Performance change of shaft lining concrete under simulated coastal ultra-deep mine environments

Construction and Building Materials 230 (2020) 116909

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Performance change of shaft lining concrete under simulated coastal ultra-deep mine environments Yu-cheng Zhou a,b,c, Juan-hong Liu a,b,c,⇑, Shun Huang a, Hai-tao Yang a,b,c, Hong-guang Ji a,b a

College of Civil and Resource Engineering, University of Science and Technology Beijing, Beijing 100083, China Beijing Key Laboratory of Urban Underground Space Engineering, University of Science and Technology Beijing, Beijing 100083, China c State Key Laboratory of High-efficient Mining and Safety of Metal Mines, Ministry of Education, University of Science and Technology Beijing, Beijing 100083, China b

h i g h l i g h t s
Residual strengths of high-performance concrete under simulated coastal ultra-deep mine environments.
Quantitative results for crystalline and amorphous phases in the hardened pastes.
DoH and DoPR of the hardened pastes.
Influence of the underground temperature on concrete durability.

a r t i c l e

i n f o

Article history: Received 3 July 2019 Received in revised form 17 August 2019 Accepted 6 September 2019

Keywords: Shaft lining concrete High temperature NaCl and Na2SO4 solution Friedel’s salt

a b s t r a c t This paper reports research on the performance change of fibre-reinforced high-performance concrete (FRHPC) and fibre-reinforced reactive powder concrete (FRRPC) subjected to simulated coastal ultradeep mine environments. The phase content, morphology, and pore structure characteristics of the hardened pastes were investigated by means of a series of quantitative methods, and the degree of hydration (DoH) and degree of pozzolanic reaction (DoPR) were calculated. The experimental results indicate that there is a great difference in DoH and DoPR between high-performance hardened paste (HPHP) and reactive powder hardened paste (RPHP) at 28 days and that they show consistent behaviour after immersion. The failure of high-performance concrete in coastal ultra-deep mines may be caused by the crystallization pressure of Friedel’s salt. Moreover, a high underground temperature can be slightly beneficial for the durability of the concrete. The conclusions obtained provide theoretical support for the construction of ultra-deep coastal mines. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Currently, many extraordinary scientific and technological developments are based on unsustainable mineral resources. Since the Industrial Revolution, the demand for mineral resources has increased five times faster than the population [1]. After hundreds of years of mining, the limited resources of the Earth are being exhausted [2]. In theory, the available metallogenic space in the interior of the Earth is distributed from the surface to 10,000 m below the surface. To meet the needs of societal development, the ability to mine deeper in the Earth is a strategic scientific and technological problem that we must solve [3,4].

⇑ Corresponding author at: College of Civil and Resource Engineering, University of Science and Technology Beijing, Beijing 100083, China. E-mail address: [email protected] (J.-h. Liu). https://doi.org/10.1016/j.conbuildmat.2019.116909 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

Shaft lining concrete in deep underground environments is affected by multiple factors. High induced stresses, osmotic pressures, in situ temperatures and corrosive environments create many requirements for concrete [5–7]. The concrete materials at the bottom of ultra-deep mines and the important structural parts, such as the ingate, must have excellent mechanical properties and durability. Multiple loading modes, including static stresses and impact disturbances, are encountered [8]. Due to the influence of mining disturbances, stress redistribution occurs in some areas. When the input energy reaches a certain level, both concrete and rock are subject to different forms of destruction [9,10]. Because of the mechanical conditions, concrete with excellent static and dynamic properties must be used in ultra-deep mine engineering. Numerous research achievements have been obtained in this field. Soulioti et al. [11] found that fibres can change the failure modes of

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The underground environment in ultra-deep mines, where many factors coexist, has a simultaneous positive and negative impact on concrete. However, there are still few studies on the structural concrete used for underground engineering. In this paper, two typical kinds of concrete with excellent mechanical properties are studied: FRHPC and FRRPC. Considering that using precast concrete in ultra-deep mine construction is not feasible, all concrete samples are cured under standard conditions. The mechanics and microstructure of the two types of concrete in simulated underground environments are studied by means of microscopic tests. The results provide theoretical and experimental support for the construction of ultra-deep mines.

