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Microstructure of ultra high performance concrete containing lithium slag
Journal of Hazardous Materials 353 (2018) 35–43
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Microstructure of ultra high performance concrete containing lithium slag a,⁎
a
Zhi-hai He , Shi-gui Du , Deng Chen a b
T
b
College of Civil Engineering, Shaoxing University, Shaoxing 312000, China College of Civil Engineering, Suzhou University of Science and Technology, Suzhou 215011, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Ultra high performance concrete Microstructure Lithium slag Pore structure Interfacial transition zone Nanoindentation
Lithium slag (LS) is discharged as a byproduct in the process of the lithium carbonate, and it is very urgent to explore an efficient way to recycle LS in order to protect the environments and save resources. Many available supplementary cementitious materials for partial replacement of cement and/or silica fume (SF) can be used to prepare ultra high performance concrete (UHPC). The effect of LS to replace SF partially by weight used as a supplementary cementitious material (0%, 5%, 10% and 15% of binder) on the compressive strengths and microstructure evolution of UHPC has experimentally been studied by multi-techniques including mercury intrusion porosimetry, scanning electron microscope and nanoindentation technique. The results show that the use of LS degrades the microstructure of UHPC at early ages, and however, the use of LS with the appropriate content improves microstructure of UHPC at later ages. The hydration products of UHPC are mainly dominated by ultrahigh density calcium-silicate-hydrate (UHD C-S-H) and interfacial transition zone (ITZ) in UHPC has similar compact microstructure with the matrix. The use of LS improves the hydration degree of UHPC and increases the elastic modulus of ITZ in UHPC. LS is a promising substitute for SF for preparation UHPC.
1. Introduction Ultra high performance concrete (UHPC) is a new type of concrete with the 28-day compressive strength in excess of 150 MPa (in China, compressive strength over 100 MPa) and has already been used in construction projects all over the world, especially containing the landmark projects [1], due to its ultra high mechanical properties and excellent durability [2]. UHPC is characterized by high binder content (over 800 kg/m3), very low water-binder ratio (w/b) and use of silica fume (SF), which results in a large amount of unhydrated cement causing waste of resources. It is well known that SF is an essential constituent for UHPC, which plays a significant role in improving the properties of UHPC. The prominent effects of SF in UHPC are filling effect, lubricating effect and pozzolanic effect [2–6]. However, the high cost, limited available quantity and uneven distribution of SF constrain greatly the application of UHPC in modern concrete construction, especially in the developing and backward countries and regions. Therefore, in order to enhance the application technology and expend the application range of UHPC, there is an urgent need for searching for other materials to substitute SF and/or cement. Recently it has been well acknowledged that the use of widely available supplementary cementitious materials (SCMs), such as fly ash and slag for partial/complete replacement of cement and SF, can also been used to prepare UHPC, which reduces obviously the cost without
⁎
sacrifice of strength, and has become the trends for the production of UHPC [2]. Li [7] reported that the ternary use of metakaolin, fly ash and cement in UHPC was of potential economic and environmental advantages. Calvo et al. [8] found that the substitution of SF by the corresponding amount of epoxy resin filled silica microcapsules and amine functionalized nanosilica increases the durability of the UHPC developed, guaranteeing a longer service life. Su et al. [9] reported that UHPC containing steel fibers and different kinds of nano materials including Nano-CaCO3, Nano-SiO2, Nano-TiO2 and Nano-Al2O3 demonstrated the superior ductility and blast resistant capacity. Alsalman et al. [10] presented an experimental investigation of UHPC using locally available materials. Their results showed that the fly ash content had an important effect on the compressive strengths of UHPC, and a fly ash content of more than 20% decreased the compressive strengths at early ages, but increased the strengths at later ages. A fly ash content of 30% produced the highest 90-day compressive strength, while a content of 20% had minimal effect on the strengths at all ages. Soliman and Tagnit-Hamou [11] prepared a green ultra-high-performance glass concrete (UHPGC) with a compressive strength of up to 220 MPa. The UHPGC properties were improved when the cement and quartz powder were replaced with nonabsorptive glass-powder (GP) particles, providing technological, economical, and environmental advantages compared to traditional UHPC. Tafraoui et al. [12] reported that it was possible to replace SF by metakaolin which did not affect the durability
Corresponding author. E-mail address: [email protected] (Z.-h. He).
https://doi.org/10.1016/j.jhazmat.2018.03.063 Received 28 November 2017; Received in revised form 30 March 2018; Accepted 31 March 2018 Available online 03 April 2018 0304-3894/ © 2018 Elsevier B.V. All rights reserved.
Journal of Hazardous Materials 353 (2018) 35–43
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of UHPC, which was used without any problem for the industry of powerful and durable UHPC [13]. Van et al. [14,15] found that rice husk ash (RHA) classified as “a highly active pozzolan” was suitable for use as a SCM to make UHPC, and the combination of 10% RHA and 10% SF proved to be optimum for achieving maximum synergic effect of UHPC. Huang et al. [16] investigated the effect of RHA on the strength and permeability of UHPC. Results from this investigation also suggested that the calcined RHA at 500 °C was a promising substitute for SF in UHPC production, and the RHA replacing SF ratio of 2/3 showed the best improving effect, increasing the compressive strength at 3, 28 and 120 days by 9.76%, 14.50%, 10.02%, respectively, compared to the traditional UHPC containing SF alone. Burroughs et al. [17] reported that the minor replacements of cement with limestone powder did not negatively affect properties of UHPC, the use of which as an inert filler in UHPC was proposed. Yazıcı et al. [18–20] prepared UHPC with Portland cement replaced with high content of granulated blast furnace slag and fly ash whose compressive strength was over 200 MPa after standard room curing, 234 MPa after steam curing and 250 MPa after autoclave curing. Peng et al. [21] found that the utilization of ultra-fine fly ash and steel slag powder in UHPC was feasible and the optimum steel slag powder/ultra-fine fly ash ratio was 1.5 for UHRC with the highest strength. Wang et al. [22] found that UHPC containing ground granulated blast furnace slag and limestone powder replacing cement achieved excellent workability with a maximum slump of 268 mm and compressive strengths of 175.8 MPa at 90 days and 182.9 MPa at 365 days. Therefore, the preparation of UHPC has already not been completely dependent on the costly and well-chosen materials, and a large number of widely available and low-cost SCMs replacing cement and/or SF all over the world can be used to prepare UHPC. Lithium slag (LS) is a byproduct, which is discharged in the process of the lithium carbonate using sulfuric acid method when the spodumene ore is calcined at high temperature of 1200 °C. According to the statistical analysis, about nine tons of LS are discharged when one ton lithium salt is produced in the production process of lithium carbonate. Today, about 8 × 105 tons of LS are discharged every year in China. LS as a promising SCM can also be used to prepare successfully different kinds of concretes and improve obviously the properties of concrete, especially the later properties, showing the similar physically filling and chemically pozzolanic effects with other SCMs such as ground granulated blast furnace slag and fly ash [23–25]. The aforementioned literature survey shows that, despite the considerable amount of work published in literature on UHPC prepared by all kinds of SCMs, and however, there is very little work on UHPC containing LS. LS is economical and abundant as a SCM, which is a promising substitute replacing SF in UHPC production. It is well known that microstructure-property relationships are at the heart of modern materials science, and the evolution of microstructure can determine the properties of UHPC [4,15,26], which may demonstrate significant difference as compared to conventional concrete. In this paper, the compressive strength variations of UHPC containing LS were measured with time using an experimental method, taking into account of the effect of different ratio of replacing SF with LS. Meanwhile, the effects of LS on the microstructure evolution of specimens were investigated by mercury intrusion porosimetry (MIP), scanning electron microscope (SEM) and nanoindentation techniques.
