES; Varian, USA) with a 40 MHz free-running radio-frequency generator and axially viewed plasma to investigate the characteristics in the initial dissolution of fly ash. This analytical technique may quantify dissolved elements in liquid solutions [27,28]. In this study, the target elements were selected as silicon, aluminum, iron, and calcium because the raw fly ash was mainly composed of aluminosilicate phases and iron oxides (see Table 1). The concentration of calcium in the paste solution may indicate the initial dissolution degree of CaO and CaCl2. The sample preparation for ICP-OES analysis was conducted as follows: each

3 days

45 40

Sample group

35

CC0 CC5 CC10

30 25

CC15

20 15 10 5 0 w/b=0.4

w/b=0.5 w/b=0.6 w/b=0.7 w/b=0.8 w/b=0.9

(a)

Compressive strength (MPa)

50 28 days

45 40

Sample group

35

CC0 CC5 CC10

30

25

CC15

20 15 10 5

0 w/b=0.4 w/b=0.5 w/b=0.6 w/b=0.7 w/b=0.8 w/b=0.9

(b) Fig. 3. Compressive strength testing results at (a) 3 days and (b) 28 days.

mixture powder was mixed with de-ionized water at a w/b = 2 to obtain a sufficient quantity of aqueous phase [27,28]; the aqueous mixture was agitated using a heating magnetic stirrer for 30 min while keeping the heating temperature at 60 °C, which is the same curing temperature used in this study; after agitating, the sample was centrifuged with 4500 rpm for 5 min; the separate aqueous phase was used for the ICP-OES measurement.

3. Results and discussion 3.1. Compressive strength The compressive strength testing results at 3 and 28 days are given in Fig. 3. Note that the samples with CaCl2 (i.e., Groups CC5, CC10, and CC15) showed remarkably higher compressive strengths at all w/b values, compared to the samples without CaCl2 (i.e., Group CC0). The CaCl2 content for the greatest strength was dependent on the w/b value. At relatively low w/b values (i.e., w/b = 0.4–0.6), the greatest strengths were obtained at 5 wt%; however, at high w/b values (i.e., w/ b = 0.7–0.9), they were achieved at 10 wt%; with more CaCl2 content above these wt% values, the strengths started to decline. Given that the CaCl2 substitution produced a significant strength improvement within only 3 days, the use of CaCl2 would significantly affect the early reaction before 3 days. However, in the earlier similar study [4], in which fly ash was activated with Ca(OH)2 and CaCl2·2H2O, strength was not notably gained until 7 days, although it significantly increased after 28 days. Thus, this suggests that CaO appears more effective for gaining a high early strength than Ca(OH)2 for the fly ash activation in the presence of CaCl2. 149

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Fig. 4. XRD patterns of the samples, after treatment using a drying procedure, with a w/b = 0.4 at (a) 3 days, and (b) 28 days.; HC: hydrocalumite, M: mullite, Q: quartz, CH: calcium hydroxide, C: calcite.

Although the atmospheric carbonation would cause the consumption of Ca(OH)2 to some extent, the whole depletion of Ca(OH)2 in the samples with CaCl2 was likely due to two following main reasons: (1) the formation of hydrocalumite and (2) the C-S-H formation from the enhanced pozzolanic reaction between amorphous phase of fly ash and Ca (OH)2. First, hydrocalumite was only identified in the samples with CaCl2, while not in the sample without CaCl2 [see ■ HC in Fig. 4]. Given that, in the samples with CaCl2, Ca(OH)2 completely disappeared, and the hydrocalumite formation might be closely related to the consumption of Ca(OH)2. Second, it is clear that the Ca(OH)2-consuming pozzolanic reaction was boosted in the samples containing CaCl2 given that these samples produced much higher strengths than did the CC0-0.4 (see Fig. 3). This second argument would be supported by Fig. 5, which presents the overlapped XRD patterns of raw fly ash, CC0-0.4, and CC5-0.4 at 28 days, and its enlargement for better visibility. By comparing these patterns, two observations were obtained: (1) the amorphous hump at 17°–27° from fly ash decreased due to the CaCl2 incorporation, and (2) the C-S-H peak near 29°–33° [29,30] largely increased in the presence of CaCl2. Given that the amorphous phase of fly ash is dissolved by OH− in a high pH environment [12,22], these should be related to the pozzolanic reaction of the amorphous phase of fly ash with Ca(OH)2 as the C-S-H peaks simultaneously increased with the reduction of the amorphous hump. Thus, the incorporation of CaCl2 in the CaO-activated fly ash system likely led to more formation of C-S-H. Note that the compressive strengths of the samples with CaCl2 were much higher than that of the sample without CaCl2 (see Fig. 3). Hence, the XRD results support the compressive strength results. Fig. 6 shows the XRD results for the samples at 3 days, which were prepared without any drying procedure. Earlier studies [25,31,32] were used to identify calcium oxychloride in this study because there was no reference pattern for calcium oxychloride in the ICDD PDF-2 database. Calcium oxychloride (denoted CAOXY) only appeared when the content