concrete and improve the structural performance of buildings. Rios et al. [12] suggested that the presence of fibres leads to an increased deformation capacity in concrete. Dong et al. [13] and Guo et al. [14] revealed that fibre-reinforced reactive powder concrete (FRRPC) has excellent impact toughness. Increasing the volume fraction of fibres within a certain range can improve the impact resistance of concrete, as reported by Verma et al. [15] and Lai et al. [16]. In general, fibre-reinforced high-performance concrete (FRHPC) and FRRPC must be selected as shaft lining materials for ultra-deep mine engineering. Based on investigations in the underground environments of several ultra-deep mines on the coast of eastern China, it was found that the Cl content is approximately 3500 mg/l, the SO2 4 content is close to 500 mg/l and the main cation is Na+. Cao et al. [17] and Shafikhani et al. [18] summarized the characteristics of Cl diffusion, and hybrid models have been proposed. Zhang et al. [19] insisted that the impact strength of steel fibrereinforced concrete decreased after 250 freeze-thaw cycles in 3.0% NaCl solution. Zhang et al. [20] indicated a coupled physical and chemical sulfate attack on concrete under drying-wetting cycles. Moreover, the in situ temperature of the underground environment below 1000 m is more than 40 °C, and the temperature below 1500 m is as high as approximately 60 °C [21]. Pichler et al. [22] showed that curing temperature has a certain influence on the early-age strength and degree of hydration (DoH) of concrete. Shen et al. [23] observed that a high temperature can increase the ultra-high density calcium silicate hydrate content, reduce the Ca/Si ratio and improve the mechanical properties of ultra-high-performance concrete.

(a)

2. Experiment This section is dedicated to the introduction of the compositions of the raw materials, mixture proportions, environmental conditions and testing methods. 2.1. Raw materials The cementitious materials consisted of Portland cement, fly ash, ground granulated blast furnace slag, and silica fume. The specific surface areas of the slag and silica fume were 520 and 18,300 m2/kg, respectively. The chemical and mineral compositions of cementitious materials obtained through X-ray fluorescence (XRF) and X-ray diffraction (XRD) with the Rietveld method are listed in Fig. 1 and Table 1. Three types of quartz sand

(b)

Caculated value and error Bragg-positions

1

1 Zincite 2 Mullite 3 Quartz

1

2

=1.52 C3S C S 2 C A 3 C AF 4 Gypsum Periclase Portland Calcite Anhydrite

0

10

20

30

40

50

60

70

2

Intensity (a.u.)

Intensity (a.u.)

1

80

1

1

3

22

3

2

3 3

3

1 3

1 1 Fly ash

3 Slag

3

Silica fume

0

10

20

2 (Degree)

30

40

50

60

70

80

2 (Degree)

Fig. 1. XRD patterns of cementitious materials: (a) cement and (b) fly ash, slag and silica fume. * The calculated value represents the diffraction peak pattern calculated on the basis of the original pattern by the XRD-Rietveld method. The error represents the difference between the calculated pattern and the original pattern. The Bragg–positions represent the positions of the crystal phase peaks.

Table 1 Chemical and mineral compositions of cementitious materials from XRF and XRD-Rietveld analysis. XRF Materials

XRD-Rietveld Cement

Main oxides

Wt.%

SiO2 Al2O3 Fe2O3 CaO MgO SO3 Na2Oeq

21.73 4.60 3.45 64.55 3.56 0.46 0.59

Fly ash

Slag

Silica fume

30.63 14.57 0.56 39.06 9.20 2.35 0.44

51.61 35.69 3.90 3.21 0.87 0.73 0.25

95.80 1.00 0.90 0.30 0.70 0.00 1.30

Cement

Fly ash

Phase name

Wt.%

Phase name

Wt.%

C3S monoclinic C2S beta C3A cubic C4AF Gypsum Periclase Portlandite Calcite Anhydrite

62.05 10.78 8.56 7.69 3.80 1.16 0.78 4.10 1.18

Mullite Quartz Magnetite Amorphous

35.27 3.44 1.29 60.00

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Y.-c. Zhou et al. / Construction and Building Materials 230 (2020) 116909

with particle sizes of 2.05–0.85, 0.42–0.18 and 0.18–0.15 mm were mixed at a mass ratio of 1:0.68:0.56. River sand with a fineness modulus of 2.8 and gravel with a size of 5–25 mm were used as aggregates. A polycarboxylate-based superplasticizer with a 30% solid content provided by Subote Ltd. was used. 2.2. Mixture proportions To reduce the hydration heat of concrete [24], improve its resistance to harmful ions [25–27] and ensure the environmental friendliness of its raw materials, supplementary cementitious materials were used in mixture proportions [28]. FRHPC and FRRPC were prepared with the addition of 1% (by volume) fibres. The mixture proportions of concrete are given in Table 2, and the specifications of the fibres are shown in Table 3. 2.3. Mixing method and environmental conditions The aggregate and fibres were mixed for 2 min in a mixer. Then, the cement and mineral admixtures were added, and the combination was re-mixed for 2 min. Finally, a mixture of water and superplasticizer was added, and the combination was mixed for approximately 4 min. This process required a total of 8 min, and the mixing time for each type of concrete was the same. Concrete cubes with dimensions of 100  100  100 mm were prepared for the determination of the compressive strength. Paste cubes with dimensions of 40  40  40 mm were manufactured for the microscopic experiments. All specimens were cured under standard curing (SC) conditions for 28 days. Then, the concrete and paste samples were immersed in two kinds of simulated environments (Table 4).