Table 1 Chemical compositions of cement, LS and SF (wt%). Materials
SO3
SiO2
Fe2O3
Al2O3
CaO
MgO
K2 O
Na2O
Loss on ignition
Cement
2.54
21.10
3.26
4.77
62.63
1.15
0.43
0.05
3.01
LS SF
7.15 0.83
53.22 88.12
1.48 0.49
17.11 0.29
10.11 0.63
0.41 3.08
0.53 3.69
0.33 1.22
8.25 1.11
Fig. 1. XRD pattern of LS.
contents of LS fall between those of SF and those of cement, respectively, and the SO3 and Al2O3 contents of LS are highest. X-Ray diffraction (XRD) pattern of LS is given in Fig. 1. The mineral compositions of LS contain mainly quartz, gypsum, calcite and lithium aluminum silicate. Fig. 2(a) shows the particle size distributions of the cement and LS determined by laser particle analysis using BT-9300 Laser Particle Analyzer, which exhibits that the size range of LS particles is wider than
2. Experimental 2.1. Materials The cement used in the present study was Portland cement with strength grade of P·I52.5 in accordance with Chinese standard GB 1752007. LS used was supplied by Sichuan lithium salt plant in China. Table 1 shows the chemical compositions of cement, SF and LS, used as the cementitious materials, which exhibits that the SiO2 and CaO
Fig. 2. (a) particle size distributions of cement and LS and (b) grading curves. 36
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Fig. 3. SEM images of LS and SF.
that of cement particles. Meanwhile, in terms of the average size, LS particle is smaller than cement particle, which is demonstrated in the grading curves of cement and LS shown in Fig. 2(b). Fig. 3 gives the SEM images of LS and SF at the magnifications of 5000×, which exhibits that the shape of LS particles is irregular, and that of SF particles is spherical. In addition, it can also be seen that the size of SF particles is much smaller than that of LS particles. The specific gravities of cement, LS and SF are about 3010, 2450 and 2180 kg/m3, respectively, which were determined according to the standard test method for cement density using the small pycnometer method in accordance with Chinese GB/T208-2014. The specific surface area of cement, LS and SF are about 370, 440 and 21,500 m2/kg, respectively, which were measured based on the nitrogen adsorption method. Quartz sand named standard sand in accordance with the international organization for standardization (ISO679) was utilized as aggregates, which was produced in Xiamen city, China. The SiO2 content of quartz sand was more than 98%. Quartz sand had particles with size less than 2 mm, whose particle distributions are shown in Table 2. The polycarboxylic superplasticizer (SP) with water-reducing range of more than 30% and solid content of about 35% by weight was carried out to adjust the workability of UHPC.
Table 3 Mix proportions of UHPC containing LS (in weight).
0 7±5 33 ± 5 67 ± 5 87 ± 5 99 ± 1
LS
Quartz sand
Water
SP
C-SF C-SF-5LS C-SF-10LS C-SF-15LS
896 896 896 896
224 168 112 56
0 56 112 168
1013 1013 1013 1013
202 202 202 202
20.2 19.6 18.8 18.0
For testing the compressive strengths of UHPC, a set of mixtures were cast into 40 mm × 40 mm × 40 mm moulds and all mixtures were vibrated for one minute. After that, the specimens were kept in moulds for one day at room temperature (20 ± 5) °C. Subsequently, they were demoulded and kept in a standard curing room of controlled temperature (20 ± 2) °C and relative humidity more than 95% until the days of testing (i.e. 3, 28 and 90 days). The UHPC specimens were selected from typical specimens to carry out the microscopic tests after strength measurement. For testing the pore size distribution and pore microstructures in UHPC specimens containing LS at the ages of 3 and 28 days, MIP and SEM techniques were performed, whose test procedures were gained through our published reference [27]. For testing the distribution of elastic modulus of matrix in UHPC specimens and characterizing the evolution of interfacial transition zone (ITZ) between the matrix and aggregate in UHPC specimens at 28 days from the nano-scale structure perspective, nanoindentation technique was carried out, whose detailed test procedures were also obtained according to our published references [28,29]. The main test procedures were as follows: the specimen was intercepted and then embedded in the epoxy resin in vacuum. After that, the specimen was ground and polished to ensure the surface roughness. Finally, the specimen was cleaned in an ultrasonic bath in order to carry out the nanoindentation test.
Table 2 Particle distributions of quartz sand.
2.0 1.6 1.0 0.5 0.16 0.08
SF
2.3. Experimental methods
It is generally acknowledged that the optimal SF content of UHPC was between 20% and 35% of cement, highly dependent on w/b [2,15]. Therefore, for all the mixtures, the total binder and cement were kept the same, 1120 kg/m3 and 896 kg/m3, respectively. The control UHPC with SF content of 20% was prepared and LS was added to replace SF by weight (5%, 10% and 15% of binder). The w/b and binder to aggregate ratio of four mixtures were kept as constants, 0.18 and 1.11, respectively, and the SP contents were adjusted to maintain the similar fluidities of UHPC mixture from 180 mm to 200 mm. The details of the components used in UHPC mixtures are shown in Table 3. All UHPC mixtures were blended in a mixer according to the
Accumulated sieving residue, %
Cement
employed mixing procedure given in Fig. 4 in order to improve the effect of interaction with the binder and SP and achieve a better uniformity.
2.2. Preparation of UHPC
Side length of square hole, mm
Types of mixture
3. Results and discussions 3.1. Compressive strength Compressive strengths of UHPC containing different LS contents to replace SF at 3, 28 and 90 days are presented in Fig. 5. Each result presented here is the average value of three tested specimens with identical mixture, and the error bar represents the variation of the 37
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Z.-h. He et al.
Fig. 4. Employed mixing procedure for producing UHPC containing LS. (Low speed: 61.5 r/min; high speed: 123 r/min).
strength of specimen containing 20% LS is lowest for all the time. The rates of strength increases become slow at later ages, i.e. beyond 28 days. In addition, the compressive strengths of all specimens at 28 days are more than 110 MPa, but less than 150 MPa. It is generally recognized that UHPC refers to cementitious materials exhibiting compressive strengths higher than 150 MPa at 28 days [2], and however, in China, UHPC is defined as cementitious materials with 28-day compressive strength over 100 MPa. Therefore, the compressive strengths of all specimens can meet the Chinese standard requirements of UHPC. The compressive strength of the control specimen containing SF only does also not reach 150 MPa, which is not consistent with the results reported elsewhere [3,13,15]. This is likely to be attributed to the different qualities of cementitious materials containing cement and SF, manufacture procedure and curing process as well as specimen sizes. The close packing of cementitious material particles plays an important role in determining the strength of UHPC. The fineness of LS falls between that of cement and that of SF, so SF, cement and LS with the appropriate content can consistently form the densest packing state to increase the compressive strength. Apart from the filling effect of LS,
Fig. 5. Effect of LS on compressive strengths of UHPC.
obtained data. It can be found that LS reduces the compressive strengths of specimens at early ages (3 days), and however, from the start of 28 days, the compressive strength of specimen containing 10% LS surpasses that of control specimen containing SF only, and the compressive
Fig. 6. SEM images of UHPC containing LS at 3 days. 38
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Fig. 7. SEM images of UHPC containing LS at 28 days.