It is worth noting that, in the hydration of portland cement, CaCl2 generally increases the early strength of cement paste [12,29]. The use of CaCl2 in the CaO-fly ash paste also showed a similar strength enhancement at 3 days (see Fig. 3 (a)), although our samples did not contain any cement compound (see Table 2). Thus, the mechanism for strength improvement from the use of CaCl2 in the CaO-activated fly ash system should be much different from that in the cement hydration. This will be discussed in further detail in Section 3.4 using the ICP-OES results. 3.2. XRD Fig. 4 presents the XRD patterns of the CaCl2-CaO-activated fly ash pastes with a w/b = 0.4 at 3 and 28 days after being treated by the solvent-exchange and drying procedure. In all samples, quartz (SiO2, ICDD PDF-2 no. 98-008-9277), mullite (3Al2O3·2SiO2, ICDD PDF-2 no. 98-004-3298), and calcite (CaCO3, ICDD PDF-2 no. 98-004-0107) were found in common. Among these phases, quartz and mullite existed in the raw fly ash (see Fig. 1), while calcite was newly formed, possibly due to carbonation. When CaCl2 was present, hydrocalumite (Ca2Al (OH)6Cl·2H2O) was newly synthesized, while Ca(OH)2 mostly disappeared. It is worth noting that Ca(OH)2, which was generated from the hydration of CaO, was present in the CC0-0.4 until 28 days, while not being found at all in the samples containing CaCl2 at 3 and 28 days [see ▲ CH with the dashed line in Fig. 4]. The consumption of Ca(OH)2 was frequently reported to be the main cause of the strength improvement of the Ca(OH)2-activated fly ash binder with additional activators [6,11]. When Na2CO3 was incorporated in the Ca(OH)2-fly ash paste, Ca(OH)2 was completely consumed in a chemical reaction within the first 3 days, resulting in a notably greater strength than that of the sample without Na2CO3 [6]. In this study, the incorporation of CaCl2 into the CaO-fly ash paste also led to the rapid consumption of Ca(OH)2. 150

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A C

B C

B

A

A B C

Fig. 5. Influence of CaCl2 on the amorphous hump at 17°–27° (2θ) and C-S-H peaks at 29°–33° (2θ) in the XRD results, which were measured for the samples with a drying process, at 28 days.

3.3. TG analysis

of CaCl2 was used over 10 wt% (see broken lines in Fig. 6). In Fig. 6, it is worth noting that, with an increasing CaCl2 content, while the reflections of hydrocalumite were not changed in the peak intensity, those of calcium oxychloride became stronger. This implies that, when CaCl2 was used over 10 wt%, the CaCl2 might be mostly used for synthesizing calcium oxychloride rather than hydrocalumite. Interestingly, the CC10-0.4 and CC15-0.4 at 3 days showed more distinct calcite peaks than those of the CC5-0.4 at 3 days (see peaks at 29.5° with arrows in Fig. 4 (a)). The decomposition of calcium oxychloride was reported to yield calcite [32]; thus, some portion of early calcite formation in the CC10-0.4 and CC15-0.4 samples at 3 days was likely related to the presence of calcium oxychloride.