using GSAS software. GSAS software can only refine 9 phases at a time, so the most important crystal phases were selected for each sample. 2.4.4. Backscatter electron (BSE) and secondary electron (SE) tests The samples were cut into small pieces, and the outer surface of the samples was sanded with 100–2000 mesh sandpapers. Then, the samples were sequentially polished with 9, 3, 1 and 0.05 mm diamond suspensions. Finally, the samples were placed in anhydrous ethanol for ultrasonic cleaning. BSE and SE images were collected by scanning electron microscopy (SEM) with a FEI Quanta 200 FEG instrument, and the elemental compositions were obtained with energy dispersive spectrometry (EDS). Gold was sprayed onto the surface of the samples before the analysis to prevent charging effects. 2.4.5. Thermogravimetric (TG) analysis The samples were ground into powder form, as described in Section 2.4.3. Then, 20–30 mg of the powders were heated to 900 °C at a constant rate of 10 °C/min under the protection of nitrogen. Considering the continuous weight loss of calcium silicoaluminate hydrate (C-A-S-H) between 50 and 600 °C, the tangential method was used to calculate each phase content (Fig. 2) [30]. 2.4.6. Mercury intrusion porosimetry (MIP) tests The pore diameter distribution of the hardened pastes was determined using an Auto Proe9520 MIP. The samples were stored at 100 °C for 1 day to ensure that there was no moisture in the pores before the tests. 3. Experimental results

2.4. Test methods

3.1. Compressive strength

2.4.1. Compressive strength tests The compressive strengths of the concrete specimens at 28 days were measured according to the Chinese National Standard GB50107-2010.

As shown in Fig. 3, the compressive strengths of the concrete were recorded every month. The compressive strengths of the FRHPC and FRRPC are similar at 28 days, but they change dramatically over time. In the early stage of concrete immersion in the simulated environments, the increase in the compressive strength of the FRRPC is higher than that of the FRHPC. In simulated envi-

2.4.2. Free water and bound water content tests The test methods for the free water and bound water contents were the same as those in described the literature [29]. 2.4.3. XRD analysis The samples were ground into powder form until they all could pass through a screen with a 45-mm diameter and were mixed with 20% zincite powder [29]. Calibration of the amorphous phase contents was performed with samples that were doped with zincite powders. XRD patterns were obtained with a TTR IIIX-ray diffractometer over a scanning range of 5–70° and an angular step of 0.02° (2h) per second. The quantitative XRD results were obtained

Table 4 Curing and simulated environments. No.

Temperature (°C)

Solution

Time (days)

SC

20 ± 1

28

Simulated environment 1 (SE1) Simulated environment 2 (SE2)

40

95% relative humidity (RH) 10%NaCl + 5%Na2SO4

60

10%NaCl + 5%Na2SO4

420

420

Table 2 Mixture proportions of concrete (kg/m3). Types

Water

Cement

Fly ash

Slags

Silica fume

Quartz sand

River sand

Stone

Fibre

Superplasticizer

FRHPC FRRPC

145 165

350 240

100 200

60 350

50 80

– 1250

650 –

1070 –

78.0 75.0

12.5 17.2

Table 3 Fibre specifications. Types

Fibre types

Tensile strength/MPa

Length/mm

Diameter/mm

FRHPC FRRPC

Steel wire end hook type steel fibres Microfilament copper plated fibres

1100 >2850

60 13

0.80 0.22

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results are affected by many factors, such as the water/binder (w/b) ratio, water/cement (w/c) ratio and mineral admixture contents. Quantitative analysis is needed to clarify the evolution process of the hardened pastes. 3.3. XRD-Rietveld analysis The quantitative analysis results for the hardened pastes were obtained by XRD-Rietveld analysis. The XRD patterns and mass fractions of the phases are shown in Fig. 5 and Table 5, respectively. The DoH of the clinkers was calculated by Eq. (1) (Table 6) [29], and the Rietveld results normalised on a total solids basis for the paste samples were corrected [30] (the effects of Cl, SO2 4 and CO2 were not considered).

    wC 3 S ðtÞþwC 2 S ðtÞþwC 3 A ðtÞþwC 4 AF ðtÞ =1wtotalwater ðtÞ ðDoHÞt 1 wC 3 S ðt0 ÞþwC 3 S ðt0 ÞþwC 3 A ðt0 ÞþwC4 AF ðt0 Þ

Fig. 2. Tangential method for the quantification of the phases.