large capillary pores and air voids. The microstructure of specimens containing LS is loose. Among them, the specimen containing 10% LS shows a relative dense microstructure. This may be attributed to the close packing of SF, cement and LS with appropriate content. Observing the later images shown in Fig. 7, it can be seen that, compared to the specimens at early ages, the microstructures of specimens containing LS are much denser. No significant CH could be found in specimens because of the pozzolanic effect of SF and LS consuming CH. In addition, the interconnected pores and air voids in specimens existed at early ages are basically filled with hydration products produced by cement hydration and secondary pozzolanic reaction of SF and LS. It is noticeably that ITZ between aggregates and paste matrix in the control specimen containing SF only seems as dense as the matrix, and both have already not been distinguished. The ITZ in the control specimen is not weakest parts, different from that of conventional concrete. The homogenous microstructure is important for the excellence performance of UHPC. In contrast, no obvious ITZ is found in specimens containing LS. Both filling effect and pozzolanic reaction of LS are less significant than those of SF, so scattered disconnected capillary pores with small sizes can still be observed in the specimen containing 5% LS, especially in the specimen containing 15% LS. However, the microstructure of the specimen containing 10% LS is denser than that of the control specimen. The unhydrated particles and aggregates in the specimen containing 10% LS are surrounded by a large number of hydration products mainly C-S-H gels, which are interconnected into homogenous microstructure. Generally speaking, the close packing density of cementitious material particles has a direct effect on the hydration reaction process and the microstructure formation especially low w/b [33–35]. The higher the close packing density is, the denser the microstructure is. The size of SF particles is much smaller than that of LS particles demonstrated in Fig.3, but the
the pozzolanic reaction of LS makes outstanding contribution to the development of compressive strength at later ages. It is generally believed that the compressive strengths of UHPC mainly depend on the close packing density and hydration of cement as well as the pozzolanic reactions of mineral admixtures [4]. The ternary cementitious materials with the appropriate proportions of different fineness of LS and SF are more likely to get the maximum close packing density, compared to the binary cementitious materials with SF only. In addition, the pozzolanic reaction of SF is fast at early ages [2], and however, that of LS is slow at early ages and relatively fast at later ages. Therefore, the appropriate combination of SF and LS in ternary cementitious materials can always guarantee the sustainability of the pozzolanic reactions to increase the compressive strengths. When the content of LS is too few (5% of binder), or too many (15% of binder), the contribution of LS to the increase of strengths will be weakened. The optimum content of LS in the ternary cementitious materials is 10% of binder. 3.2. SEM analysis In general, SEM is recognized as a good tool to evaluate the microstructure of materials including UHPC [30–32]. Figs. 6 and 7 give the SEM images of specimens containing LS at the magnifications of 10,000× at 3 days and 28 days, respectively. As shown in Fig. 6, it is clear that the internal microstructure of UHPC at early ages is mainly comprised of unhydrated cementitious material particles, hydration products, aggregate, pores and air voids. The main hydration products in the control specimen containing SF only are homogeneous calcium-silicate-hydrate (C-S-H) gels, and some small calcium hydroxide (CH) crystals can also be found. Owing to the lower activity of LS compared to SF showing the filling effect only at early ages, LS degrades the microstructures of specimens with some 39
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Table 4 Pore size distributions in UHPC containing LS at 3 days (mL/g). Number
< 4.5 nm
(4.5–50) nm
(50–100) nm
> 100 nm
Total porosity
C-SF C-SF-5LS C-SF-10LS C-SF-15LS
0.00013 0.0011 0.00019 0.0006
0.00767 0.0095 0.00751 0.0055
0.003 0.0021 0.0029 0.0026
0.0259 0.0396 0.0306 0.0510
0.0367 0.0523 0.0412 0.0597
Table 5 Pore size distributions in UHPC containing LS at 28 days (mL/g).
Fig. 8. Effect of LS on cumulative porosity of UHPC at 3 days.
close packing density of ternary cementitious material system containing cement, SF and LS with the appropriate content may exceed that of binary system containing cement and SF. This is likely to explain why the use of 10% LS can improve the microstructure of specimens at later ages.
Number
< 4.5 nm
(4.5–50) nm
(50–100) nm
> 100 nm
Total porosity
C-SF C-SF-5LS C-SF-10LS C-SF-15LS
0.0011 0.0002 0.00063 0.0002
0.0133 0.012 0.01317 0.0096
0.0018 0.0031 0.0017 0.0032
0.0136 0.0158 0.01 0.0241
0.0298 0.0311 0.0255 0.0371
to C-S-H to fill large pores. However, 5% and 15% LS increase the total porosities and the contents of > 100 nm pore of the specimens, and those of the specimen containing 15% LS are highest. This is likely to be attributed mainly to the fact that the appropriate content LS may increase the close packing density of ternary system, exceeding that of binary system containing cement and SF, to accelerate the hydration process. There is an optimal content of LS for the maximum close packing density of ternary system, and however, too few or too many LS contents influence the increase of close packing density to hinder the hydration process.
3.3. Pore structure The microstructure of concrete represented by its pore structure, i.e., the porosity and pore size distribution, plays a decisive role in evolving the properties of concrete [36]. MIP is the most widely adopted useful method for the study of pore structure characteristics of cementitious materials [36–38]. The cumulative porosities are measured using MIP for specimens containing LS cured for 3 days and 28 days, and the results are shown in Figs. 8 and 9, respectively. In addition, the corresponding pore size distributions are shown in Tables 4 and 5, respectively. It is clear that low w/b of UHPC results in low porosity. The pores of cementitious materials can be classified into four levels based on the pore diameters: < 4.5 nm, 4.5–50 nm, 50–100 nm and > 100 nm [39]. According to the effect of pores with different diameters on the properties of cementitious materials, the pore > 100 nm is definitely harmful. As it is shown in Fig. 8 and Table 4 that the total porosities of specimens containing LS are higher than that of the control specimen containing SF only at early ages, and especially the contents of > 100 nm pores are also increased with the addition of LS. This indicates that LS degrades the pore structures of UHPC specimens at early ages, which may be due to the lower activity of LS compared to SF. By comparison with Figs. 8 and 9, the porosities of all specimens are further reduced with the increase of ages. As it is shown in Fig. 9 and Table 5 that 10% LS decreases the contents of different size pores including the total porosity at later ages, compared to those of the control specimen. Therefore, LS with the appropriate content can refine the pore structure of specimen due to its pozzolanic reaction and physical filling effect. LS can consume most of the CH crystals and converts them
3.4. Nanoindentation investigation Nanoindentation is an effective and powerful tool to detect the local elastic properties and hardness of each phase of materials which can character the evolution of material microstructure and nanostructure including cementitious materials [40,41]. The working principle of nanoindentation is relatively simple: push a very sharp tip into the surface of the material and investigate the mechanical behavior of the material from the response of the tip. Firstly, the matrix of specimens was found with the help of the optical microscope of nanoindentor, and a 10 × 10 grid containing 100 points was carried out on each specimen, and the distance between two adjacent test points was 20 μm. The area was chosen to be statistically representative of the matrix of each specimen. A nanoindentation test consisted of establishing contact between a specimen and a tip of known geometry and then continuously measuring the change in indentation depth h as a function of increasing indentation load P. After the nanoindentation test was carried out, each indentation P-h curve was examined to determine the validity of experimental data. Fig. 10 shows the irregular discarded typical P-h curves in UHPC
Fig. 9. Effect of LS on cumulative porosity of UHPC at 28 days.