The TG curves with differential thermogravimetry (DTG) curves, a differential form of TG, at 28 days are presented in Fig. 7. The TG results confirmed the formation of C-S-H, hydrocalumite, Ca(OH)2, and calcite. Unlike CC0-0.4, the samples with CaCl2 showed significant weight loss below 160 °C, which is attributed to the dehydration of both C-S-H and hydrocalumite [5,12,33,34]. Hydrocalumite is known to undergo three-step thermal decompositions in TG: (1) dehydration below 120 °C, (2) dehydroxylation around 290 °C, and (3) anion decomposition above 670 °C [33–35]. Although, below 160 °C, the weight losses of the hydrocalumite and C-S-H overlapped with each other, the weight loss in 237–360 °C was only attributed to the dehydroxylation of hydrocalumite. The samples containing CaCl2 distinctively exhibited

Fig. 6. XRD patterns of the samples at 3 days without a drying procedure; HC: hydrocalumite, CAOXY: calcium oxychloride. 151

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Fig. 7. TG and DTG results for the samples with a w/b = 0.4 at 28 days.

dehydroxylation (i.e., at 237–360 °C) of hydrocalumite, in TG at 28 days are presented in Fig. 8. The weight losses for the dehydroxylation of hydrocalumite were quite similar among the samples containing CaCl2, even though the weight loss slightly increased with increasing CaCl2 content (see Fig. 8 (a)). Therefore, the quantities of hydrocalumite are likely quite similar between the samples with CaCl2. This is consistent with the XRD results shown in Fig. 6, which showed that the peak intensities for hydrocalumite were similar among the samples despite the higher CaCl2 content. Fig. 8 (b) shows the weight losses in 60–160 °C, which are due to the decomposition of both C-S-H and hydrocalumite. The weight loss in 60–160 °C clearly increased as the CaCl2 content increased. Thus, given that hydrocalumite showed only a slight difference in its formed quantity between the samples with CaCl2, Fig. 8 (b) demonstrates that

the hydrocalumite DTG peaks around 290 °C, while the CC0-0.4 did not show any hydrocalumite peak, consistent with the XRD results. The large DTG peak around 450 °C, which was the consequence of the decomposition of Ca(OH)2, was only found in CC0-0.4; this indicates that the presence of CaCl2 might accelerate the consumption of Ca(OH)2. The other DTG peaks above 600 °C were related to the decomposition of calcite and hydrocalumite [5,34]. One might conjecture that filling of pores by the calcite formation could lead some strength improvement in this study; however, it should be noted that the TG and compressive strength results indicated that although the sample without CaCl2 produced more calcite than the other samples, it gained lower strengths. Thus, the filling of pores with calcite was not likely the cause for the strength gain in this study. The weight losses, related to dehydration (i.e., at 60–160 °C) and

Fig. 8. Weight losses in TG at 28 days: (a) at 237–360 °C (dehydroxylation of hydrocalumite) and (b) at 60–160 °C (dehydration of hydrocalumite and C-S-H). 152

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0.08

0.45

Si

0.07

Concentration (mg/L)

Concentration (mg/L)

0.4 0.35 0.3 0.25

0.2 0.15 0.1

Al

0.06 0.05

0.04 0.03 0.02

0.05

0.01

0

0

(a)

(b) 30,000

0.018

Fe

Ca Concentration (mg/L)

Concentration (mg/L)

0.016

0.014 0.012 0.01 0.008 0.006 0.004

0.002 0

25,000

20,000 15,000 10,000 5,000

N.D.

0

(c)

(d)

Fig. 9. Ionic concentrations in the diluted paste sample (w/b = 2) at 60 °C after 35 min for (a) silicon, (b) aluminum, (c) iron, and (d) calcium; N.D.: non-detected.

CaCl2 is relatively high (136.8 g/100 mL at 60 °C [36]), compared to that of CaO (0.116 g/100 mL at 60 °C [36]). Thus, as mentioned earlier, CaCl2 likely played notably different roles in between portland cement and the CaO-fly ash system. Although the exact mechanism for the accelerating effect of CaCl2 on cement hydration is not clearly understood, it has been reported that CaCl2 in portland cement paste accelerates the hydration of tricalcium silicates (C3S) and induces the early formation of C-S-H, resulting in a higher strength at 28 days [14,15,37]. However, the use of CaCl2 in the CaO-fly ash system of this study promotes the pozzolanic reaction by increasing the dissolution of fly ash, resulting in strength enhancement (see Figs. 3 and 5).