100% ð1Þ

Compressive strength (MPa)

160 FRHPC-SE1 FRHPC-SE2 FRRPC-SE1 FRRPC-SE2

140 120 100 80 60 40

0

60

120

180

240

300

360

420

480

Times (Days) Fig. 3. Change in compressive strengths of the FRHPC and FRRPC immersed in SE1 and SE2.

ronment 1 (SE1), the compressive strength of the FRHPC subsequently begins to decrease at approximately 360 days, and that of FRRPC decreases slightly at 390 days. Moreover, there is no decrease in the FRRPC strength in simulated environment 2 (SE2). 3.2. Free water and bound water contents The free water and bound water contents of the hardened pastes in SE1 and SE2 are shown in Fig. 4. After immersion in the simulated environments for 420 days, the free water contents of the hardened pastes decrease obviously, while their bound water contents increase. This phenomenon shows that the reaction of the clinkers in the hardened pastes is not complete at 28 days and that a large amount of free water is retained inside the pastes. However, the free water gradually reacts with some components in the hardened paste to form bound water during immersion in the simulated environments. Moreover, the change in SE2 is more obvious than that in SE1. This finding may be because the high temperature in SE2 can accelerate the hydration of cement, resulting in an increase in the bound water content. The differences between the two types of hardened pastes are also very obvious. The bound water content of the reactive powder hardened paste (RPHP) is 36.00% lower than that of the high-performance hardened paste (HPHP) at 28 days, but the difference decreases to 15.91–19.56% after 420 days in the simulated environments. These

where w(t0) represents the mass fractions of phases in the total anhydrous cementitious materials and w(t) represents the mass fractions of phases in the hardened paste at age t. The DoH of the HPHP is already high at 28 days (61.47%), and the increasing trend is no longer obvious in the period from 28 to 420 days (65.40% in SE1 and 68.71% in SE2). In contrast, the DoH of RPHP is low after SC (32.92%), and it grew rapidly in the simulated environments (69.62% in SE1 and 73.39% in SE2). After a period of time in the simulated environments, the DoH of RPHP even surpasses that of HPHP. The RPHP has a low w/b ratio, resulting in a low degree of early reaction under SC. However, the w/c ratio of the RPHP is higher than that of the HPHP, and after 28 days, the RPHP still retains a portion of water, which can be used for the hydration of the cement. This phenomenon also confirms the rapid increase the compressive strengths of the FRRPC. The mullite in the fly ash almost does not react during SC for 28 days, but the mullite contents decrease after immersion in both environments. Compared with that of the RPHP, the portlandite content of the HPHP is high at 28 days because of the higher cement content and the less extensive pozzolanic reaction of the mineral admixture in the HPHP [31]. It is well known that gypsum in cement leads to the formation of ettringite, which adheres to the surface of C3A to prevent excessive hydration [32]. Changes in the ettringite content are found to be almost negligible after immersion for 420 days. Jin et al. [25] believed that the diffusion rate of  SO2 4 is slower than that of Cl and that the solubility of ettringite in chloride solution is approximately three times that of ettringite in water. Both Maes et al. [33] and Zhang et al. [34] suggested that the presence of Cl prevents SO2 4 attack on concrete and reduces the formation of ettringite. In addition, Cao et al. [35] showed that  SO2 4 can inhibit the diffusion of Cl . In that study, the concentra2 tion of SO4 was relatively low, and the concrete had high compactness, which may have resulted in inhibition of its corrosion in the simulated environments. Wang et al. [36] found that physical damage to concrete induced by NaCl crystallization occurred during freeze-thaw cycles in a NaCl solution. However, the solubility of the salt was high in both simulated environments, and the salt crystal contents were less than 1%. The Friedel’s salt content in the HPHP is higher than that in the RPHP after immersion in the simulated environments, and the contents are negatively correlated with the ambient temperature. Balonis [37] believed that Friedel’s salt loses its dominant position in the system above 55 °C and that monosulfoaluminate subsequently begins to dominate the system. In addition, a high DoH may suppress the formation of Friedel’s salt.

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Y.-c. Zhou et al. / Construction and Building Materials 230 (2020) 116909

2.5

22

(a)

2.31

20.36

20

Bound water content (wt.%)

Free water content (wt.%)

(b) 20.30

2.14

2.0

1.5

1.0 0.56

0.48

0.5

0.49

0.34

18

17.61

17.12 16.33

16 14 12

11.27

10 8

0.0

HPHP-SC HPHP-SE1 HPHP-SE2

RPHP-SC

6

RPHP-SE1 RPHP-SE2

HPHP-SC HPHP-SE1 HPHP-SE2

RPHP-SC

RPHP-SE1 RPHP-SE2

Fig. 4. Free water and bound water contents in HPHP and RPHP: (a) free water contents and (b) bound water contents.

Caculated value and error:

RPHP-SC

HPHP-SC

RPHP-SE1

HPHP-SE1

RPHP-SE2

HPHP-SE2

Bragg-positions

2

Intensity (a.u.)

1.465 1.759 1.675 1.488 1.556

1.584 Zincite Mullite C3S C2S C3A C4AF Portlandite Calcite Freidel's salt Ettringite NaCl Na2SO4

0

10

20

30

40

2

50

60

70

80

degree

Fig. 5. XRD patterns of the hardened pastes.