Fig. 10. Irregular discarded typical P-h curves in UHPC matrix. 40
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Porosity LD C-S-H HD C-S-H CH or UHD C-S-H Unhydrated particle
Indentation load, mN
2.0 1.5 1.0 0.5 0.0
0
100 200 300 400 500 600 700 Indentation depth, nm
Fig. 11. Typical P-h curves of phases in UHPC matrix.
matrix which can not provide any information in determining the nanomechanical properties. (a) P-h curve may be due to the presence of a large void caused by the specimen preparation.(b) P-h curve should be attributed to the indenter piercing the weak specimen surface and then penetrating into the harder under layer. When the indenter lies in the surface cracking during the load-driven the nanoindentation test, (c) Ph curve is obtained. Generally speaking, UHPC matrix mainly consists of porosity, hydration products and unhydrated particles, and the typical P-h curves of these phases are presented in Fig. 11. It is clear that depending on the nano-mechanical properties of indented phases, the maximum indentation depth ranges from 89.32 nm to 699.71 nm. According to the information provided by each P-h curve, the elastic modulus of each indentation point can be obtained. Each phase has a corresponding elastic modulus ranges which are intrinsic mechanical properties. The main hydration products of cement are C-S-H and CH. C-S-H phases can be classified into inner C-S-H and outer C-S-H or low density C-S-H (LD C-S-H) and high density C-S-H (HD C-S-H). In addition, a recent study revealed the presence of a new phase at low w/b which has the similar structure with C-S-H and similar mechanical performance with CH, so it is defined as ultra-high density (UHD C-S-H) and considered as the third C-S-H [42,43]. It is well known that the elastic modulus of LD C-S-H, HD C-S-H and CH/ UHD C-S-H are generally stable regardless of different curing methods, w/b, and so on, in the range of 14–24 GPa, 24–35 GPa and 35–50 GPa, respectively [44–46]. Generally, the elastic modulus of porosity is less than 14 GPa, and that of unhydrated particles is more than 50 GPa. In addition, no CH crystals can be observed in UHPC owing to the low w/b and pozzolanic reaction with mineral admixtures [46,47], so the phase with the elastic modulus of 35–50 GPa in UHPC should mainly be UHD C-S-H rather than CH. Fig. 12 displays the frequency histogram of elastic modulus in matrix of the control specimen containing SF only and the specimen containing 10% LS at 28 days. The bin size of the histogram is 10 GPa. The proportions of indentations for different phases which can be determined from the histogram are also given in Table 6 and can be considered the approximate volume fractions of different phases in cementitious materials [48]. It is observed that the main phases in UHPC matrix are UHD C-S-H and unhydrated particles, and about 70% of the hydration products in UHPC are UHD C-S-H, which indicates that the hydration products of UHPC have much higher mechanical properties than those of conventional concrete whose hydration products are LD C-S-H and/or HD C-SH mainly [48]. In addition, there are few the porosity and LD C-S-H in UHPC implying very denser internal structure of UHPC, and the quantity of HD C-S-H is two or three times as many as that of LD C-S-H. Compared to the proportions of constituent phases of the control specimen obtained from nanoindentation tests, 10% LS improves the hydration degree of the specimen and increases the quantity of C-S-H especially HD C-S-H and UHD C-S-H obviously, and reduces the
Fig. 12. Effect of LS on frequency histogram of elastic modulus of UHPC at 28 days. Table 6 Volume fractions of constituent phases in UHPC matrix (%). Specimen
Porosity
LD C-S-H
HD C-S-H
UHD C-S-H
Unhydrated particle
C-SF C-SF-10LS
3.2 3.3
3.2 4.5
7.4 13.0
36.2 46.6
50.0 32.6
unhydrated particles. LS can consume CH produced by the cement hydration and generate additional C-S-H. It can be inferred that C-S-H generated by the secondary pozzolanic reaction of LS is higher mechanical HD C-S-H and UHD C-S-H mostly in UHPC, which can improve effectively the macroscopic properties of UHPC. Concrete is a composite materials and consists of matrix, aggregate and ITZ. ITZ is considered as a loose phase with higher porosity compared to the bulk matrix, which plays an important role in determining the properties of concrete. Nanoindentation also proves to be a very sensitive and appropriate experimental tool to characterize the properties of ITZ quantitatively [49–51]. Nanoindentation tests of ITZ in specimens began every 10 μm from the aggregate surface until 60 μm, which worked in a direction perpendicular to the aggregate surface. Cementitious materials were heterogeneous, and therefore another direction of nanoindentation tests was every 10 μm along the aggregate surface until 40 μm in order to include all phase results. The properties of ITZ including the effective thickness and mechanical properties vary with the mix proportions, preparation, curing and so on. There is not a clear boundary between ITZ and matrix in concrete. Generally speaking, the elastic modulus shows a stable trend, indicating the test areas are from ITZ to matrix, and therefore the thickness of ITZ between 41
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• The hydration products of UHPC are mainly dominated by UHD C-S-
•
H, and the use of LS improves the hydration degree of the UHPC and increases the quantities of HD C-S-H and UHD C-S-H obviously. ITZ in UHPC has similar compact microstructure and mechanical properties with the matrix, and the use of LS also increases the elastic modulus of ITZ in UHPC. It indicates that UHPC containing LS with the appropriate content has the higher mechanical properties. LS is suitable for use as a SCM to substitute SF for preparation UHPC.
Acknowledgment The authors would like to acknowledge the National Natural Science Foundation of China (Grant Nos. 51602198 and 41427802) for their financial support to the work present in this paper. Fig. 13. Elastic modulus evolution of ITZ in specimens.
References aggregate and matrix is the distance from the aggregate surface to the stable indentation point with the elastic modulus. In order to exclude the effect of unhydrated particles on the ITZ, the results of elastic modulus over 50 GPa obtained from nanoindentation tests are eliminated. Fig. 13 shows the elastic modulus evolution of ITZ in the control specimen and the specimen containing 10% LS at 28 days, and the error bar represents the variation of the obtained data. It can be seen that the elastic modulus across the whole test zone appears to be stable and uniform, and meanwhile there is no distinct trough near the aggregate surface. Zhao and Sun [47] also obtained the similar results when investigating the nano-mechanical behavior of a green UHPC formula which contained high volumes of fly ash and used natural river sand as fine aggregate. It implies ITZ in the UHPC specimens is improved thoroughly and has similar compact microstructure and mechanical properties with the matrix, and there is a tight bond between the aggregate and matrix. These results are corresponding well with the results shown in SEM images. In addition, compared to the control specimen, LS also increases the elastic modulus across the whole test zone of the specimen. This is likely to be attributed to the more additional HD C-S-H and UHD C-S-H with higher mechanical properties generated by the secondary pozzolanic reaction of LS. Xie et al. [52] found that the relationship between porosity and elastic modulus agreed well with empirical formulas stemming mostly from macroscopic tests by comparing the results of backscattered electron and nanoindentation at identical regions, which bridged the microstructure with the mechanical properties of concrete. The denser the microstructure was, the higher the elastic modulus of the same area was and macroscopic mechanical properties of cementitious materials were. Therefore, it indicates that the specimen containing 10% LS with the higher elastic modulus obtained by nanoindentation tests corresponds to the higher mechanical properties including the results shown in Fig. 5.
