The effect of curing regimes on the mechanical properties, nano-mechanical properties and microstructure of ultra-high performance concrete

Cement and Concrete Research 118 (2019) 1–13

Contents lists available at ScienceDirect

Cement and Concrete Research journal homepage: www.elsevier.com/locate/cemconres

The effect of curing regimes on the mechanical properties, nano-mechanical properties and microstructure of ultra-high performance concrete ⁎

T

⁎

Peiliang Shena,b, Linnu Lua, , Yongjia Hea, Fazhou Wanga, , Shuguang Hua a b

State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Luoshi Road 122, Wuhan 430070, China School of Materials Science & Engineering, Wuhan Institute of Technology, Wuhan 430205, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Ultra-high performance concrete Curing regimes Mechanical properties Microstructure Flexural/tensile to compressive strength ratio

This study addresses the effect of curing regimes on the mechanical properties, hydration and microstructure of ultra-high performance concrete (UHPC). The results demonstrate that the mechanical properties are strengthened by increasing curing temperature, but the flexural/tensile to compressive strength ratio shows an unusual increasing tendency with increasing temperature and compressive strength, which is opposite to normal concrete. The nano-mechanical properties are also enhanced by heat treatment. The ultra-high density phase is dominated hydrates. Microstructure observation indicates that heat treatment promotes the formation of additional hydrates with high-packing density and stiffness such as tobermorite and xonotlite, enhancement of transition zone around steel fiber, quartz and clinker, average chain length of hydrates and pozzolanic reaction between quartz/silica fume and Ca(OH)2. The evolution of hydrates and microstructure due to curing regimes and the presence of quartz play key roles in controlling the unusual behavior of the strength ratio and improvement of mechanical properties.

1. Introduction Ultra high performance concrete (UHPC) is a type of advanced cement based material, that shows good permeability resistance [1], excellent freezing and erosion resistance [2], and high fire endurance temperature [3–5]. According to Richard and Cheyrezy, UHPC represents the highest development of high performance concrete [6]. UHPC is normally heat treated (by steam or autoclave curing) at an early age to achieve high strength [7]. The heat treatment of UHPC can accelerate the hydration of clinker, pozzolanic reaction of silica fume and the reaction of quartz powders, and further would increase the amount of C-S-H phases [8–10]. With the enhancement of heat treatments, the precast plants and engineers are interested in understanding the mechanical properties and microstructure of UHPC under different curing regimes, aiming at guiding to find an optimal curing regime to enhance the performance and microstructure of a UHPC [11,12]. Therefore, it is important to better understand the implications of different curing regimes on the mechanical properties, hydration and microstructure of UHPC. The effects of different heat treatments on the performance and microstructure of UHPC have been investigated in some previous studies. Detlef et al. investigated the effect of heat treatment method on the properties of UHPC [10]; the experimental results showed that heat ⁎

treatment of UHPC resulted in very high strength in a short time period. Additionally, the pozzolanic reaction of silica fume was promoted to form additional C-S-H phases. Hélène et al. studied the hydration and pozzolanic reaction of UHPC using 29Si NMR [8]; the experimental results showed that the silica fume and quartz powder consumption were highly dependent on curing temperature and curing duration. The effects of different curing regimes and duration on the strength and durability of UHPC were also studied; the test results showed that the time at which thermal treatment applied was closely related to performance [13]. Zemei et al. investigated the effect of three kinds of curing regimes (standard, hot water and steam curing) on flexural strength of UHPC containing different supplementary cementitious materials; the results indicated that a standard curing over 28d led to comparable flexural properties as those shown by hot water and steam curing [14]. The early strength development of UHPC was also investigated under different curing regimes; the test results showed that UHPC with hot water curing gained higher strength compared to other curing methods [12]. Yazici et al. [15,16] developed UHPC using different curing regimes, the autoclave pressures and temperature on strength was discussed. The maximum strength was achieved using autoclave curing with 2 MPa pressure. Steam curing and autoclaving process parameters on the mechanical properties of UHPC were also considered; preset time, target temperature and holding time were

Corresponding authors. E-mail addresses: [email protected] (L. Lu), [email protected] (F. Wang).

https://doi.org/10.1016/j.cemconres.2019.01.004 Received 1 September 2017; Received in revised form 18 November 2018; Accepted 16 January 2019 0008-8846/ © 2019 Elsevier Ltd. All rights reserved.

Cement and Concrete Research 118 (2019) 1–13

P. Shen et al.

considered. The factors having the strongest effect on mechanical properties were the target temperature of steaming and autoclaving [16,17]. Heat curing has a strong influence on the strength development of UHPC. The development of dense microstructure with the formation of C-S-H gels contributes to the improvement of mechanical properties. In addition, the mechanical properties improvement of UHPC has indicated heat treatment at 250–300 °C to be the most beneficial curing conditions to achieve higher UHPC compressive strength followed by autoclave, steam, and standard curing [18,19]. But the effects may rely on the mix design of UHPC.To develop high strength UHPC, combined curing conditions have been utilized [4,20–22]. Yunsheng et al. used combined curing regimes to improve the mechanical properties of UHPC, and the interfacial bond strength between the fiber and matrix was significantly affected by the curing process [23]. Generally, the microstructure, rate of strength gain and hydration process of UHPC are significantly enhanced by heat treatment; the reduction of curing time leads to rapid construction [24,25]. Pozzolanic hydration products such as tobermorite and xonotlite have been confirmed in heat treated specimens [12,22]. It has been established that crushed quartz acts as pozzolanic materials, the reactivity of silica fume is improved at high temperatures (higher than 90 °C), and crushed quartz enhances the number of secondary hydration products [26]. Heat treatment modifies the chemical composition of hydration products [11,12,17] by reducing calcium oxide/silicon dioxide ratio and the water/calcium oxide ratio, leading to the formation of tobermorite at temperatures between 100 °C and 150 °C and xonotlite at temperature between 200 °C and 250 °C [12]. As a result, the early strength and durability of UHPC can be enhanced by heat treatment [27]. It has already been shown that the mechanical properties and microstructure characteristics of UHPC depend on the curing regimes [8,10,16,28–31]. However, the current knowledge of the differences in the effects of curing regimes on compressive, flexural and tensile strength is still limited. Also, the underlying mechanisms that govern the differences in the improvement of different mechanical properties are still unclear. Although the mechanical improvement of high temperature curing has been experimentally investigated at macroscopic scale [32], the effect of curing regimes on micro-mechanical properties of UHPC and its response on strength are still not well understood. Additionally, the difference (including mechanical and microstructure) between standard cured and steam-cured UHPC were usually investigated, without studying the effects of autoclave curing at temperature higher than 100 °C. Hence, it can be predicted that the effects of the most common curing regimes on the mechanical properties, micro-mechanical properties and microstructure development of UHPC still need further attention. In this study, an investigation was carried out to understand the effect of different curing regimes on the mechanical properties, nanomechanical properties and microstructure of UHPC. Five types of typical curing regimes (standard curing at 20 °C for 28 d, steam curing at 60 °C and 90 °C for two days and autoclave curing at 200 °C and 250 °C for 8 h) were studied. The macroscopic mechanical and nano-mechanical properties were tested, and the relations between the improvement of mechanical properties and curing regimes were discussed. This study focused on the microstructure development of UHPC cured at different curing regimes. Then 29Si MAS NMR spectroscopy was used to determine the hydration of cement and silica fume as well as the chain length of the C-S-H phases. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to identify the hydration products and microstructure. The BSE-EDAX was used to evaluate the microstructure of polished UHPC paste and the hydration products around quartz powders, clinker and steel fiber. Additionally, the thermosgravimetric (TG) and differential scanning calorimetry (DSC) were further employed to evaluate the hydration degree of UHPC paste.