Table 5 Mass fractions of crystalline and amorphous phases in the hardened pastes analysed by XRD (wt.%). Types

HPHP

Phase name

SC

SE1

SE2

RPHP SC

SE1

SE2

C3S C2S C3A C4AF Mullite Portlandite Calcite Friedel’s salt Ettringite NaCl Na2SO4 Amorphous phases

10.84 2.26 2.10 2.02 4.62 2.01 2.07 – 8.21 – – 65.87

10.80 2.17 – 0.18 3.86 – – 3.88 8.30 0.13 0.83 69.85

10.78 2.11 – 0.90 3.10 – – 3.36 8.36 0.17 0.63 70.59

9.55 2.35 1.34 0.96 6.88 0.25 1.16 – 4.05 – – 73.46

4.83 0.83 – 0.53 4.94 – – 2.67 4.09 0.13 0.81 81.17

4.22 0.76 – 0.40 4.76 – – 2.36 4.21 0.18 0.60 82.51

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Y.-c. Zhou et al. / Construction and Building Materials 230 (2020) 116909

Table 6 DoH of clinkers in hardened pastes (%). Types

HPHP-SC

HPHP-SE1

HPHP-SE2

RPHP-SC

RPHP-SE1

RPHP-SE2

DoH

61.47

65.40

68.71

32.92

69.62

73.39

3.4. TG analysis Fig. 6 shows the differential thermogravimetry (DTG) curves of the hardened pastes before and after immersion in the simulated environments. C-A-S-H forms in a large temperature range (50– 600 °C) of water loss due to interlayer water movement and dehydroxylation. Ettringite, Friedel’s salt and portlandite are dehydrated at approximately 260, 270 and 460 °C, respectively. Moreover, calcite loses carbon dioxide near 650 °C. At 810 °C, the small peaks indicate the dehydroxylation of C-A-S-H with a low Ca/Si ratio or tobermorite to form wollastonite crystals [38]. The phase contents were calculated with the tangential method by combining the DTG and weight loss curves (Fig. 7) of the hardened pastes (Table 7). The quantitative results of TG analysis are consistent with those of the XRD-Rietveld analysis. Because GSAS software can refine only 9 phases, the phase contents of portlandite and calcite are obtained from the TG analysis. The C-A-S-H (due to the quantitative dehydroxylation behaviour of C-A-S-H near 810 °C not being clear, this part of the C-A-S-H content was not taken into account) and amorphous SiO2 and Al2O3 contents were calculated by Eqs. (2) and (3), respectively, by incorporating portlandite and calcite contents into the XRD-Rietveld results. The degree of pozzolanic reaction (DoPR) of mineral admixtures can be calculated by Eq. (4) (Table 8).

wCASH ðtÞ ¼

ð3Þ

wamorphous SiO2 and Al2 O3 ðtÞ=½1  wtotal water ðtÞÞ wamorphous SiO2 and Al2 O3 ðt 0 Þ

ð4Þ

where MW is the molecular weight of the phases. 0.00 0.02

C-A-S-H Friedel's salt

0.04

Calcite

Portlandite

0.06 0.08 HPHP-SC HPHP-SE1 HPHP-SE2 RPHP-SC RPHP-SE1 RPHP-SE2

0.10 0.12 C-A-S-H, Ettringite

0.14

Free Water

0.16

0

3.5. Morphology The microstructure of the hardened pastes in different environments was investigated with BSE imaging (Fig. 8). Cement, fly ash, slag and silica fume can be distinguished by their different grey levels. The HPHP was dense at 28 days, and its surface was mainly composed of C-A-S-H and a small amount of unhydrated cement and mineral admixture (Fig. 8(a)). After the HPHP was immersed

wweight at 600 C ðtÞ  wbound waterðPortlanditeÞ ðtÞ  wbound waterðFreidel0 ssaltÞ ðtÞ  wbound waterðEttringiteÞ ðtÞ MW water loss of CASH =MW CASH

wamorphous SiO2 and Al2 O3 ðtÞ ¼ wamorphous phases ðtÞ  wCASH ðtÞ ðDoPRÞt ¼

It is obvious that there is still a low portlandite content in the HPHP after immersion in the simulated environments for 420 days, while there is no portlandite in the RPHP. This result is attributed to the cement/binder (c/b) ratio of RPHP being much lower than that of HPHP, which limits the portlandite content that can be produced. Both the DoH and DoPR of the HPHP are much higher than those of the RPHP. There is a large amount of C-A-S-H in the bulk that provides strength to the HPHP, while amorphous SiO2 and Al2O3 coexist with C-A-S-H in the RPHP. After immersion for 420 days, the RPHP reached a DoPR and C-A-S-H content that were close to those in the HPHP. In addition, amorphous SiO2 and Al2O3 particles were retained in the RPHP, which may also improve its strength. The Friedel’s salt content and relative amounts from TG analysis agree with the XRD results; thus, Friedel’s salt may have an important influence on the performance of hardened pastes.

150

300

450

600

Fig. 6. DTG curves of hardened pastes.