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4. Conclusions In this paper, the effect of LS to replace SF partially by weight on the compressive strengths and microstructure evolution of UHPC has experimentally been studied by multi-techniques. The following conclusions can be drawn:
• The use of LS to replace SF in UHPC reduces the compressive •
strengths at early ages, and however, the use of 10% LS can increase the compressive strength at later ages surpassing that of the control UHPC containing SF only. SEM images and pore structure analysis show that the use of LS degrades the microstructure of UHPC at early ages, but the use of 10% LS improves the microstructure of UHPC at later ages. This should be attributed to the filling effect and pozzolanic reaction of LS. 42
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[39] P.K. Mehta, P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials, third edition, McGraw-Hill, New York, 2006, pp. 26–32. [40] N. Venkovic, L. Sorelli, F. Martirena, Nanoindentation study of calcium silicate hydrates in concrete produced with effective microorganisms-based bioplasticizer, Cem. Concr. Compos. 49 (2014) 127–139. [41] C. Hu, Z. Li, A review on the mechanical properties of cement-based materials measured by nanoindentation, Constr. Build. Mater. 90 (2015) 80–90. [42] M. Vandamme, F.J. Ulm, P. Fonollosa, Nanogranular packing of C-S-H at substochiometric conditions, Cem. Concr. Res. 40 (2010) 14–26. [43] M. Vandamme, F.J. Ulm, Nanogranular origin of concrete creep, Proc. Natl. Acad. Sci. U. S. A. 106 (2009) 10552–10557. [44] 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. [45] E.M. Foley, J.J. Kim, M.M.R. Taha, Synthesis and nano-mechanical characterization of calcium-silicate-hydrate (CSH) made with 1.5 CaO/SiO2 mixture, Cem. Concr. Res. 42 (2012) 1225–1232. [46] Z.D. Rong, W. Sun, H.J. Xiao, W. Wang, Effect of silica fume and fly ash on hydration and microstructure evolution of cement based composites at low waterbinder ratios, Constr. Build. Mater. 51 (2014) 446–450. [47] S. Zhao, W. Sun, Nano-mechanical behavior of a green ultra-high performance concrete, Constr. Build. Mater. 63 (2014) 150–160. [48] G. Constantinides, F.J. Ulm, The nanogranular nature of C-S-H, J. Mech. Phys. Solids 55 (2007) 64–90. [49] W. Li, J. Xiao, Z. Sun, S. Kawashima, S.P. Shah, Interfacial transition zones in recycled aggregate concrete with different mixing approaches, Constr. Build. Mater. 35 (2012) 1045–1055. [50] J. Xiao, W. Li, Z. Sun, D.A. Lange, S.P. Shah, Properties of interfacial transition zones in recycled aggregate concrete tested by nanoindentation, Cem. Concr. Compos. 37 (2013) 276–292. [51] X.H. Wang, S. Jacobsen, J.Y. He, Z.L. Zhang, S.F. Lee, H.L. Lein, Application of nanoindentation testing to study of the interfacial transition zone in steel fiber reinforced mortar, Cem. Concr. Res. 39 (2009) 701–715. [52] Y. Xie, D.J. Corr, F. Jin, H. Zhou, S.P. Shah, Experimental study of the interfacial transition zone (ITZ) of model rock-filled concrete (RFC), Cem. Concr. Compos. 55 (2015) 223–231.
[25] Z. He, L. Li, S. Du, Mechanical properties, drying shrinkage, and creep of concrete containing lithium slag, Constr. Build. Mater. 147 (2017) 296–304. [26] C. Shi, D. Wang, L. Wu, Z. Wu, The hydration and microstructure of ultra highstrength concrete with cement-silica fume-slag binder, Cem. Concr. Compos. 61 (2015) 44–52. [27] Z. He, L. Li, S. Du, Creep analysis of concrete containing rice husk ash, Cem. Concr. Compos. 80 (2017) 190–199. [28] Z. He, C. Qian, S. Du, M. Huang, M. Xia, Nanoindentation characteristics of cement paste and interfacial transition zone in mortar with rice husk ash, J. Wuhan Univ. Technol. 32 (2017) 417–421. [29] Z. He, C. Qian, Y. Zhang, F. Zhao, Y. Hu, Nanoindentation characteristics of cement with different mineral admixtures, Sci. China Technol. Sci. 56 (2013) 1119–1123. [30] X. Wang, R. Yu, Z. Shui, Q. Song, Z. Zhang, Mix design and characteristics evaluation of an eco-friendly ultra-high performance concrete incorporating recycled coral based materials, J. Clean. Prod. 165 (2017) 70–80. [31] W. Meng, K.H. Khayat, Mechanical properties of ultra-high-performance concrete enhanced with graphite nanoplatelets and carbon nanofibers, Compos. Part B: Eng. 107 (2016) 113–122. [32] H. Paiva, A.S. Silva, A. Velosa, P. Cachim, V.M. Ferreira, Microstructure and hardened state properties on pozzolan-containing concrete, Constr. Build. Mater. 140 (2017) 374–384. [33] M.A. Elrahman, B. Hillemeier, Combined effect of fine fly ash and packing density on the properties of high performance concrete: an experimental approach, Constr. Build. Mater. 58 (2014) 225–233. [34] F. Lange, H. Mörtel, V. Rudert, Dense packing of cement pastes and resulting consequences on mortar properties, Cem. Concr. Res. 27 (1997) 1481–1488. [35] T. Zhang, Q. Yu, J. Wei, P. Zhang, A new gap-graded particle size distribution and resulting consequences on properties of blended cement, Cem. Concr. Compos. 33 (2011) 543–550. [36] B.B. Das, B. Kondraivendhan, Implication of pore size distribution parameters on compressive strength, permeability and hydraulic diffusivity of concrete, Constr. Build. Mater. 28 (2012) 382–386. [37] R. Kumar, B. Bhattacharjee, Study on some factors affecting the results in the use of MIP method in concrete research, Cem. Concr. Res. 33 (2003) 417–424. [38] C.S. Poon, S.C. Kou, L. Lam, Compressive strength, chloride diffusivity and pore structure of high performance metakaolin and silica fume concrete, Constr. Build. Mater. 20 (2006) 858–865.
43
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Microstructure of ultra high performance concrete containing lithium slag a,⁎
a
Zhi-hai He , Shi-gui Du , Deng Chen a b
T
b
College of Civil Engineering, Shaoxing University, Shaoxing 312000, China College of Civil Engineering, Suzhou University of Science and Technology, Suzhou 215011, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Ultra high performance concrete Microstructure Lithium slag Pore structure Interfacial transition zone Nanoindentation
Lithium slag (LS) is discharged as a byproduct in the process of the lithium carbonate, and it is very urgent to explore an efficient way to recycle LS in order to protect the environments and save resources. Many available supplementary cementitious materials for partial replacement of cement and/or silica fume (SF) can be used to prepare ultra high performance concrete (UHPC). The effect of LS to replace SF partially by weight used as a supplementary cementitious material (0%, 5%, 10% and 15% of binder) on the compressive strengths and microstructure evolution of UHPC has experimentally been studied by multi-techniques including mercury intrusion porosimetry, scanning electron microscope and nanoindentation technique. The results show that the use of LS degrades the microstructure of UHPC at early ages, and however, the use of LS with the appropriate content improves microstructure of UHPC at later ages. The hydration products of UHPC are mainly dominated by ultrahigh density calcium-silicate-hydrate (UHD C-S-H) and interfacial transition zone (ITZ) in UHPC has similar compact microstructure with the matrix. The use of LS improves the hydration degree of UHPC and increases the elastic modulus of ITZ in UHPC. LS is a promising substitute for SF for preparation UHPC.