the weight loss of C-S-H followed the order: CC15-0.4 (largest) > CC10-0.4 > CC5-0.4 > CC0-0.4 (smallest), indicating that the samples with more CaCl2 produced more C-S-H phase. However, considering that C-S-H is the primary reaction product determining the strength, this TG result might appear inconsistent with the strength testing results because the greatest strength was obtained at a 5 wt% CaCl2 content. Thus, this inconsistency suggests the presence of an additional strong factor for strength determination in the CaO-CaCl2-fly ash system, which is discussed in Section 3.6. 3.4. ICP-OES Fig. 9 illustrates the results of ICP-OES for the diluted samples (w/ b = 2), which were prepared at 60 °C, at 35 min. The silicon, aluminum, and iron elements tended to be more dissolved with an increasing CaCl2 content, as shown in Fig. 9 (a)–(c). Note that those elements only originated from fly ash (see Tables 1 and 3); thus, the increased concentrations of Si, Al, and Fe indicate that more dissolution of fly ash occurred when CaCl2 was present. This is consistent with the XRD results in Fig. 5. The concentration of calcium also increased as the CaCl2 content increased (see Fig. 9 (d)). As stated above, as fly ash was more dissolved in the presence of CaCl2, a portion of Ca ions was produced from the dissolved fly ash. However, a significant portion of Ca was also likely created from the dissolution of CaCl2 itself because the solubility of

3.5. MIP The cumulative and density pore size distributions of hardened pastes at 28 days are presented from the MIP results in Fig. 10. In Fig. 10 (a), compared to the sample without CaCl2 (i.e., CC0-0.4), all the samples with CaCl2 exhibited large reductions in the total pore volume; however, the total pore volumes were quite similar to each other, although the strengths largely varied between these samples (see Fig. 3). In Fig. 10 (b), the average pore sizes of the samples with CaCl2 were also largely reduced compared to that of the CC0-0.4. However, although most pores in CC5-0.4 were smaller than 50 nm, the other samples (i.e., CC10-0.4 and CC15-0.4) had large volumes of big pores 153

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0.45 Cumulativ e Intrusion (mL/g)

0.40 0.35

of hardened cement pastes, but pores smaller than 50 nm hardly affect the strength [12]. Thus, the formation of these pores, possibly cracks, was likely the main cause of strength reduction when the CaCl2 content was over 10 wt%. These pores will be discussed in further detail in Section 3.6.

28 days Large reduction in total pore volume

CC0-0.4 CC5-0.4 CC10-0.4 CC15-0.4

0.30 0.25

3.6. Phase diagram analysis

0.20

Fig. 11 (a) shows the CaCl2-Ca(OH)2-H2O phase diagram [28] that contains the compositions of the samples at a w/b = 0.4 and 0.7, used in this study. According to the phase diagram, calcium oxychloride phase (denoted CAOXY in the figure) forms in most compositions except for regions I, II, and V. Earlier studies [24,32,39] have reported that the formation of calcium oxychloride (3Ca(OH)2·CaCl2·12H2O) due to a CaCl2 attack would generate severe hydraulic pressure in the cement paste matrix, resulting in the expansion and deterioration of cement concrete. When the w/b = 0.4, CC0-0.4 and CC5-0.4 belong to region II, in which calcium oxychloride does not form, while CC10-0.4 and CC150.4 were in regions III and IV, respectively, where calcium oxychloride should form. It should be noted that the MIP results identified that CC10-0.4 and CC15-0.4 contained relatively large pores (1–10 μm) (see Fig. 10 (b)); these large pores were likely the expansive cracking due to the formation of calcium oxychloride. However, when the w/b = 0.7, only CC15-0.7 was involved in region III, but the others (i.e., CC0-0.7, CC5-0.7, and CC10-0.7) were in region II. The strength data in Fig. 11 (b) illustrate that the strength reduction of the samples started from the formation of calcium oxychloride. Thus, although more use of CaCl2 produced more C-S-H, as discussed in the TG analysis, the excessive quantity of CaCl2 resulted in calcium oxychloride formation, leading to expansive cracks with equivalent pore sizes of 1–10 μm and reducing strength. Thus, Fig. 11 explains the dependency of the best CaCl2 content on the w/b value to obtain the greatest strength.

0.15 0.10 0.05 0.00 0.001

0.01

0.1

1

10

100

1000

50 nm

Pore size (μm)

(a) 0.5

50 nm

28 days

0.4

CC0-0.4

0.3

Average pore size - 43.7 nm

0.2 0.1 0.0 0.5

28 days

Pore-size refinement

CC5-0.4

Average pore size - 12.9 nm

Log Differential Intrusion (mL/g)