750

900

ð2Þ

in SE1 and SE2 for 420 days, a large number of cracks appeared on the surface (Fig. 8(b) and (c)). A large number of unhydrated particles remained on the surface of the RPHP at 28 days (Fig. (d)). A few fine cracks appeared in the RPHP in SE1 (Fig. 8(e)), but it was difficult to find visible cracks in the RPHP in SE2 (Fig. 8(f)). It is obvious that the number of cracks in the hardened pastes is positively correlated with the Friedel’s salt content. Moreover, the number of unhydrated particles on the surface of the two types of hardened pastes decreases to varying degrees after immersion. The microstructure of the hardened pastes was observed by SE imaging (Fig. 9). This kind of complete polygonal flake crystal is slow-forming Friedel’s salt from C3A (Fig. 9(a)) [39]. There have been many reports on concrete cracks caused by the crystallization of Friedel’s salt [40]. In a study on the failure behaviour of cement paste in a sodium chloride environment, Qiao [41] speculated that these cracks may be caused by the crystallization pressure of Friedel’s salt. Therefore, the damage in the two kinds of hardened pastes is also likely to be caused by Friedel’s salt after excluding other factors. Some ettringite was also formed in the pores (Fig. 9(b)), but it may have been formed during the curing stage and is not part of the corrosion products that were identified by XRD and TG analysis.

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Two-dimensional (2D) scatter plots of EDS analyses of the C-AS-H on the surface of the hardened pastes are shown in Fig. 10. The average of the scatter points was calculated by Eqs. (5) and (6) (Fig. 11). The average Ca/Si ratio of the C-A-S-H produced by the RPHP was significantly lower than that produced by the HPHP at 28 days. This difference may be due to the pozzolanic reaction of a large amount of mineral admixture in the RPHP. The average Ca/Si ratios of the C-A-S-H decrease to varying degrees after immersion. Moreover, this decrease is positively correlated with temperature. Liu et al. [42] indicated that the C-A-S-H formed by a high-volume mineral admixture of FRRPC had an extremely low Ca/Si ratio and was ‘‘cloud shaped” with high compactness. The formation of C-A-S-H with a low Ca/Si ratio is consistent with a fast increase in the compressive strength and the excellent corrosion resistance of the concrete specimens.

100 HPHP-SC HPHP-SE1 HPHP-SE2 RPHP-SC RPHP-SE1 RPHP-SE2

Weight (wt.%)

95

90

85

80

75

0

150

300

450

600

750

900

Si=CaCASH ¼ Fig. 7. Weight loss curves of hardened pastes.

ASi ðpÞ ACa ðpÞ

ð5Þ

Table 7 Mass fractions of phases in the hardened pastes according to TG analysis (wt.%). Types

HPHP

Phase name

SC

SE1

SE2

RPHP SC

SE1

SE2

Portlandite Calcite Friedel’s salt C-A-S-H Amorphous SiO2 and Al2O3

2.28 1.89 0.00 56.74 9.13

0.63 1.19 4.09 >60.77 <7.81

0.61 1.18 3.81 >62.41 <6.92

0.41 1.15 0.00 38.61 34.85

0.00 0.69 2.33 >60.38 <20.23

0.00 0.82 2.08 >63.20 <18.63

Table 8 DoPR of amorphous SiO2 and Al2O3 (%). Types

HPHP-SC

HPHP-SE1

HPHP-SE2

RPHP-SC

RPHP-SE1

RPHP-SE2

DoPR

69.33

>71.82

>75.18

40.73

>64.09

>66.98

(a)

(c)

(b) Fly ash Cracks

Cement Slag

(d)

Fly ash

Cracks

(e)

(f) Slag

Slag Silica fume

Fine crack

Cement

Fig. 8. BSE images of hardened pastes: (a) HPHP-SC, (b) HPHP-SE1, (c) HPHP-SE2, (d) RPHP-SC, (e) RPHP-SE1, and (f) RPHP-SE2.

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Y.-c. Zhou et al. / Construction and Building Materials 230 (2020) 116909

(a)

Element

(b)

At%

C

17.46

O

63.54

Al

5.63

Si

1.81

Cl

2.42

Ca

9.14

Ettringite

Fig. 9. Part of products in hardened pastes: (a) Friedel’s salt and (b) ettringite.

0.7 0.6

HPHP-SC

RPHP-SC

HPHP-SE1

RPHP-SE1

HPHP-SE2

RPHP-SE2

0.5

Al/Ca

0.4 0.3 0.2 0.1 0.0 0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

Si/Ca Fig. 10. The 2D scatter plots of EDS analyses of C-A-S-H.

those in the reference samples at 28 days. This phenomenon is consistent with previous results. It has been shown that hydration, pozzolanic reactions and Friedel’s salt formation are constantly occurring in samples to fill the pores in the two kinds of simulated environments [43]. The main peaks of the pore diameters are determined by the pressure at which mercury enters the samples. The related peaks at hundreds of micrometres are most likely caused by cracks on the surface of the samples. Moreover, it is clear that the distribution of micron-sized pores is related to the Friedel’s salt content. The RPHP and HPHP have similar porosities at 28 days, but the pore diameters of the RPHP are slightly smaller than those of the HPHP (Fig. 12(b)). The reason for this phenomenon is that the surface of the RPHP is filled with a large amount of unreacted mineral admixture, including nanometre-sized silica fume. Obviously, the RPHP has a better pore structure distribution than the HPHP. Without considering the micron-sized pores, both the ratio of cementitious materials and the ambient temperature have a certain effect on the gel pore volume of the samples.