1. Introduction Ultra high performance concrete (UHPC) is a new type of concrete with the 28-day compressive strength in excess of 150 MPa (in China, compressive strength over 100 MPa) and has already been used in construction projects all over the world, especially containing the landmark projects [1], due to its ultra high mechanical properties and excellent durability [2]. UHPC is characterized by high binder content (over 800 kg/m3), very low water-binder ratio (w/b) and use of silica fume (SF), which results in a large amount of unhydrated cement causing waste of resources. It is well known that SF is an essential constituent for UHPC, which plays a significant role in improving the properties of UHPC. The prominent effects of SF in UHPC are filling effect, lubricating effect and pozzolanic effect [2–6]. However, the high cost, limited available quantity and uneven distribution of SF constrain greatly the application of UHPC in modern concrete construction, especially in the developing and backward countries and regions. Therefore, in order to enhance the application technology and expend the application range of UHPC, there is an urgent need for searching for other materials to substitute SF and/or cement. Recently it has been well acknowledged that the use of widely available supplementary cementitious materials (SCMs), such as fly ash and slag for partial/complete replacement of cement and SF, can also been used to prepare UHPC, which reduces obviously the cost without
⁎
sacrifice of strength, and has become the trends for the production of UHPC [2]. Li [7] reported that the ternary use of metakaolin, fly ash and cement in UHPC was of potential economic and environmental advantages. Calvo et al. [8] found that the substitution of SF by the corresponding amount of epoxy resin filled silica microcapsules and amine functionalized nanosilica increases the durability of the UHPC developed, guaranteeing a longer service life. Su et al. [9] reported that UHPC containing steel fibers and different kinds of nano materials including Nano-CaCO3, Nano-SiO2, Nano-TiO2 and Nano-Al2O3 demonstrated the superior ductility and blast resistant capacity. Alsalman et al. [10] presented an experimental investigation of UHPC using locally available materials. Their results showed that the fly ash content had an important effect on the compressive strengths of UHPC, and a fly ash content of more than 20% decreased the compressive strengths at early ages, but increased the strengths at later ages. A fly ash content of 30% produced the highest 90-day compressive strength, while a content of 20% had minimal effect on the strengths at all ages. Soliman and Tagnit-Hamou [11] prepared a green ultra-high-performance glass concrete (UHPGC) with a compressive strength of up to 220 MPa. The UHPGC properties were improved when the cement and quartz powder were replaced with nonabsorptive glass-powder (GP) particles, providing technological, economical, and environmental advantages compared to traditional UHPC. Tafraoui et al. [12] reported that it was possible to replace SF by metakaolin which did not affect the durability
Corresponding author. E-mail address: [email protected] (Z.-h. He).
https://doi.org/10.1016/j.jhazmat.2018.03.063 Received 28 November 2017; Received in revised form 30 March 2018; Accepted 31 March 2018 Available online 03 April 2018 0304-3894/ © 2018 Elsevier B.V. All rights reserved.
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of UHPC, which was used without any problem for the industry of powerful and durable UHPC [13]. Van et al. [14,15] found that rice husk ash (RHA) classified as “a highly active pozzolan” was suitable for use as a SCM to make UHPC, and the combination of 10% RHA and 10% SF proved to be optimum for achieving maximum synergic effect of UHPC. Huang et al. [16] investigated the effect of RHA on the strength and permeability of UHPC. Results from this investigation also suggested that the calcined RHA at 500 °C was a promising substitute for SF in UHPC production, and the RHA replacing SF ratio of 2/3 showed the best improving effect, increasing the compressive strength at 3, 28 and 120 days by 9.76%, 14.50%, 10.02%, respectively, compared to the traditional UHPC containing SF alone. Burroughs et al. [17] reported that the minor replacements of cement with limestone powder did not negatively affect properties of UHPC, the use of which as an inert filler in UHPC was proposed. Yazıcı et al. [18–20] prepared UHPC with Portland cement replaced with high content of granulated blast furnace slag and fly ash whose compressive strength was over 200 MPa after standard room curing, 234 MPa after steam curing and 250 MPa after autoclave curing. Peng et al. [21] found that the utilization of ultra-fine fly ash and steel slag powder in UHPC was feasible and the optimum steel slag powder/ultra-fine fly ash ratio was 1.5 for UHRC with the highest strength. Wang et al. [22] found that UHPC containing ground granulated blast furnace slag and limestone powder replacing cement achieved excellent workability with a maximum slump of 268 mm and compressive strengths of 175.8 MPa at 90 days and 182.9 MPa at 365 days. Therefore, the preparation of UHPC has already not been completely dependent on the costly and well-chosen materials, and a large number of widely available and low-cost SCMs replacing cement and/or SF all over the world can be used to prepare UHPC. Lithium slag (LS) is a byproduct, which is discharged in the process of the lithium carbonate using sulfuric acid method when the spodumene ore is calcined at high temperature of 1200 °C. According to the statistical analysis, about nine tons of LS are discharged when one ton lithium salt is produced in the production process of lithium carbonate. Today, about 8 × 105 tons of LS are discharged every year in China. LS as a promising SCM can also be used to prepare successfully different kinds of concretes and improve obviously the properties of concrete, especially the later properties, showing the similar physically filling and chemically pozzolanic effects with other SCMs such as ground granulated blast furnace slag and fly ash [23–25]. The aforementioned literature survey shows that, despite the considerable amount of work published in literature on UHPC prepared by all kinds of SCMs, and however, there is very little work on UHPC containing LS. LS is economical and abundant as a SCM, which is a promising substitute replacing SF in UHPC production. It is well known that microstructure-property relationships are at the heart of modern materials science, and the evolution of microstructure can determine the properties of UHPC [4,15,26], which may demonstrate significant difference as compared to conventional concrete. In this paper, the compressive strength variations of UHPC containing LS were measured with time using an experimental method, taking into account of the effect of different ratio of replacing SF with LS. Meanwhile, the effects of LS on the microstructure evolution of specimens were investigated by mercury intrusion porosimetry (MIP), scanning electron microscope (SEM) and nanoindentation techniques.
Table 1 Chemical compositions of cement, LS and SF (wt%). Materials
SO3
SiO2
Fe2O3
Al2O3
CaO
MgO
K2 O
Na2O
Loss on ignition
Cement
2.54
21.10
3.26
4.77
62.63
1.15
0.43
0.05
3.01
LS SF
7.15 0.83
53.22 88.12
1.48 0.49
17.11 0.29
10.11 0.63
0.41 3.08
0.53 3.69
0.33 1.22
8.25 1.11
Fig. 1. XRD pattern of LS.
contents of LS fall between those of SF and those of cement, respectively, and the SO3 and Al2O3 contents of LS are highest. X-Ray diffraction (XRD) pattern of LS is given in Fig. 1. The mineral compositions of LS contain mainly quartz, gypsum, calcite and lithium aluminum silicate. Fig. 2(a) shows the particle size distributions of the cement and LS determined by laser particle analysis using BT-9300 Laser Particle Analyzer, which exhibits that the size range of LS particles is wider than
2. Experimental 2.1. Materials The cement used in the present study was Portland cement with strength grade of P·I52.5 in accordance with Chinese standard GB 1752007. LS used was supplied by Sichuan lithium salt plant in China. Table 1 shows the chemical compositions of cement, SF and LS, used as the cementitious materials, which exhibits that the SiO2 and CaO
Fig. 2. (a) particle size distributions of cement and LS and (b) grading curves. 36
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Fig. 3. SEM images of LS and SF.
that of cement particles. Meanwhile, in terms of the average size, LS particle is smaller than cement particle, which is demonstrated in the grading curves of cement and LS shown in Fig. 2(b). Fig. 3 gives the SEM images of LS and SF at the magnifications of 5000×, which exhibits that the shape of LS particles is irregular, and that of SF particles is spherical. In addition, it can also be seen that the size of SF particles is much smaller than that of LS particles. The specific gravities of cement, LS and SF are about 3010, 2450 and 2180 kg/m3, respectively, which were determined according to the standard test method for cement density using the small pycnometer method in accordance with Chinese GB/T208-2014. The specific surface area of cement, LS and SF are about 370, 440 and 21,500 m2/kg, respectively, which were measured based on the nitrogen adsorption method. Quartz sand named standard sand in accordance with the international organization for standardization (ISO679) was utilized as aggregates, which was produced in Xiamen city, China. The SiO2 content of quartz sand was more than 98%. Quartz sand had particles with size less than 2 mm, whose particle distributions are shown in Table 2. The polycarboxylic superplasticizer (SP) with water-reducing range of more than 30% and solid content of about 35% by weight was carried out to adjust the workability of UHPC.