0.4

0.3

Removal of pores, which were extensively present in CC0-0.4

0.2 0.1

4. Conclusions

0.0 0.5

28 days

This study reports the significant enhancing effect of CaCl2 on the strength development of the CaO-activated Class F fly ash system. This strength enhancement was mainly achieved because the presence of CaCl2 promoted (1) a higher degree of fly ash dissolution, (2) increasing C-S-H formation, and (3) an overall reduction of pore size and volume. However, although a higher content of CaCl2 induced more C-S-H formation, leading to strength improvement, an excessive dosage of CaCl2 was detrimental for the strength development because the strength was significantly reduced by the formation of calcium oxychloride, which generally causes expansive cracking and depends on w/ b; thus, the best quantity of CaCl2 for the greatest strength was determined differently by the w/b ratio. In addition, this study suggests that the CaO-activated Class F fly ash system can be used as a high strength structural binder for the production of construction products (e.g., cementless high strength bricks and blocks) when CaCl2 was incorporated in the system. Detailed conclusions are given as follows:

CC10-0.4

0.4

0.3

Average pore size - 13.7 nm

Pore-size refinement

0.2 Newly formed pores, possibly cracks

0.1 0.0

28 days

0.5 0.4

CC15-0.4

Average pore size - 19.6 nm

Pore-size refinement

0.3 0.2 Newly formed pores, possibly cracks

0.1 0.0

0.001

0.01

0.1 50 nm

1

10

100

1. The use of CaO for fly ash activation appears significantly effective in gaining a high early strength in the presence of CaCl2. 2. In XRD, when CaCl2 was present, Ca(OH)2 was mostly rapidly consumed mainly due to the new formation of hydrocalumite, and more C-S-H formation from the enhanced pozzolanic reaction between Ca(OH)2 and the amorphous phase of fly ash. When CaCl2 was used more than a certain limit, calcium oxychloride formed. 3. In TG, as the CaCl2 content increased, although the quantities of hydrocalumite did not significantly change, the quantity of C-S-H increased. 4. The ICP-OES results showed that the fly ash was significantly more

1000

Pore size (μm)

(b) Fig. 10. Pore size distributions of hardened samples at 28 days: (a) cumulative pore volume and (b) pore size density distributions.

near 1–10 μm, which were 37–38% of the total pore volumes. These big pores appeared only when the CaCl2 dosage was over 10 wt%. In general, capillary pores larger than 50 nm significantly reduce the strength 154

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H2O (Mol %)

H2O (Mol % )

100

100

0

II I

90

IV

80 70

I

VII

II

30

60 50

W/B=0.7

20 III

0

V10

90

40 VI

IV

10

W/B=0.4

V

50

40

60

30

70

20

80

III

80

10

VII

90

0 100 90 80 70 60 50 40 30 20 10 Ca(OH)2 (Mol %)

20

Samples with CAOXY Samples without CAOXY

100 0 CaCl 2 (Mol %)



Compositional region

Compatible phases

I

Aq

II

Ca(OH)2 + Aq

Relevant samples of this study to each region

w/b = 0.4

w/b = 0.7

CC0-0.4 CC5-0.4

CC0-0.7 CC5-0.7 CC10-0.7 CC15-0.7

III

Ca(OH)2 + Aq + CAOXY(3:1:12)

CC10-0.4

IV

Aq + CAOXY(3:1:12)

CC15-0.4

V

CaCl2∙6H 2O + Aq

VI

Ca(OH)2 + CAOXY(3:1:12) or -(1:1:1)

VII

Aq + CAOXY(3:1:12) or -(1:1:1)

* Aq = aqueous solution * CAOXY(3:1:12) = calcium oxychloride (3Ca(OH) 2∙CaCl2∙12H2O) * CAOXY(1:1:1) = calcium oxychloride (Ca(OH)2∙CaCl2∙H2O)

(a) Samples with CAOXY Samples without CAOXY 50

W/B=0.4 Compressive strength (MPa)

45

28 days

40 35 30

Strength reduction

25

W/B=0.7

20 15 10 5 0

0 5 10 15 0 5 10 15 CaCl2 content

(b) Fig. 11. (a) Ternary phase diagram of the CaCl2-Ca(OH)2-H2O system [38], marking the compositions of the samples at a w/b = 0.4 and 0.7 in this study and (b) relevant strength testing results.

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dissolved at an early age (i.e., < 35 min) with more CaCl2 content. This observation differs from the accelerating effect of CaCl2 on the cement hydration. 5. In the MIP and phase diagram analysis, the samples with the synthesis of calcium oxychloride had relatively large pores near 1–10 μm, which were approximately 37–38% of the total pore volumes. These pores were likely the results of expansive cracking due to the formation of calcium oxychloride.

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