2.5

4. Discussion 2.0 1.51 Ca/Si

1.5

1.43 1.19 1.05

0.97

1.0

0.93

0.5

0.0

HPHP-SC HPHP-SE1 HPHP-SE2

RPHP-SC

RPHP-SE1 RPHP-SE2

Fig. 11. Average Ca/Si ratios of C-A-S-H.

Al=SiCASH ¼

AAl ðpÞ ASi ðpÞ

ð6Þ

where A(p) represents the atomic percentage of elements in the hardened paste at the position of p. 3.6. Pore structure Fig. 12 shows the pore structure of the hardened pastes. The differential pore volumes in the samples at 420 days are lower than

In environments where high temperature and corrosion ions coexist, concrete samples undergo not only favourable hydration and pozzolanic reactions but also corrosion product formation. The composition distributions in the concrete samples are shown in Fig. 13. Because the pore structure in hardened paste samples was studied, the interfacial pores between the paste and aggregate were not considered. The slight increase in the compressive strengths in the simulated environments may be mainly caused by the formation of Friedel’s salt. However, the process of Friedel’s salt growth causes crystallization pressure on the matrix and may lead to the development of cracks. The reason for the great increase in the compressive strengths of the FRRPC in the simulated environments is the hydration of the cement and the pozzolanic reaction of the mineral admixtures. Pyo et al. [44] revealed that ultra-HPHP can effectively prevent Cl attack from fibre corrosion in a series of experiments and that corrosioninduced microcracking was inconsequential. The C-A-S-H content of the RPHP at 28 days is low, but the proportion of C-A-S-H in the FRRPC is not lower than that in the FRHPC. In addition, there are very few weak areas in the FRRPC sample with only fine aggregates. Moreover, the interior of the FRRPC can maintain a large number of cracks under a load [45]. For these reasons, the compressive strength of the FRRPC is higher than that of the FRHPC at 28 days. After immersion in the simulated environments, the internal composition of the FRHPC chan-

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Y.-c. Zhou et al. / Construction and Building Materials 230 (2020) 116909

0.09

0.045

(a)

HPHP-SC HPHP-SE1 HPHP-SE2 RPHP-SC RPHP-SE1 RPHP-SE2

0.07 0.06

(b)

0.040

0.05 0.04 0.03 0.02 0.01

>300 m

0.00391

100nm-300 m

0.00355

0.00260

0.035 Pore volume (ml/g)

Differental pore volume (ml/g)

0.08

50-100nm

0.00277

<50nm

0.00018

0.030

0.00118

0.025 0.020 0.03455

0.01890

0.03361

0.015

0.02461 0.00081

0.010 0.00281

0.00

0.00561

0.005 0.01

0.1

1

10

100

0.00547

1000

0.000

Pore diameter ( m)

0.00355

0.00235 0.00318

0.00227

HPHP-SC HPHP-SE1 HPHP-SE2

RPHP-SC

0.00323

RPHP-SE1 RPHP-SE2

Fig. 12. Pore structure of hardened pastes: (a) differential pore volume and (b) pore volume of different pore diameter ranges.

Pore

140

120

Weight (wt.%)

Ca/Si =1.51

Fibre

Amorphous SiO2 and Al2O3

Sand

Ettringite, et al. crystal

Stone

78.7MPa 100

80

C3S, C2S, C3A and C4AF

C-S-A-H gel

52.7MPa

2.25 14.70

Ca/Si

2.07 4.91 3.43 3.12

85.8MPa

1.65 Ca/Si 16.54

16.11 =1.43

2.37 4.38 4.46 3.12

63.4MPa

1.64

=1.19

1.84 4.54 3.59 3.12

60 25.97

25.97

25.97

115.8MPa

Ca/Si

15.58

=1.05

131.9MPa 0.85

1.10

3.49 Ca/Si

25.82

=0.97 14.06

Ca/Si 27.18 =0.93

8.65

4.98 5.74 3.18

5.66 2.62 3.18

8.01 5.52 2.29 3.18

52.97

52.97

52.97

40

20

0

42.75

42.75

42.75

FRHPC-SC FRHPC-SE1 FRHPC-SE2 FRRPC-SC FRRPC-SE1 FRRPC-SE2 Fig. 13. Composition distributions of concrete samples.

ged little, but a large amount of C-A-S-H with a low Ca/Si ratio was formed in the FRRPC. Nevertheless, there are still many problems that require a solution. Additional experiments are needed to demonstrate the crystallization pressure of Friedel’s salt and the mechanical effects of the large number of unhydrated particles that remained in the FRRPC.