Table 3 Mix proportions of UHPC containing LS (in weight).
0 7±5 33 ± 5 67 ± 5 87 ± 5 99 ± 1
LS
Quartz sand
Water
SP
C-SF C-SF-5LS C-SF-10LS C-SF-15LS
896 896 896 896
224 168 112 56
0 56 112 168
1013 1013 1013 1013
202 202 202 202
20.2 19.6 18.8 18.0
For testing the compressive strengths of UHPC, a set of mixtures were cast into 40 mm × 40 mm × 40 mm moulds and all mixtures were vibrated for one minute. After that, the specimens were kept in moulds for one day at room temperature (20 ± 5) °C. Subsequently, they were demoulded and kept in a standard curing room of controlled temperature (20 ± 2) °C and relative humidity more than 95% until the days of testing (i.e. 3, 28 and 90 days). The UHPC specimens were selected from typical specimens to carry out the microscopic tests after strength measurement. For testing the pore size distribution and pore microstructures in UHPC specimens containing LS at the ages of 3 and 28 days, MIP and SEM techniques were performed, whose test procedures were gained through our published reference [27]. For testing the distribution of elastic modulus of matrix in UHPC specimens and characterizing the evolution of interfacial transition zone (ITZ) between the matrix and aggregate in UHPC specimens at 28 days from the nano-scale structure perspective, nanoindentation technique was carried out, whose detailed test procedures were also obtained according to our published references [28,29]. The main test procedures were as follows: the specimen was intercepted and then embedded in the epoxy resin in vacuum. After that, the specimen was ground and polished to ensure the surface roughness. Finally, the specimen was cleaned in an ultrasonic bath in order to carry out the nanoindentation test.
Table 2 Particle distributions of quartz sand.
2.0 1.6 1.0 0.5 0.16 0.08
SF
2.3. Experimental methods
It is generally acknowledged that the optimal SF content of UHPC was between 20% and 35% of cement, highly dependent on w/b [2,15]. Therefore, for all the mixtures, the total binder and cement were kept the same, 1120 kg/m3 and 896 kg/m3, respectively. The control UHPC with SF content of 20% was prepared and LS was added to replace SF by weight (5%, 10% and 15% of binder). The w/b and binder to aggregate ratio of four mixtures were kept as constants, 0.18 and 1.11, respectively, and the SP contents were adjusted to maintain the similar fluidities of UHPC mixture from 180 mm to 200 mm. The details of the components used in UHPC mixtures are shown in Table 3. All UHPC mixtures were blended in a mixer according to the
Accumulated sieving residue, %
Cement
employed mixing procedure given in Fig. 4 in order to improve the effect of interaction with the binder and SP and achieve a better uniformity.
2.2. Preparation of UHPC
Side length of square hole, mm
Types of mixture
3. Results and discussions 3.1. Compressive strength Compressive strengths of UHPC containing different LS contents to replace SF at 3, 28 and 90 days are presented in Fig. 5. Each result presented here is the average value of three tested specimens with identical mixture, and the error bar represents the variation of the 37
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Fig. 4. Employed mixing procedure for producing UHPC containing LS. (Low speed: 61.5 r/min; high speed: 123 r/min).
strength of specimen containing 20% LS is lowest for all the time. The rates of strength increases become slow at later ages, i.e. beyond 28 days. In addition, the compressive strengths of all specimens at 28 days are more than 110 MPa, but less than 150 MPa. It is generally recognized that UHPC refers to cementitious materials exhibiting compressive strengths higher than 150 MPa at 28 days [2], and however, in China, UHPC is defined as cementitious materials with 28-day compressive strength over 100 MPa. Therefore, the compressive strengths of all specimens can meet the Chinese standard requirements of UHPC. The compressive strength of the control specimen containing SF only does also not reach 150 MPa, which is not consistent with the results reported elsewhere [3,13,15]. This is likely to be attributed to the different qualities of cementitious materials containing cement and SF, manufacture procedure and curing process as well as specimen sizes. The close packing of cementitious material particles plays an important role in determining the strength of UHPC. The fineness of LS falls between that of cement and that of SF, so SF, cement and LS with the appropriate content can consistently form the densest packing state to increase the compressive strength. Apart from the filling effect of LS,
Fig. 5. Effect of LS on compressive strengths of UHPC.
obtained data. It can be found that LS reduces the compressive strengths of specimens at early ages (3 days), and however, from the start of 28 days, the compressive strength of specimen containing 10% LS surpasses that of control specimen containing SF only, and the compressive
Fig. 6. SEM images of UHPC containing LS at 3 days. 38
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Fig. 7. SEM images of UHPC containing LS at 28 days.
large capillary pores and air voids. The microstructure of specimens containing LS is loose. Among them, the specimen containing 10% LS shows a relative dense microstructure. This may be attributed to the close packing of SF, cement and LS with appropriate content. Observing the later images shown in Fig. 7, it can be seen that, compared to the specimens at early ages, the microstructures of specimens containing LS are much denser. No significant CH could be found in specimens because of the pozzolanic effect of SF and LS consuming CH. In addition, the interconnected pores and air voids in specimens existed at early ages are basically filled with hydration products produced by cement hydration and secondary pozzolanic reaction of SF and LS. It is noticeably that ITZ between aggregates and paste matrix in the control specimen containing SF only seems as dense as the matrix, and both have already not been distinguished. The ITZ in the control specimen is not weakest parts, different from that of conventional concrete. The homogenous microstructure is important for the excellence performance of UHPC. In contrast, no obvious ITZ is found in specimens containing LS. Both filling effect and pozzolanic reaction of LS are less significant than those of SF, so scattered disconnected capillary pores with small sizes can still be observed in the specimen containing 5% LS, especially in the specimen containing 15% LS. However, the microstructure of the specimen containing 10% LS is denser than that of the control specimen. The unhydrated particles and aggregates in the specimen containing 10% LS are surrounded by a large number of hydration products mainly C-S-H gels, which are interconnected into homogenous microstructure. Generally speaking, the close packing density of cementitious material particles has a direct effect on the hydration reaction process and the microstructure formation especially low w/b [33–35]. The higher the close packing density is, the denser the microstructure is. The size of SF particles is much smaller than that of LS particles demonstrated in Fig.3, but the
the pozzolanic reaction of LS makes outstanding contribution to the development of compressive strength at later ages. It is generally believed that the compressive strengths of UHPC mainly depend on the close packing density and hydration of cement as well as the pozzolanic reactions of mineral admixtures [4]. The ternary cementitious materials with the appropriate proportions of different fineness of LS and SF are more likely to get the maximum close packing density, compared to the binary cementitious materials with SF only. In addition, the pozzolanic reaction of SF is fast at early ages [2], and however, that of LS is slow at early ages and relatively fast at later ages. Therefore, the appropriate combination of SF and LS in ternary cementitious materials can always guarantee the sustainability of the pozzolanic reactions to increase the compressive strengths. When the content of LS is too few (5% of binder), or too many (15% of binder), the contribution of LS to the increase of strengths will be weakened. The optimum content of LS in the ternary cementitious materials is 10% of binder. 3.2. SEM analysis In general, SEM is recognized as a good tool to evaluate the microstructure of materials including UHPC [30–32]. Figs. 6 and 7 give the SEM images of specimens containing LS at the magnifications of 10,000× at 3 days and 28 days, respectively. As shown in Fig. 6, it is clear that the internal microstructure of UHPC at early ages is mainly comprised of unhydrated cementitious material particles, hydration products, aggregate, pores and air voids. The main hydration products in the control specimen containing SF only are homogeneous calcium-silicate-hydrate (C-S-H) gels, and some small calcium hydroxide (CH) crystals can also be found. Owing to the lower activity of LS compared to SF showing the filling effect only at early ages, LS degrades the microstructures of specimens with some 39
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Table 4 Pore size distributions in UHPC containing LS at 3 days (mL/g). Number
< 4.5 nm
(4.5–50) nm
(50–100) nm
> 100 nm
Total porosity
C-SF C-SF-5LS C-SF-10LS C-SF-15LS
0.00013 0.0011 0.00019 0.0006
0.00767 0.0095 0.00751 0.0055
0.003 0.0021 0.0029 0.0026
0.0259 0.0396 0.0306 0.0510
0.0367 0.0523 0.0412 0.0597
Table 5 Pore size distributions in UHPC containing LS at 28 days (mL/g).