Table 1 Oxide compositions of the Portland cement and silica fume (wt%). Oxide

SiO2

Al2O3

CaO

Cement Silica fume

19.37 88.29

3.92 0.14

66.30 0.92

Fe2O3

SO3

MgO

Na2O

K2O

LOI

3.69 0.19

2.81 1.51

1.61 3.21

0.13 0.14

0.59 0.17

1.09 5.26

Table 2 Physical properties of Portland cement and silica fume. Blaine fineness (m2/kg)

Properties

Density (g/cm3)

Setting time (min) Initial

Cement Silica fume

419.7 15,248.0

3.10 2.48

Final

237 /

298 /

2. Material and methods 2.1. Materials (1) Raw materials A type I Portland cement produced by Huaxin cement Co. LTD was used to prepare the UHPC and paste. The silica fume produced by Wuhan Iron & Steel Group was used. The chemical compositions of the Portland cement and silica fume are listed in the Table 1. Their technical properties are listed in the Table 2. Quartz sand with a diameter ranging from 0.125 to 1.18 mm was used as aggregates. Steel fiber (length = 13 mm, diameter = 0.22 mm), superplasticizer (water reducing ratio of 35%) and quartz powder were used as received. Clean tap water was used for all mixtures. (2) Specimens preparation The mix proportion of UHPC is shown in Table 3. The volume proportion of steel fiber was 2%. After mixing, the mixtures were poured into the molds and consolidated by using a vibrating table. The cement paste mixtures were also prepared. The slump flow of UHPC with 2% steel fiber is about 160 mm. After casting, all specimens were stored in a room (20 °C and 100% RH) for 24 h prior to demolding. Then, they were cured under different curing regimes. (3) Curing regimes Five types of curing regimes were employed in this study: 1) Standard curing at 20 °C (RH 100%) for 28 days; 2) Steam curing at 60 °C for 48 h; 3) Steam curing at 90 °C for 48 h; 4) Autoclave curing at 200 °C and 1.7 MPa pressure for 8 h; 5) Autoclave curing at 250 °C and 2.1 MPa pressure for 8 h. 2.2. Methods 2.2.1. Mechanical properties (1) Flexural strength and compressive strength Specimens sizing of 40 × 40 × 160 mm3 were prepared for flexural Table 3 Mix proportion of the UHPC (Mass). Cement

1.00 a

2

Silica fume 0.30

Quartz powder 0.30

Quartz sand

Water/binder ratioa

superplasticizer

1.10

0.20

0.015

The binder consists of cement and silica fume.

Cement and Concrete Research 118 (2019) 1–13

P. Shen et al.

Fig. 1. The tensile specimen geometry.

strength and compressive strength measurement. Three specimens were performed to further calculate the average strength.

for XRD worded with Cu Kα radiation (λ = 1.5405 Å) at 35 kV and 30 mA. The 2-Theta values ranging from 5° to 75° was chosen.

(2) Direct tensile strength

2.2.4. Scanning electron microscopy (SEM) and backscattered electron (BSE) imaging A FEI QUANTA FEG 450 ESEM fitted with a BSE detector was used. The microscope with an accelerating voltage of 15 kV and a Bruker AXS XFlash Detector for energy dispersive X-ray analysis was used. The hydration was stopped by soaking in ethanol. Once dried, specimens were prepared for BSE-SEM test. Before testing, the specimens were stored in a drying vessel.

The geometry of the test specimen is shown in Fig. 1. The section of specimens is 30 × 60 mm2. A universal testing machine running in displacement control was used to conduct the tensile test. A special test frame was used to measure the elongation from both sides by using two linear variable differential transformers (LVDTs). The speed of displacement during testing was 0.4 mm/min. Six specimens were prepared for each batch. The average value was used as the final tensile strength.

2.2.5. Nuclear magnetic resonance spectroscopy (NMR) A Bruker Solid-State NMR Spectrometer (400 MHz), model AVANCE III was used to obtain 29Si MAS NMR spectra of UHPC pastes prepared under different curing regimes, and the spinning speed was 12,000 Hz.

2.2.2. Nano-mechanical properties The surface of sample was polished and a relatively flat surface finish was used for indentation test to obtain reliable results. The specimens were impregnated with epoxy resin and polished with 600, 800, 1200 and 2500 grit polishing papers and then polished with diamond abrasives sizing of 3, 1 and 0.25 um. Care should be taken during the polishing not to damage the surface of the specimens and no water being used during polishing. Nanoindentation tests were carried out with a Bruker Nanoindentation Tester using a diamond Berkovich tip. A line with a length of 1 mm was chosen on the BSE image, then 50 indentations was performed on this line. The distance between two points was set as 20 μm to avoid the indentation tests. And the image of every position of indentations test was performed by SEM. Based on the SEM images, the phase that the test performed could be confirmed. Load controlled indentation measurements were performed up to a maximum load of 2 mN at a load rate of 1 mN/s following a holding time of 5 s and a 5 s unloading period. The load process is shown in Fig. 2.

2.2.6. Thermal analysis A Power-Compensation Differential Scanning Calorimeter was used to obtain the thermos-gravimetric (TG) and differential scanning calorimetry (DSC) curves of UHPC cured under different regimes. The thermal tests were conducted at a heating rate of 10 °C/min from 50 °C to 1000 °C under nitrogen atmosphere. 3. Results and interpretation 3.1. Effect of curing regimes on the mechanical properties of UHPC 3.1.1. The development of mechanical properties (1) Compressive strength and flexural strength The compressive strength and flexural strength of UHPC with 2% steel fiber under different curing regimes are measured and shown in Fig. 3. The UHPC cured under the standard curing condition has the lowest strength. However, the compressive strength is obviously increased by steam curing and autoclave curing. The specimens cured under autoclave curing show higher strength enhancement than those of steam curing. It can be seen from Fig. 3 that a compressive strength of 180 MPa and flexural strength of 70.32 MPa are achieved through 8hour autoclave curing at 250 °C. Based on the above observations, it can be concluded that an elevated temperature can accelerate the hydration, and enhance the compact microstructure, resulting in high early compressive and flexural strength. This is in agreement with previous works [33,34]. From the compressive strength and flexural strength test results, the mechanical properties are enhanced by heat treatment. But the improvement of flexural strength of UHPC is more obvious than that of compressive strength, especially for the UHPC under autoclave curing.

2.2.3. X-ray diffraction analysis The X-ray diffraction (XRD) was used to identify the hydration products of UHPC cured under different curing regimes. The UHPC pastes without quartz sand were used. The measuring instrument used

2.5

Load (mN)

2 1.5 1 0.5 0 0

2

4

6

8 10 Time (s)

12

14

16

(2) Direct tensile strength

Fig. 2. The load process of indentation measurements. 3

Cement and Concrete Research 118 (2019) 1–13

200

80 Flexural strength (MPa)

Compressive strength (MPa)

P. Shen et al.

180 160 140 120 100

70 60 50 40 30 20

80 20

60 90 200 Temperature (oC)

20

250

(a) compressive strength

60 90 200 Temperature (oC)

250

(b) flexural strength

Fig. 3. Effect of curing regimes on the compressive and flexural strength of UHPC.

0.45

Flexural to compressive strength ratio

Tensile strength (MPa)

20 18

0.4

16

0.35

14

0.3

12

0.25

10 20

60 90 200 Temperature (oC)

250

0.2 20

Fig. 4. Effect of curing regimes on the tensile strength of UHPC.

60 90 200 Temperature (oC)

250

Fig. 5. Effect of curing regimes on the flexural to compressive strength ratio of UHPC.

Fig. 4 shows the tensile strength of UHPC with 2% steel fiber cured under different curing regimes. It can be observed that the tensile strength is improved by steam curing and autoclaved curing processes. The autoclave curing is the most effective factor in improving tensile strength. In general, tensile strength is mainly influenced by binder reaction, compressive strength of paste and interaction between steel fibers and matrix [35,36]. Steam curing and autoclave curing accelerates the hydration and pozzolanic reaction of paste, resulting in the improvement of the hydration degree and microstructure [37]. As a result, the tensile strength is enhanced by increasing curing temperature. Increased curing temperature improved not only the tensile and flexural strength of concrete but also the compressive strength. Also, it should be noted that the improvement in compressive strength is less than that of flexural and tensile strength.

than 90 °C) specimens furnish higher values. This phenomenon may be attributed to the change of hydration products, microstructure and steel fibers. The effects of curing regimes on nano-mechanical properties, hydration products, and microstructure will be shown in Sections 3.2–3.4. According to Yunsheng's work, the interfacial bonding strength after autoclave curing is 14.2 MPa [40], which is much higher than that after steam curing and standard curing (3–5 MPa). This high bonding strength leads to an obvious increase in flexural strength, resulting in the increasing of flexural to compressive strength ratio. The effect of curing regimes on the development of this ratio will be further discussed in Section 4. 3.1.3. Relations between tensile and compressive strength Fig. 6 shows the ratio of tensile to compressive strength of UHPC cured under different curing regimes. It can be seen from Figs. 3 and 4 that tensile strength increases with increasing compressive strength. Tensile to compressive strength ratio

3.1.2. Relations between flexural and compressive strength The flexural to compressive strength ratio of UHPC is shown in Fig. 5. It can be seen that the ratio depends on the curing regimes. This ratio first decreases at steam curing at 60 °C, then increases as the curing temperature increases. For normal concrete, the ratio of flexural to compressive strength depends to some extent on the level of compressive strength, and the ratio usually decreases as the compressive strength of concrete increases. According to previous research, the ratio of flexural to compressive strength can be used to indicate the brittleness of concrete [38]. The higher compressive strength is, the lower the ratio would be. So the ratio of normal or high strength concrete with or without steel fiber decreases with the increasing strength. Usually, the steam-cured and autoclave-cured concrete tends to be rather brittle [39], so a lower flexural to compressive ratio than that of standard cured concrete is expected. The main reason for this brittleness is that the hydration products become crystalline at high temperature [12]. The main products of UHPC cured at high temperature are different from the hydration products of concrete cured at 20 °C. However, the test results show an unusual behavior. Here the heat treated (higher