5. Conclusion In this paper, the durability of two typical kinds of concrete with excellent mechanical properties, namely, FRHPC and FRRPC, was studied by the simulation of underground environments in coastal ultra-deep mines. In these environments, the compressive strengths of the concrete samples began to decrease. Several

microscopic tests were performed to explore the internal factors. The following conclusions can be drawn: (1) The DoH and DoPR of the RPHP are lower than those of the HPHP at 28 days, and the internal phases of the RPHP consist of C-A-S-H and amorphous SiO2 and Al2O3. However, the proportion of C-A-S-H in the FRRPC is slightly higher than that in the FRHPC. The compressive strength of the FRRPC is not inferior to that of the FRHPC at 28 days. (2) Analysis of the performance evolution behaviour of concrete in simulated coastal ultra-deep mine underground environments shows that there is little hydration and pozzolanic reaction in the FRHPC, accompanied by the formation of many corrosion products. This effect results in a decrease in the compressive strength of the FRHPC. However, the hydration and pozzolanic reaction behaviours in the FRRPC

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Y.-c. Zhou et al. / Construction and Building Materials 230 (2020) 116909

are obvious, and the corrosion products are less pronounced than those in the FRHPC. The compressive strengths of the FRRPC are still in a stable stage. (3) A high ambient temperature can improve the DoH and DoPR and reduce the porosity of concrete. High temperatures in ultra-deep underground environments may be beneficial to the development of concrete. (4) The cracks in BSE images and the peaks of the micron-sized pores in the pore structure curves are related to the Friedel’s salt content. The failure of high-performance concrete in coastal ultra-deep mines is most likely caused by the crystallization pressure of Friedel’s salt. (5) The application of FRRPC in coastal ultra-deep mines has more advantages than that of FRHPC.

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. Acknowledgments The research described herein was sponsored by the National Natural Science Foundation of China (Nos. 51678049 and No. 51834001). This research was also supported by grants from the National Key Research and Development Project of China (Grant No. 2016YFC0600803). References [1] C. Fairhurst, Some challenges of deep mining, Eng. (2017) 527–537. [2] M.F. Cai, E.T. Brown, Challenges in the mining and utilization of deep mineral resources, Eng. 3 (2017) 432–433. [3] H.P. Xie, Y. Ju, F. Gao, et al., Groundbreaking theoretical and technical conceptualization of fluidized mining of deep underground solid mineral resources, Tunn. Undergr. Sp. Tech. 67 (2017) 68–70. [4] P.G. Ranjith, J. Zhao, M.H. Ju, et al., Opportunities and Challenges in Deep Mining: A Brief Review, Eng. 3 (2017) 546–551. [5] X.T. Feng, J.P. Liu, B.R. Chen, et al., Monitoring, warning, and control of Rockburst in deep metal mines, Eng. 3 (2017) 538–545. [6] Q.Y. Han, Y.L. Zhang, K.Q. Li, et al., Computational evaluation of cooling system under deep hot and humid coal mine in China: a thermal comfort study, Tunn. Undergr. Sp. Tech. 90 (2019) 394–403. [7] M. Bomberg, H. Miettinen, M. Wahlstrom, et al., Post operation inactivation of acidophilic bioleaching microorganisms using natural chloride-rich mine water, Hydrometallurgy 180 (2018) 236–245. [8] X.B. Li, F.Q. Gong, M. Tao, et al., Failure mechanism and coupled static-dynamic loading theory in deep hard rock mining: a review, J. Rock Mech. Geotech. Eng. 9 (2017) 767–782. [9] M.H. Ren, G.S. Zhao, G.Q. Zhou, et al., Using strain dynamics for fracture warning of shaft lining, Phys. Stat. Mech. Appl. 507 (2018) 406–413. [10] C.S. Zhang, F. Hu, S. Zhou, Effects of blast induced vibrations on the fresh concrete lining of a shaft, Tunn. Undergr. Sp. Tech. 20 (2005) 356–361. [11] D. Soulioti, N.M. Barkoula, A. Paipetis, et al., Acoustic emission behavior of steel fibre reinforced concrete under bending, Constr. Build. Mater. 23 (2009) 3532– 3536. [12] J.D. Riosa, H. Cifuentes, C. Leiva, et al., Analysis of the mechanical and fracture behavior of heated ultra-high-performance fiber-reinforced concrete by X-ray computed tomography, Cem. Concr. Res. 119 (2019) 77–88. [13] S.F. Dong, B.G. Han, X. Yu, et al., Dynamic impact behaviors and constitutive model of super-fine stainless wire reinforced reactive powder concrete, Constr. Build. Mater. 184 (2018) 602–616. [14] Z.K. Guo, W.X. Chen, Y.Y. Zhang, et al., Postfire blast-resistances of RPC-FST columns using improved Grigorian model, Int. J. Impact Eng. 107 (2017) 80– 95. [15] M. Verma, P.R. Prem, J. Rajasankar, et al., On low-energy impact response of ultra-high performance concrete(UHPC) panels, Mater. Design 92 (2016) 853– 865.

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