0.105 0.1 0.095 0.09 0.085 0.08 20

60 90 200 Temperature (oC)

250

Fig. 6. Effect of curing regimes on the tensile to compressive strength ratio of UHPC. 4

Cement and Concrete Research 118 (2019) 1–13

P. Shen et al.

maximum indentation depth with the residual depth of penetration [44,45]. The modulus of elasticity of hydrates is much lower than those of unreacted raw materials. The maximum indentation depths of hydrates range from 300 nm to 500 nm, which are much larger than those of the raw materials (such as cement and quartz sand). All indentation curves in this study are checked to remove aberrant results. Every indentation result corresponds to a BSE image, and the critical BSE images of quartz sand, cement and hydrates are shown in Fig. 8. In these BSE images, the unreacted cement and silica fume can be observed as having bright signatures, followed by the less-bright signature of quartz and hydrates. The voids are the darkest in the pictures. Based on BSE images of every indentation test, the differences among these phases are clear, and a clear correlation between the compositions and nano-mechanical measurements can be built. Nano-indentations have been widely used to study the nano-mechanical properties of cement-based materials during the past decade [46]. In this study, nano-indentation was applied to investigate the effect of curing regimes on the nano-mechanical properties of UHPC. From the typical load vs indentation depth curves shown in Fig. 7, the nano-mechanical properties can be determined, and the indention modulus is defined as follows [47,48]:

This indicates that the tensile strength and compressive strength are closely related, but there is no direct proportionality. Actually, tensile strength increases more slowly than compressive strength, so the ratio decreases with increasing temperature. This observation is not in agreement with the tendency of tensile to compressive strength ratio of normal concrete [41]. The ratio of tensile to compressive strength increases with increasing compressive strength and curing temperature. Usually, for normal and high strength concrete, the tensile to compressive strength ratio decreases with an increasing strength level of concrete, and the ratio of high strength concrete is less than 0.07 [41]. However, the tensile to compressive strength ratio is much higher than that of high strength concrete. This result is mainly attributed to the presence of steel fiber and absence of coarse aggregate. It should be noted that the relationship between compressive and tensile strengths shows an opposite tendency compared to that of normal concrete. The samples cured under autoclave curing obtain the largest tensile to compressive strength ratio. The tensile to compressive strength ratio depends on various factors, such as curing age, mixture proportion, water to cement ratio, type of aggregate and admixtures [39]. Generally, the tensile strength depends on the properties of matrix and transition zone. The decrease of porosity and transition zone can improve both compressive and tensile strength. A magnitude of increase in tensile strength can be obtained when the intrinsic strength of hydration products comprising the transition zone is improved at the same time [41]. The transition zone can be enhanced by the pozzolanic reaction and reaction between quartz sand and Ca(OH)2. Steam curing above 90 °C and autoclave curing promotes these two reactions. The matrix and transition zone enhancement is probably the reason for the relatively big increase in the tensile strength of UHPC. Meanwhile, the interfacial bonding strength of steel fibers is improved by the pozzolanic reaction of silica fume leading to an enhancement of the interfacial bonding strength [37,42,43]. The interfacial bonding strength of steam and autoclave curing samples was about 12–14 MPa [23], which is two times higher than that of samples cured under standard curing, and a higher tensile strength is expected. Therefore, an improvement in the transition zone of grains and interfacial bonding strength of steel fiber leads to the increasing of ration of tensile to compressive strength. Other reasons, such as hydration products and microstructure will be discussed in Section 4.1.

M=

1 1 − v2 1 − v12 = + M E E1

Load (uN)

2000

HD C-S-H Cement

1500 1000 500 0 0

100

200

300 400 Depth (nm)

500

600

(1)

(2)

where E is the modulus of elasticity of the test materials, v is the Poisson's ratio of the test materials, E1 is the modulus of elasticity of the indented tip, and v1 is the Poisson's ratio of the indented tip. Because a diamond tip was used in this study, E1 and V1 have values of 1141 GPa and 0.007, respectively. The modulus of elasticity of hydrates in UHPC cured under different regimes was determined. All modulus of elasticity values were calculated from the unloading indentation curves. Fig. 9 shows the indentation pattern of lines of hydrates in specimens cured under different curing regimes. The results show that the modulus of elasticity of the hydration products is significantly influenced by the curing regimes. Autoclave-cured specimens furnish the highest results followed by steam-cured specimens and then samples cured at 20 °C. The modulus of elasticity of hydrates in standard cured specimens ranges between 17.1 GPa and 51.0 GPa, and the average value is 28.7 GPa. The average modulus of elasticity shows a significant increase after steam curing. The average modulus of elasticity of specimens cured at 60 °C is 43.4 GPa. For specimens cured at 90 °C, the modulus of elasticity ranges between 37.1 GPa and 64.6 GPa, with an average value of 51.7 GPa. Similar average modulus of elasticity were obtained by V.Y. Garas [11] and Sorelli et al. [32], where the average modulus of elasticity are 47.6 and 49.4 GPa, respectively. For autoclave-cured UHPC, the modulus of elasticity is a little higher than that of steamcured UHPC (an average value of 55.8 GPa corresponds to 250 °C), This is different from the results obtained by U. Müller et al. [34]. The modulus of elasticity of specimens cured at 200 °C (54.9 GPa) is similar to that of specimen cured under 250 °C. In order to clearly show the effects of different curing regimes on the modulus of elasticity of specimens, the test results are not present.

3.2.1. Nano-mechanical properties results Fig. 7 shows the depth-load curves of typical phases in the UHPC. The quartz, cement clinker, and hydrates were separated based on SEMBSE tests. Unlike normal cement-based materials, there are three different hydrates in UHPC; low density (LD) C-S-H, high density (HD) CS-H and ultra-high density phase (UHD) C-S-H in UHPC. This ultra-high density phase only appears in UHPC. Generally, the elastic behavior of these phases can be reflected from depth-load curves by comparing the UHD C-S-H LD C-S-H Quartz powder

π⎞ h − h max A⎠

where p and h are the indentation load and indentation depth recorded by nano-indentation, h max is the maximum indentation depth, and A is the projected contact area that can be extrapolated from the indentation depth h through Oliver and Pharr's method [49]. The indentation modulus is closely related to the modulus of elasticity and the Poisson's ratio of the test material. Then, the modulus of elasticity of the test materials can be calculated:

3.2. Effect of curing regimes on the nano-mechanical properties of UHPC

2500

1 ⎛ dp 2 ⎝ dh

700

Fig. 7. Typical load vs indentation depth curves for hydrates, quartz and cement. 5

Cement and Concrete Research 118 (2019) 1–13

P. Shen et al.

(a) The test line

(b) Hydration products

(c) Clinker

(d) Quartz sand Fig. 8. Illustrations of nano-indentation test.

20

60

90

100%

250

80%

60

Proportion

Modulus of elasticity (GPa)

80

40

60% LD 40%

HD UHD

20%

20

0% 20

0 0

200 400 600 800 1000 1200 Distance from the first point (μm)

60 90 200 Curing temperature (oC)

250

Fig. 10. Effect of curing regimes on the fractions of the hydrationphases.

Fig. 9. Effect of curing regimes on modulus of hydrates.

volume proportion of HD C-S-H and UHD phase would be produced after heat treatment. Fig. 10 shows the effect of curing regimes on the fractions of LD C-SH, HD C-S-H and UHD phases in UHPC. The low density of C-S-H is characterized by phase modulus E = 22.5 ± 5.0GPa. The high density C-S-H is characterized by phase modulus E = 30.4 ± 2.9GPa. The ultra high density phase is characterized by phase modulus E = 40.9 ± 7.7GPa [51]. From the frequency of the elastic modulus of UHPC cured under different curing regimes, the content of the different C-S-H in UHPC can be calculated [54]. Unlike the hydration products in normal concrete, more than 90% of the hydrates in UHPC cured at 20 °C are HD C-S-H and UHD phase. In addition, the content of UHD phase increases from 11.5% to 100% when the curing temperature increases from 20 °C to 250 °C. The LD CS-H phase disappears in UHPC cured above 90 °C. For autoclave-cured UHPC, almost all of the hydrates are UHD phase. This indicates that the hydration products have much higher mechanical properties over those of sample cured at low temperature. The packing density distribution of the hydration products is closely affected by heat treatment, and the heat treatment favors the formation of UHD phase. Therefore, the UHPC cured at higher temperature has higher compressive strength and higher nano-mechanical properties [32,50]. This increasing modulus of

3.2.2. Summary For normal cement-based materials with a water to cement (w/c) ratio of 0.5, the hydrates consist of 57% LD C-S-H, 30% HD C-S-H and 13% Ca(OH)2 [50]. The volume of HD C-S-H increases significantly with decreasing w/c [51], which leads to a higher average modulus of elasticity than that of normal concrete. The LD C-S-H and HD C-S-H are generally regarded as the main hydrates in normal cement-based materials, but a kind of ultra-high density phase is present especially in samples with a low w/c ratio [51]. When the w/c is below 0.2, the volume of UHD phase increases dramatically [50]. The modulus of elasticity of UHD phase ranges from 40GPa to 60 GPa [52,53], which is much higher than that of LD C-S-H and HD C-S-H. Therefore, the average modulus of elasticity of UHPC shows much higher average values than normal concrete [32,50]. From the indentation results shown in Fig. 9, hydrates in UHPC after steam and autoclave curing have much higher average modulus of elasticity than that of standard cured specimens, and these values increase with the increasing curing temperature. Considering the modulus of elasticity of the hydrates in UHPC under different curing regimes, it seems that the heat treatment promotes the production of HD C-S-H and UHD phase, thus, a larger 6

Cement and Concrete Research 118 (2019) 1–13

P. Shen et al.

Fig. 12. The Fig. 11. The XRD analysis of UHPC cured at different temperatures.

29

Si MAS NMR analysis of UHPC cured with different regimes.

WINNMR software using a Lorent curve. The deconvolution results are shown in Table 4. The Q4 values are not included in this table. There are fewer Q0 units in UHPC pastes cured at high temperature than at low temperature. This is related to the acceleration of hydration kinetics with temperature and the conversion of Q0 to Q1, Q2, and Q3. This indicates that the amount of hydration products in UHPC increases with increasing curing temperature. The proportion of Q2 increases with increasing curing temperature (except for 250 °C), while the Q1 units decrease significantly, leading to a raising Q2/Q1 ratio. Also the Q3-defect unit appears in UHPC specimens cured at 90 °C and its content increases with increasing curing temperature. This phenomenon is attributed to the presence of crystal hydration products. In addition, the proportion of unreacted silica fume is relatively high in specimens cured at 20 °C, which is reflected by the Q4 in Fig. 12. For the autoclave and steamcured specimens, the pozzolanic reaction of silica fume is promoted. As a result, the structure of C-S-H phases depends on the curing regimes, which sre responsible for the increase in compressive strength [58]. Nevertheless, compared with samples cured at 200 °C and 250 °C, a significant microstructural change is observed with the appearance of a higher Q3 peak at 250 °C. This peak is mainly attributed to the presence of xonotlite [8]. The structure implies the presence of a silicon‑oxygen tetrahedral connected to three neighbor tetrahedral, which is confirmed by the appearance of xonotlite by XRD measurement (Fig. 11). By fitting the intensity of the different 29Si NMR signals, it is possible to calculate the degree of cement and the main chain length of the C-S-H. The main chain length of the C-S-H phases and hydration formation ratio can be calculated as follows, and the results are shown in Fig. 13.

elasticity with increasing heat treatment may also contribute to the increase of macroscopic mechanical properties of UHPC [39]. 3.3. Hydration products 3.3.1. XRD analysis The X-ray diffraction of UHPC paste cured under different curing regimes is shown in Fig. 11. For the standard cured specimen, it reveals that the clinker reacts with water to form amorphous C-S-H and porlandite. The silica fume reacts with portlandite to form additional C-SH. The ettringite can be observed in specimens cured at 20 °C and 60 °C, and no appreciable reaction of quartz powder is observed. But for specimens cured at 90 °C, the crystalline phase and tobermorite are already present [55]. Additionally, it can be observed that the intensity of SiO2 peaks decreases with the increase of curing temperature. The portlandite is hard to detect in steam and autoclave-cured specimens. This indicates that the reaction between silica fume/quartz powder and portlandite is promoted by heat treatment. When specimens are cured at 200 °C and 250 °C, crystalline tobermorite [56] and xonotlite are detected, which have an advantageous effect on material mechanical properties [15,16]. During the heat treatment, the hydration of clinker and the pozzolanic reaction of the silica fume both continue, resulting in further hardening of the UHPC. Additionally, new products such as xonotlite can be observed in specimens under autoclave curing. Compared with the xonotlite in UHPC cured at 200 °C, the sharpness and area of its peak become obvious. This indicates the increase of xonotlite content and crystallinity. In addition, there is a large amount of un-hydrated C3S and C2S, which exist because of the low water to binder ratio. Compared to standard cured specimens, the intensity of peaks of un-hydrated clinker and Ca(OH)2 decreases significantly with increasing curing temperature.

C = 2 (Q1 + Q 2 + Q 3 − defect )/(Q1)

(3)

H = Q1 + Q 2 + Q 3

(4)

where C is the chain length and H is the hydration formation ratio that describes the fraction of hydrates in cementitious materials. It can be observed from Fig. 13 that the average C-S-H chain length is short when the curing temperature is 20 °C [23], and the calculated chain length increases significantly with increasing curing temperature. The calculated chain length of C-S-H in specimen cured at 250 °C is 12.14, which is more than four times higher than that of standard cured specimens. This reveals higher gel polymerization (increased chain length), which would entail a lower Ca/Si ratio. Also, it should be noted that there is an important effect of curing temperature on the hydrates formation ratio of UHPC; it increases from 33.7% to 72.1% between 20 °C and 250 °C. Thus, it can be concluded that curing temperature promotes the hydration of UHPC, leading to the formation of long chain C-S-H. This formation is attributed to the continuing hydration of cement and increasing pozzolanic activity of silica fume and quartz powder. Therefore, the increasing hydration degree of UHPC and the

3.3.2. 29Si MAS NMR The 29Si NMR spectra of UHPC pastes cured with different curing regimes are presented in Fig. 12. The 29Si NMR spectra of specimens cured at different curing regimes are performed according the QnQuotation, where Q represents a SiO4 tetrahedron unit and n corresponds to its degree of connectivity. Generally, the unreacted cement displays a peak at about −71.0 ppm corresponding to Q0 units, the C-SH phases present Q1 and Q2 units from their chain structure at about −80.0 ppm and − 85.0 ppm, respectively. The Q3 indicates the connection of two chains, or the precipitation of silica gels [57] displays two peaks at about −92.4 ppm and − 97.0 ppm respectively. And the silica fume and quartz powders exhibit Q4 species at about −110.6 ppm and − 107.2 ppm, respectively. The signal patterns of the spectra are deconvoluted with Bruker 7

Cement and Concrete Research 118 (2019) 1–13

P. Shen et al.

Table 4 Deconvolution data of

29

Si NMR spectra of UHPC paste.

Curing regime

Q0

Q1

Q2

20 °C

−71.7 ppm I = 52.7% −71.6 ppm I = 48.7% −71.6 ppm I = 40.3% −71.7 ppm I = 33.5% −71.7 ppm I = 28.0%

−79.5 ppm I = 25.2% −79.9 ppm I = 21.3% −79.9 ppm I = 17.0% −79.2 ppm I = 11.6% −79.2 ppm I = 10.2%

−84.5 ppm I = 8.5% −84.8 ppm I = 30.0% −84.8 ppm I = 37.2% −84.6 ppm I = 43.5% −84.6 ppm I = 41.8%

60 °C 90 °C 200 °C 250 °C

Q3-defect

Q3

H

–

–

33.7%

–

–

51.3%

−92.3 ppm I = 6.3% −92.4 ppm I = 8.0% −92.4 ppm I = 9.9%

–

60.5%

−96.8 ppm I = 3.4% −96.8 ppm I = 10.2%

66.5% 72.1%

between Ca(OH)2 and quartz powder and silica fume in specimens are accelerated by temperature. Also, the wall effect and the formation of Ca(OH)2 are hindered in UHPC [8,65], so the amount of Ca(OH)2 in UHPC cured at 20 °C is still very low. For specimens cured at 20 °C, the mass loss between 650 °C and 800 °C becomes more significant, representing a more intensive decomposition process due to carbonation of UHPC during standard curing process. Additionally, there is also a weak peak at about 570 °C because of the transformation of quartz (αSiO2 to β-SiO2). It can be noted that the hydration degree is promoted by heat treatment. The amount of Ca(OH)2 decreases with the increasing curing temperature. This is mainly attributed to high pozzolanic reaction degree at high temperature. Based on the XRD and thermal analysis, it can be concluded that the Ca(OH)2 is exhausted in UHPC cured at temperatures above 90 °C. Many new products such as tobermorite and xonolite are formed. The high curing temperature improves the hydration degree of UHPC, and makes the microstructure dense. As a result, a significant increase of mechanical properties and nano-mechanical properties can be observed. In conclusion, the hydration products of UHPC differ from common concrete where larger crystals of Ca(OH)2 are limited. It is believed that the very low water to binder ratio of UHPC results in a low porosity where there is little space available for the growth of Ca(OH)2 crystals. For steam-cured and autoclave-cured specimens, the high content of silica fume, together with addition of quartz powders and relatively high curing temperature, create an effective pozzolanic environment that would consume most of the Ca(OH)2 in UHPC [9]. It should be noted that steam curing and autoclave curing cause a nearly total consumption of Ca(OH)2.

Fig. 13. Effect of curing regimes on the main chain length of hydrates.

average C-S-H chain length at high temperatures result in a dense microstructure and high strength. From Eq. (3), it should be noticed that the content of Q2 and Q3 increases more significant with increasing curing temperature than Q1. The Q3 peak in Fig. 12 refers to the presence of crystal hydrate, tobermorite and xonotlite. The structure of these phases indicates the presence of SO4 tetraheda connected to three neighbor tetrahedral. As the reactivity of silica fume and quartz is enhanced by heat treatment, the total content of crystal hydrate, tobermorite and xonotlite increases with increasing curing temperature (Fig. 11). As a result, the connectivity ratio of hydrate species is significantly increased, leading to an increasing average chain length. In addition, the increasing average chain length indicates that the SiO4 tetrahedra chain length increases with temperature. After 28 days of standard curing at 20 °C, the average chain length of C-S-H in UHPC has about three links; at 60 °C, around five links; and at 90 °C, around eight links. The autoclave curing further increases the amount of links created in a short time, and it has nearly eleven links. In other words, the polymerization of hydrates is closely related to the curing temperature. Usually, at high temperatures the CS-H gel is interspersed in matrix, forming a solid solution [59]. The formation of hydrates with very long chains hinders the development of cracks, while the compressive strength of concrete is positively correlated with average chain length [60]. This hydrate fromation would also explain the greater compressive strength found in specimens cured at high temperatures.

3.4. Microstructure 3.4.1. SEM analysis Fig. 15 shows the effect of different curing regimes on the SEM images of UHPC. UHPC presents a densely compacted microstructure due to low water to binder ratio and substantial pozzolanic reaction of reactive mineral admixtures. The interfacial transition zone observed in UHPC is of very small thickness concerning common concrete [66], and there is no obvious interfacial transition zone in autoclave-cured UHPC. The small width of the interfacial transition zone of UHPC indicates a well-developed bond between the paste and the aggregate surface. In standard curing condition, amorphous or poorly crystalline C-S-H being rigid in nature are formed. In the SEM images of steam-cured specimens, large portlandite cannot be observed due to the acceleration of hydration, suggesting a higher hydration degree of UHPC than that of standard-cured specimens. For autoclaved cured specimens, etched appearances of the borders of quartz powders can be observed, indicating the reaction of quartz with cement. It can be seen from the SEM images of autoclave-cured samples that the spherical pores have been filled with needle-like tobermorite and jennite-like xonotlite. These particles appear to be ball-

3.3.3. Thermal analysis Fig. 14 shows the thermal analysis curves of UHPC pastes cured under different curing regimes. It can be seen from TG curve that all samples exhibit significant mass losses at temperature between 100 °C and 200 °C. This is mainly attributed to the dehydration of C-S-H gel and physically bond water [61,62]. This mass loss is significantly improved by heat treatment. The mass loss between 410 °C and 510 °C indicates the decomposition of Ca(OH)2 [63,64]. From the DSC curves, a weak peak at about 410–510 °C can be observed in UHPC cured at 20 °C, while there is no obvious peak in other UHPCs cured under steam and autoclave curing conditions. This confirms that the reaction 8

Cement and Concrete Research 118 (2019) 1–13

0.5

104

0

100 Mass loss (%)

DSC/(mW/mg)

P. Shen et al.

-0.5 20ȭ 60ȭ 90ȭ 200ȭ 250ȭ

-1 -1.5

20ȭ 60ȭ 90ȭ 200ȭ 250ȭ

96 92 88 84

-2 0

200

400 600 800 Temperature (oC)

0

1000

(a) DSC

200

400 600 Temperature (oC)

800

1000

(b) TG

Fig. 14. The thermal analysis of UHPC cured with different regimes.

like fibrous crystals with hollow cores filled with fibrous crystals of xonotlite. However, the fraction of these two products in UHPC cured at 200 °C differs from that of the sample cured at 250 °C, in which tobermorite is the main products in the spherical pores. It can be observed that a much lager amount of jennite-like xonolite is found in specimens cured at 250 °C, and the XRD results also support this observation. Also, there are no calcium hydroxide crystals that can be observed in the autoclave-cured sample due to the high pozzolanic activity of silica fume and the presence of quartz powder [59]. Generally, UHPC shows a dense microstructure at high temperature, forming crystalline hydrates. The changes of hydrates enhance the strength of the specimens.

separated shells. Usually, the shell is separated from the partially reacted cores by a gap of approximately 1–2 μm [67] due to the reaction of aluminate phase at high temperature. In addition, no interfacial transition zone is found in any of the specimens cured by the different curing regimes. This is attributed to the pozzolanic reaction and the absence of coarse aggregate. Also, increasing curing temperatures seems to strengthen these effects. The morphologies between clinker and hydrates in UHPC cured under different curing regimes differ from one another. There are some variations corresponding to the relationship between composition and morphology. In order to investigate the effect of curing regimes on the hydrates around clinkers, the composition of the hydrates around clinker (about 5 μm from the clinker) was tested by the BSE-EDAX method. The test results are shown in Fig. 17. The test results show that the Ca/Si ratio of hydrates around clinker decreases with increasing curing temperature. The Ca/Si ratio of the standard cured specimens is much higher than that of samples cured at high temperature. Also, the hydrates around the clinker in steam and autoclave-cured UHPC have lower Ca/Si ratios than those of farther away from the clinker [26]. Compared to the specimens cured at ambient conditions, autoclavecured specimens have a stronger pozzolanic reaction, resulting in a lower Ca/Si ratio, a reduction of pores in the nanometer range and an increased compressive strength. Additionally, steam curing and autoclave curing can promote the formation of crystalline hydration

3.4.2. SEM-BSE analysis Fig. 16 shows the micro texture of the samples in the form of polished cross sections. The microstructures of all samples are dense. Compared with UHPC cured at 20 °C, a monolithic region surrounding the clinker can be observed in the specimens cured at 60 °C. This region is attributed to the increasing hydration rate during steam curing. The hydration products of this monolithic region mainly form at 20 °C, which differs from products in other regions. The 90 °C steam-cured specimens show a micro-crack between clinker and hydrates in the form of separated hydration shells. Additionally, the autoclaved cured specimens also show slightly more porous microstructure with pores concentrated between clinker and hydration products in the form of

(a) 20 oC 28d

(b) 60 oC 48h

(c) 90 oC 48h

(d) 200 oC 8h

(e) 250 oC 8h Fig. 15. The SEM images of UHPC under different curing regimes. 9

Cement and Concrete Research 118 (2019) 1–13

P. Shen et al.

(a) 20oC 28d

(b) 60 oC 48h

(c) 90 oC 48h

(d) 200 oC 8h

(e) 250 oC 8h Fig. 16. The BSE images of UHPC under different curing regimes.

20 60 90 200 250

0.08

Al/Si

0.12 Al/Ca

0.1

20 Inner 60 Inner 90 Inner 200 Inner 250 Inner

0.16

0.08 0.04

0.06 0.04 0.02 0

0 0.6

1

1.4

1.8

2.2

0.1

2.6

0.6

1.1

1.6

2.1

2.6

Ca/Si

Ca/Si

Fig. 17. EDAX analysis of the inner hydrates around clinker.

Fig. 18. EDAX analysis of the hydrates around quartz sand.

products. The Ca/Si ratios of steam and autoclave-cured UHPC suggest the formation of hydration phases with low Ca/Si ratio such as xonotlite and tobermorite. The presence of tobermorite and xonotlite can be confirmed by the XRD results in Fig. 11. The decreasing Ca/Si ratio of hydrates reveals higher gel polymerization, leading to longer C-S-H chains (shown in Fig. 13). In order to evaluate pozzolanic reaction of the quartz in UHPC, the hydration products around quartz powders (about 5 μm from the quartz) was investigated by EDAX. Fig. 18 shows the test results. It can be observed that the Ca/Si ratio of hydration products around quartz decreases by with increasing curing temperature. It can be concluded that the quartz powders react with Ca(OH)2 in heat cured specimens [34]. The test results prove that the quartz powders in UHPC cured at 20 °C are absolutely inert even after 28 days' storage. It should be noted

that the reaction between quartz powders and Ca(OH)2 occurs after two-days of steam curing at 60 °C. This is different from the result in ref. [26]. The Ca/Si ratio decreases significantly after steam and autoclave curing. The Ca/Si ratios of hydrates in standard cured specimens are similar to that of the matrix. However, the Ca/Si ratio of hydrates around quartz powder in steam and autoclave-cured specimens is much lower than hydrates far away from quartz powder and clinker [26]. This difference is attributed to the pozzolanic reaction between quartz powder and Ca(OH)2, which increases the silicon content of hydrates. It should be noted that the quartz powder starts to hydrate during twoday steam curing at 60 °C. This reaction can improve the bond strength between quartz and paste, resulting in a dense interfacial transition zone. As a result, increases in compressive, flexural and tensile strength 10

Cement and Concrete Research 118 (2019) 1–13

P. Shen et al.

(a) Standard curing at 20oC

(b) steam curing

(c) autoclave curing

Fig. 19. The BSE images of interfacial transition between steel fiber and matrix of UHPC under different curing regimes.

can be obtained, as well as an increase in the ratio of flexural/tensile to compressive strength.

decrease in porosity and an increase in compressive strength [70]. Tobermorite and xonotlite become dominant components of autoclavecured UHPC (as shown in Fig. 12 and Fig. 18), and the hydration products of autoclave-cured concrete are mostly coarse and largely microcrystalline. The presence of tobermorite and xonotlite will play an important role in the development of the tensile/flexural ratio. Moreover, the high curing temperature can lead to an increase of C-S-H chain length so as to secure higher strength hydrates [71] and possibly increase ratio vs. curing temperature. The tobermorite phase is no longer observed at 200 °C. The xonotlite has acicular morphology (shown in Fig. 15), but the tobermorite phase may have platelets, lathes and acicular morphology [72,73]. This acicular morphology plays an important role in preventing crack development under tensile or flexural stress. From the SEM and EDS results, the reaction between silica and paste enhances the transition zone of the silica. The hydrates around cement clinkers show low Ca/Si ratio [74], a reduction of pores in the nanometer range and an increased bonding strength. Therefore, the interfacial transition zone of grains in UHPC is improved by high temperature curing, and microstructure plays an important role in the improvement of tensile/flexural to compressive ratio. Generally, when micro-cracks propagate through the UHPC paste due to load, steel fibers can bridge the micro-cracks [39,75–77]. Interfacial bonding strength after autoclave curing is much higher than that of standard cured UHPC [40]. This is one of the key factors contributing to increasing flexural/tensile to compressive ratio. Moreover, the appearance of tobermorite and xonotlite, the enhancement of the interfacial transition zone between quartz and clinker and matrix, and the morphology of hydrates and average chain length are closely related to the increasing tensile/flexural to compressive ratio.

3.4.3. SEM-BSE analysis of transition between steel and fiber Fig. 19 shows the SEM-BSE micrographs of the fiber/matrix interface of UHPC cured with different curing regimes. It can be seen that characterization of UHPC subjected to different curing regimes shows differences in the microstructure at the interfacial transition zone between fiber and matrix. For UHPC cured at 20 °C, a porous zone around the fiber is observed, and the size of the porous zone is about 5 μm. In the heat curing, either by steam curing or autoclave curing, no porous zone can be observed. This is in agreement with the characterization of heat-cured UHPC seen by previous studies [11,32]. It can be concluded that enhancement of the interfacial transition zone is attributed to the heat treatment, which could promote further reaction of paste, leading to increased density around the fiber and increasing interfacial bond strength. Furthermore, the improvement of the interfacial transition zone contributes to the increasing ratio of flexural/tensile to compressive strength. 4. Discussion 4.1. Curing regime vs. ratio of flexural/tensile to compressive strength The ratio of flexural/tensile to compressive strength of UHPC is highly dependent on the curing regimes and compressive strength. For normal and high strength concrete, the ratio of flexural/tensile to compressive strength is closely related to compressive strength. As steam curing and autoclave curing can promote the early strength development of concrete, the ratio shows a decreasing trend with increasing compressive strength [41]. However, the ratios vs. compressive strength of UHPC in this study show a different trend. The ratios of UHPC cured under temperature higher than 90 °C are higher than standard cured UHPC. Apart from the improvement in bonding strength of steel fiber, the microstructure and hydrates evolution plays important role in the increasing ratio vs. curing temperature. After steam curing and autoclave curing, the pozzolanic reaction between silica fume is promoted. This has been confirmed by the XRD test results (Fig. 11). The intensity of Ca(OH)2 peaks decrease significantly. The reaction between silica and Ca(OH)2 can be observed at 90 °C, which can be confirmed by the intensity of the silica peaks of the XRD test results. The reaction of silica can also be supported by SEM results (Fig. 18) and NMR results (Figure12). Therefore, a large amount of additional C-S-H is formed by pozzolanic reaction, which is responsible for the increased compressive strength and flexural/tensile to compressive strength ratio. The Ca(OH)2 is almost entirely consumed in UHPC under autoclave curing, with only trace amounts being detected in XRD test results. The decomposition of ettringite takes place at about 70 °C, converting to monosulfoaluminate (AFm) and vanishes at about 85 °C [68,69]. This process increases the strength. A large amount of tobermorite can be detected in UHPC cured at 90 °C. The tobermorite phase has a larger volume of structure than a-C2SH phase, causing a

4.2. Curing regime vs. nano-mechanical properties The nano-mechanical property of hydrates and the content of the ultra-high density phase are strongly related to the curing regimes. This relationship is highly dependent on the hydration evolution, composition of hydrates and microstructure. It should be noted that new hydration products forms as the curing temperature increases. For UHPC cured at 90 °C (as shown in Fig. 12), some C-S-H gel is replaced by tobermorite. Xonotlite forms and becomes dormant when the curing temperature reaches 200 °C (as shown in Figs. 12 and 18). The average stiffness of tobermorite is about 77GPa, and the average stiffness of xonotlite is 106GPa, which is significantly higher than that of C-S-H gel [78,79]. The stiffness of the hydrates contributes to the enhancement of the nano-mechanical properties of UHPC. In UHPC cured above 200 °C, the LD C-S-H and HC-S-H gel disappear, and the hydrates are almost entirely transformed to UHD phases. Thus, the modulus of elasticity of hydrates in UHPC increases with increasing curing temperature. Regarding the presence of silica fume and silica powders, the pozzolanic reaction between them and Ca(OH)2 modifies the initial composition of hydrates and microstructure, and the Ca/Si ratio and average chain length of hydrates are significantly increased with increasing curing temperature. This reveals higher gel polymerization in UHPC under steam curing and standard curing. But the hydrates of 11

Cement and Concrete Research 118 (2019) 1–13

P. Shen et al.

References

autoclave-cured UHPC are coarse and largely microcrystalline with very low specific surface area and high packing density [80]. As a consequence, the hardness and modulus of elasticity of hydrates are improved. The interface properties between silica and hydrates are improved by pozzolanic reaction. Hydrates around silica shows a denser microstructure and lower Ca/Si than that of hydrates in other areas, leading to a further improvement in nano-mechanical properties.

[1] N. Roux, C. Andrade, M. Sanjuan, Experimental study of durability of reactive powder concretes, J. Mater. Civ. Eng. 8 (1996) 1–6. [2] V. Matte, M. Moranville, Durability of reactive powder composites: influence of silica fume on the leaching properties of very low water/binder pastes, Cem. Concr. Compos. 21 (1999) 1–9. [3] Y.-S. Tai, H.-H. Pan, Y.-N. Kung, Mechanical properties of steel fiber reinforced reactive powder concrete following exposure to high temperature reaching 800 °C, Nucl. Eng. Des. 241 (2011) 2416–2424. [4] C.-T. Liu, J.-S. Huang, Fire performance of highly flowable reactive powder concrete, Constr. Build. Mater. 23 (2009) 2072–2079. [5] W. Zheng, H. Li, Y. Wang, Compressive behaviour of hybrid fiber-reinforced reactive powder concrete after high temperature, Mater. Des. 41 (2012) 403–409. [6] P. Richard, M. Cheyrezy, Composition of reactive powder concretes, Cem. Concr. Res. 25 (1995) 1501–1511. [7] R. Yu, P. Spiesz, H.J.H. Brouwers, Mix design and properties assessment of UltraHigh Performance Fibre Reinforced Concrete (UHPFRC), Cem. Concr. Res. 56 (2014) 29–39. [8] H. Zanni, M. Cheyrezy, V. Maret, S. Philippot, P. Nieto, Investigation of hydration and pozzolanic reaction in Reactive Powder Concrete (RPC) using 29Si NMR, Cem. Concr. Res. 26 (1996) 93–100. [9] M.M. Reda, N.G. Shrive, J.E. Gillott, Microstructural investigation of innovative UHPC, Cem. Concr. Res. 29 (1999) 323–329. [10] D. Heinz, L. Urbonas, T. Gerlicher, Effect of heat treatment method on the properties of UHPC, Proceedings of Hipermat 2012 3rd International Symposium on UHPC and Nanotechnology for High Performance Construction Materials, 2012, pp. 283–290. [11] V.Y. Garas, K.E. Kurtis, L.F. Kahn, Creep of UHPC in tension and compression: effect of thermal treatment, Cem. Concr. Compos. 34 (2012) 493–502. [12] P.N. Hiremath, S.C. Yaragal, Effect of different curing regimes and durations on early strength development of reactive powder concrete, Constr. Build. Mater. 154 (2017) 72–87. [13] T.M. Ahlborn, D.L. Misson, E.J. Peuse, C.G. Gilbertson, Durability and strength characterization of ultra-high performance concrete under variable curing regimes, in: E. Fehling, M. Schmidt, S. Stürwald (Eds.), Proc 2nd Int Symp on Ultra High Performance Concrete, 2008, pp. 197–204 (Kassel, Germany). [14] Z. Wu, C. Shi, W. He, Comparative study on flexural properties of ultra-high performance concrete with supplementary cementitious materials under different curing regimes, Constr. Build. Mater. 136 (2017) 307–313. [15] H. Yazıcı, M.Y. Yardımcı, S. Aydın, A.Ş. Karabulut, Mechanical properties of reactive powder concrete containing mineral admixtures under different curing regimes, Constr. Build. Mater. 23 (2009) 1223–1231. [16] H. Yazıcı, E. Deniz, B. Baradan, The effect of autoclave pressure, temperature and duration time on mechanical properties of reactive powder concrete, Constr. Build. Mater. 42 (2013) 53–63. [17] T. Zdeb, An analysis of the steam curing and autoclaving process parameters for reactive powder concretes, Constr. Build. Mater. 131 (2017) 758–766. [18] M. Ipek, K. Yilmaz, M. Sümer, M. Saribiyik, Effect of pre-setting pressure applied to mechanical behaviours of reactive powder concrete during setting phase, Constr. Build. Mater. 25 (2011) 61–68. [19] S. Philippot, S. Masse, H. Zanni, P. Nieto, V. Maret, M. Cheyrezy, 29Si NMR study of hydration and pozzolanic reactions in reactive powder concrete (RPC), Magn. Reson. Imaging 14 (1996) 891–893. [20] K.M. Ng, C.M. Tam, V.W. Tam, Studying the production process and mechanical properties of reactive powder concrete: a Hong Kong study, Mag. Concr. Res. 62 (2010) 647–654. [21] A. Loukili, P. Richard, J. Lamirault, A study on delayed deformations of an ultra high strength cementitious material, Special Publication, 179 1998, pp. 929–950. [22] D. Mostofinejad, M.R. Nikoo, S.A. Hosseini, Determination of optimized mix design and curing conditions of reactive powder concrete (RPC), Constr. Build. Mater. 123 (2016) 754–767. [23] Z. Yunsheng, S. Wei, L. Sifeng, J. Chujie, L. Jianzhong, Preparation of C200 green reactive powder concrete and its static–dynamic behaviors, Cem. Concr. Compos. 30 (2008) 831–838. [24] C.M. Tam, V. Wing, T. Yan, Microstructural behaviour of reactive powder concrete under different heating regimes, Mag. Concr. Res. 64 (2013) 259–267. [25] G. Long, X. Wang, Y. Xie, Very-high-performance concrete with ultrafine powders, Cem. Concr. Res. 32 (2002) 601–605. [26] M. Courtial, M.N. de Noirfontaine, F. Dunstetter, M. Signes-Frehel, P. Mounanga, K. Cherkaoui, A. Khelidj, Effect of polycarboxylate and crushed quartz in UHPC: microstructural investigation, Constr. Build. Mater. 44 (2013) 699–705. [27] S.L. Yang, S.G. Millard, M.N. Soutsos, S.J. Barnett, T.T. Le, Influence of aggregate and curing regime on the mechanical properties of ultra-high performance fibre reinforced concrete (UHPFRC), Constr. Build. Mater. 23 (2009) 2291–2298. [28] M. Helmi, M.R. Hall, L.A. Stevens, S.P. Rigby, Effects of high-pressure/temperature curing on reactive powder concrete microstructure formation, Constr. Build. Mater. 105 (2016) 554–562. [29] B. Han, S. Dong, J. Ou, C. Zhang, Y. Wang, X. Yu, S. Ding, Microstructure related mechanical behaviors of short-cut super-fine stainless wire reinforced reactive powder concrete, Mater. Des. 96 (2016) 16–26. [30] D.-Y. Yoo, N. Banthia, Mechanical properties of ultra-high-performance fiber-reinforced concrete: a review, Cem. Concr. Compos. 73 (2016) 267–280. [31] Z. Wu, C. Shi, W. He, L. Wu, Effects of steel fiber content and shape on mechanical properties of ultra high performance concrete, Constr. Build. Mater. 39 (103)

5. Conclusions A comprehensive investigation of the effect of curing regimes on the macroscopic properties, nano-mechanical properties and microstructure of UHPC was presented. Based on this experimental investigation, the following conclusions can be drawn. (1) The flexural, compressive and tensile strengths increase with increasing curing temperature. The mechanical properties are strengthened by temperature. However, the improvement of compressive strength is less than that of flexural and tensile strength. Therefore, the flexural/tensile to compressive strength ratio shows an unusual increasing tendency with increasing curing temperature and compressive strength, while this is opposite to that of normal concrete. (2) The nano-mechanical properties are also enhanced by heat treatment. Autoclave-cured specimens furnish the highest results, followed by steam-cured specimens and then samples cured at 20 °C. The hydrates in UHPC consist of LD C-S-H, HD C-S-H and UHD phases. The content of the UHD phase increases from 11.5% to 100% when curing temperature increases from 20 °C to 250 °C. The LD C-S-H phase disappears in UHPC cured above 90 °C. The UHD phase with high-packing density is dominated by the hydrates in UHPC, leading to increasing nano-mechanical properties of UHPC. (3) The hydration products differ from normal concrete in that large crystals of Ca(OH)2 are absent. A large amount of additional hydrate is formed from pozzolanic reaction after heat treatment. The tobermorite and xonotlite become dominant components of autoclave-cured UHPC. Meanwhile, the structure of C-S-H phases depends on the curing regimes, and the content of Q2 and Q3 increases more significantly with increasing curing temperature compared to Q1. As a result, the connectivity ratio of hydrate species is significantly increased, leading to an increased average chain length. (4) The microstructure of UHPC depends on the curing regime. Needlelike tobermorite and jennite-like structure are observed in autoclave-cured specimens. Separated hydration shells around the clinker can be found in UHPC cured under high temperature (above 90 °C). The improvement of the interfacial transition zone between matrix and aggregate and steel fiber due to increasing curing temperature indicates a well-developed bond. Meanwhile, the matrix around clinker and quartz is enhanced because of pozzolanic reaction, resulting in a decreasing Ca/Si ratio of hydrates. (5) The formation of additional hydrates with high packing density and stiffness such as tobermorite and xonotlite, enhancement of the transition zone around steel fiber, quartz and clinker, average chain length of hydrates and pozzolanic reaction between quartz/silica fume and Ca(OH)2 due to heat treatment and the presence of quartz plays all key roles in controlling the unusual behavior of the flexural/tensile to compressive strength ratio and improvement of mechanical properties of UHPC. Acknowledgements This work was financially supported by the National “13th FiveYear” Plan for Science & Technology Support of China (No. 2017YFB0310001) and the National Natural Science Foundation of China (No. 51772226). 12

Cement and Concrete Research 118 (2019) 1–13

P. Shen et al.

[57] A.G.P. Colombet, H. Zanni, P. Sozzani, Nuclear Magnetic Resonance Spectroscopy of Cement-Based Materials, (2012). [58] H. Yazıcı, The effect of curing conditions on compressive strength of ultra high strength concrete with high volume mineral admixtures, Build. Environ. 42 (2007) 2083–2089. [59] H. Zanni, M. Cheyrezy, V. Maret, S. Philippot, P. Nieto, Investigation of hydration and pozzolanic reaction in reactive powder concrete (RPC) using 29 Si NMR, Cem. Concr. Res. 26 (1996) 93–100. [60] L. Wang, H.Q. Yang, S.H. Zhou, E. Chen, S.W. Tang, Hydration, mechanical property and C-S-H structure of early-strength low-heat cement-based materials, Mater. Lett. 217 (2018). [61] F.M. Lea, The Chemistry of Cement and Concrete, (1970). [62] D.L. Kong, J.G. Sanjayan, Effect of elevated temperatures on geopolymer paste, mortar and concrete, Cem. Concr. Res. 40 (2010) 334–339. [63] Y. Fang, J. Chang, Microstructure changes of waste hydrated cement paste induced by accelerated carbonation, Constr. Build. Mater. 76 (2015) 360–365. [64] J. Chang, Y. Li, M. Cao, Y. Fang, Influence of magnesium hydroxide content and fineness on the carbonation of calcium hydroxide, Constr. Build. Mater. 55 (2014) 82–88. [65] A. Cwirzen, The effect of the heat-treatment regime on the properties of reactive powder concrete, Adv. Cem. Res. 19 (2007) 25–33. [66] B.P. Trinh, N. Van Chanh, Study on the properties of ultra high performance fiber reinforced concrete, Advanced Materials Research, Trans Tech Publ, 2012, pp. 3305–3312. [67] K.L. Scrivener, Backscattered electron imaging of cementitious microstructures: understanding and quantification, Cem. Concr. Compos. 26 (2004) 935–945. [68] H.F.W. Taylor, Cement Chemistry, 2nd edition, T. Telford, London, 1997. [69] M. Paul, F.P. Glasser, Impact of prolonged warm (85 °C) moist cure on Portland cement paste, Cem. Concr. Res. 30 (2000) 1869–1877. [70] W. Wongkeo, P. Thongsanitgarn, K. Pimraksa, A. Chaipanich, Compressive strength, flexural strength and thermal conductivity of autoclaved concrete block made using bottom ash as cement replacement materials, Mater. Des. 35 (2012) 434–439. [71] S. Masse, H. Zanni, J. Lecourtier, J.C. Roussel, A. Rivereau, 29Si solid state NMR study of tricalcium silicate and cement hydration at high temperature, Cem. Concr. Res. 23 (1993) 1169–1177. [72] N.Y. Mostafa, A.A. Shaltout, H. Omar, S.A. Abo-El-Enein, Hydrothermal synthesis and characterization of aluminium and sulfate substituted 1.1 nm tobermorites, J. Alloys Compd. 467 (2009) 332–337. [73] H. Maeda, K. Abe, E.H. Ishida, Hydrothermal synthesis of aluminum substituted tobermorite by using various crystal phases of alumina, J. Ceram. Soc. Jpn. 119 (2011) 375–377. [74] N. Meller, K. Kyritsis, C. Hall, The mineralogy of the CaO–Al2O3–SiO2–H2O (CASH) hydroceramic system from 200 to 350 °C, Cem. Concr. Res. 39 (2009) 45–53. [75] Z. Wu, C. Shi, W. He, L. Wu, Effects of steel fiber content and shape on mechanical properties of ultra high performance concrete, Constr. Build. Mater. 103 (2016) 8–14. [76] K. Huang, M. Deng, L. Mo, Y. Wang, Early age stability of concrete pavement by using hybrid fiber together with MgO expansion agent in high altitude locality, Constr. Build. Mater. 48 (2013) 685–690. [77] B. Bissonnette, M. Pigeon, Tensile creep at early ages of ordinary, silica fume and fiber reinforced concretes, Cem. Concr. Res. 25 (1995) 1075–1085. [78] R.J.-M. Pellenq, A. Kushima, R. Shahsavari, K.J.V. Vliet, M.J. Buehler, S. Yip, F.J. Ulm, Z.P. Bažant, A. Realistic Molecular, Model of cement hydrates, Proc. Natl. Acad. Sci. U. S. A. 106 (2009) 16102–16107. [79] H. Manzano, S. Moeini, F. Marinelli, A.C.T.V. Duin, F.J. Ulm, J.M. Pellenq, Confined water dissociation in microporous defective silicates: mechanism, dipole distribution, and impact on substrate properties, J. Am. Chem. Soc. 134 (2012) 2208–2215. [80] K.J. Krakowiak, J.J. Thomas, S. Musso, S. James, A.T. Akono, F.J. Ulm, Nanochemo-mechanical signature of conventional oil-well cement systems: effects of elevated temperature and curing time, Cem. Concr. Res. 67 (2015) 103–121.

(2016) 8–14. [32] L. Sorelli, G. Constantinides, F.-J. Ulm, F. Toutlemonde, The nano-mechanical signature of Ultra High Performance Concrete by statistical nanoindentation techniques, Cem. Concr. Res. 38 (2008) 1447–1456. [33] S. Aydin, H. Yazıcı, M. Yardimci, H. Yigiter, Effect of aggregate type on mechanical properties of reactive powder concrete, ACI Mater. J. 107 (2010) 441–449. [34] U. Müller, B. Meng, H.-C. Kühne, J. Nemecek, P. Fontana, E. Fehling, Micro texture and mechanical properties of heat treated and autoclaved Ultra High Performance Concrete (UHPC), 2nd International Symposium on Ultra High Performance Concrete, 2008, pp. 213–220. [35] K. Habel, M. Viviani, E. Denarié, E. Brühwiler, Development of the mechanical properties of an Ultra-High Performance Fiber Reinforced Concrete (UHPFRC), Cem. Concr. Res. 36 (2006) 1362–1370. [36] J. Liu, F. Han, G. Cui, Q. Zhang, J. Lv, L. Zhang, Z. Yang, Combined effect of coarse aggregate and fiber on tensile behavior of ultra-high performance concrete, Constr. Build. Mater. 121 (2016) 310–318. [37] M. Cheyrezy, V. Maret, L. Frouin, Microstructural analysis of RPC (Reactive Powder Concrete), Cem. Concr. Res. 25 (1995) 1491–1500. [38] H.Z. Cui, T.Y. Lo, S.A. Memon, W. Xu, Effect of lightweight aggregates on the mechanical properties and brittleness of lightweight aggregate concrete, Constr. Build. Mater. 35 (2012) 149–158. [39] A.M. Neville, Properties of Concrete, 4th edition, Wiley, New York, 1996. [40] Z. Rong, W. Sun, Y. Zhang, Dynamic compression behavior of ultra-high performance cement based composites, Int. J. Impact Eng. 37 (2010) 515–520. [41] P.K. Mehta, P.J.M. Monteiro, I. Ebrary, Concrete: Microstructure, Properties, and Materials, McGraw-Hill, New York, 2006. [42] M.D. Cohen, A. Goldman, W.-F. Chen, The role of silica fume in mortar: transition zone versus bulk paste modification, Cem. Concr. Res. 24 (1994) 95–98. [43] S. Bhanja, B. Sengupta, Influence of silica fume on the tensile strength of concrete, Cem. Concr. Res. 35 (2005) 743–747. [44] C. Hu, Nanoindentation as a tool to measure and map mechanical properties of hardened cement pastes, MRS Commun. 5 (2015) 83–87. [45] C. Hu, Z. Li, Property investigation of individual phases in cementitious composites containing silica fume and fly ash, Cem. Concr. Compos. 57 (2015) 17–26. [46] C. Hu, Z. Li, A review on the mechanical properties of cement-based materials measured by nanoindentation, Constr. Build. Mater. 90 (2015) 80–90. [47] L.A. Galin, H. Moss, I.N. Sneddon, Contact problems in the theory of elasticity, North Carolina State Univ Raleigh School of Physical Sciences and Applied Mathematics, 1961. [48] I.N. Sneddon, The relation between load and penetration in the axisymmetric Boussinesq problem for a punch of arbitrary profile, Int. J. Eng. Sci. 3 (1965) 47–57. [49] W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (1992) 1564–1583. [50] S. Zhao, W. Sun, Nano-mechanical behavior of a green ultra-high performance concrete, Constr. Build. Mater. 63 (2014) 150–160. [51] M. Vandamme, F.-J. Ulm, P. Fonollosa, Nanogranular packing of C–S–H at substochiometric conditions, Cem. Concr. Res. 40 (2010) 14–26. [52] P. Mondal, S.P. Shah, L. Marks, A reliable technique to determine the local mechanical properties at the nanoscale for cementitious materials, Cem. Concr. Res. 37 (2007) 1440–1444. [53] E.M. Foley, J.J. Kim, M.R. Taha, Synthesis and nano-mechanical characterization of calcium-silicate-hydrate (CSH) made with 1.5 CaO/SiO 2 mixture, Cem. Concr. Res. 42 (2012) 1225–1232. [54] G. Constantinides, F.J. Ulm, The nanogranular nature of C–S–H, J. Mech. Phys. Solids 55 (2007) 64–90. [55] T. Mitsuda, H. Taylor, Influence of aluminium on the conversion of calcium silicate hydrate gels into 11 Å tobermorite at 90 °C and 120 °C, Cem. Concr. Res. 5 (1975) 203–209. [56] H. Yazıcı, M.Y. Yardımcı, H. Yiğiter, S. Aydın, S. Türkel, Mechanical properties of reactive powder concrete containing high volumes of ground granulated blast furnace slag, Cem. Concr. Compos. 32 (2010) 639–648.

13