Fig. 8. Effect of LS on cumulative porosity of UHPC at 3 days.
close packing density of ternary cementitious material system containing cement, SF and LS with the appropriate content may exceed that of binary system containing cement and SF. This is likely to explain why the use of 10% LS can improve the microstructure of specimens at later ages.
Number
< 4.5 nm
(4.5–50) nm
(50–100) nm
> 100 nm
Total porosity
C-SF C-SF-5LS C-SF-10LS C-SF-15LS
0.0011 0.0002 0.00063 0.0002
0.0133 0.012 0.01317 0.0096
0.0018 0.0031 0.0017 0.0032
0.0136 0.0158 0.01 0.0241
0.0298 0.0311 0.0255 0.0371
to C-S-H to fill large pores. However, 5% and 15% LS increase the total porosities and the contents of > 100 nm pore of the specimens, and those of the specimen containing 15% LS are highest. This is likely to be attributed mainly to the fact that the appropriate content LS may increase the close packing density of ternary system, exceeding that of binary system containing cement and SF, to accelerate the hydration process. There is an optimal content of LS for the maximum close packing density of ternary system, and however, too few or too many LS contents influence the increase of close packing density to hinder the hydration process.
3.3. Pore structure The microstructure of concrete represented by its pore structure, i.e., the porosity and pore size distribution, plays a decisive role in evolving the properties of concrete [36]. MIP is the most widely adopted useful method for the study of pore structure characteristics of cementitious materials [36–38]. The cumulative porosities are measured using MIP for specimens containing LS cured for 3 days and 28 days, and the results are shown in Figs. 8 and 9, respectively. In addition, the corresponding pore size distributions are shown in Tables 4 and 5, respectively. It is clear that low w/b of UHPC results in low porosity. The pores of cementitious materials can be classified into four levels based on the pore diameters: < 4.5 nm, 4.5–50 nm, 50–100 nm and > 100 nm [39]. According to the effect of pores with different diameters on the properties of cementitious materials, the pore > 100 nm is definitely harmful. As it is shown in Fig. 8 and Table 4 that the total porosities of specimens containing LS are higher than that of the control specimen containing SF only at early ages, and especially the contents of > 100 nm pores are also increased with the addition of LS. This indicates that LS degrades the pore structures of UHPC specimens at early ages, which may be due to the lower activity of LS compared to SF. By comparison with Figs. 8 and 9, the porosities of all specimens are further reduced with the increase of ages. As it is shown in Fig. 9 and Table 5 that 10% LS decreases the contents of different size pores including the total porosity at later ages, compared to those of the control specimen. Therefore, LS with the appropriate content can refine the pore structure of specimen due to its pozzolanic reaction and physical filling effect. LS can consume most of the CH crystals and converts them
3.4. Nanoindentation investigation Nanoindentation is an effective and powerful tool to detect the local elastic properties and hardness of each phase of materials which can character the evolution of material microstructure and nanostructure including cementitious materials [40,41]. The working principle of nanoindentation is relatively simple: push a very sharp tip into the surface of the material and investigate the mechanical behavior of the material from the response of the tip. Firstly, the matrix of specimens was found with the help of the optical microscope of nanoindentor, and a 10 × 10 grid containing 100 points was carried out on each specimen, and the distance between two adjacent test points was 20 μm. The area was chosen to be statistically representative of the matrix of each specimen. A nanoindentation test consisted of establishing contact between a specimen and a tip of known geometry and then continuously measuring the change in indentation depth h as a function of increasing indentation load P. After the nanoindentation test was carried out, each indentation P-h curve was examined to determine the validity of experimental data. Fig. 10 shows the irregular discarded typical P-h curves in UHPC
Fig. 9. Effect of LS on cumulative porosity of UHPC at 28 days.
Fig. 10. Irregular discarded typical P-h curves in UHPC matrix. 40
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Porosity LD C-S-H HD C-S-H CH or UHD C-S-H Unhydrated particle
Indentation load, mN
2.0 1.5 1.0 0.5 0.0
0
100 200 300 400 500 600 700 Indentation depth, nm
Fig. 11. Typical P-h curves of phases in UHPC matrix.
matrix which can not provide any information in determining the nanomechanical properties. (a) P-h curve may be due to the presence of a large void caused by the specimen preparation.(b) P-h curve should be attributed to the indenter piercing the weak specimen surface and then penetrating into the harder under layer. When the indenter lies in the surface cracking during the load-driven the nanoindentation test, (c) Ph curve is obtained. Generally speaking, UHPC matrix mainly consists of porosity, hydration products and unhydrated particles, and the typical P-h curves of these phases are presented in Fig. 11. It is clear that depending on the nano-mechanical properties of indented phases, the maximum indentation depth ranges from 89.32 nm to 699.71 nm. According to the information provided by each P-h curve, the elastic modulus of each indentation point can be obtained. Each phase has a corresponding elastic modulus ranges which are intrinsic mechanical properties. The main hydration products of cement are C-S-H and CH. C-S-H phases can be classified into inner C-S-H and outer C-S-H or low density C-S-H (LD C-S-H) and high density C-S-H (HD C-S-H). In addition, a recent study revealed the presence of a new phase at low w/b which has the similar structure with C-S-H and similar mechanical performance with CH, so it is defined as ultra-high density (UHD C-S-H) and considered as the third C-S-H [42,43]. It is well known that the elastic modulus of LD C-S-H, HD C-S-H and CH/ UHD C-S-H are generally stable regardless of different curing methods, w/b, and so on, in the range of 14–24 GPa, 24–35 GPa and 35–50 GPa, respectively [44–46]. Generally, the elastic modulus of porosity is less than 14 GPa, and that of unhydrated particles is more than 50 GPa. In addition, no CH crystals can be observed in UHPC owing to the low w/b and pozzolanic reaction with mineral admixtures [46,47], so the phase with the elastic modulus of 35–50 GPa in UHPC should mainly be UHD C-S-H rather than CH. Fig. 12 displays the frequency histogram of elastic modulus in matrix of the control specimen containing SF only and the specimen containing 10% LS at 28 days. The bin size of the histogram is 10 GPa. The proportions of indentations for different phases which can be determined from the histogram are also given in Table 6 and can be considered the approximate volume fractions of different phases in cementitious materials [48]. It is observed that the main phases in UHPC matrix are UHD C-S-H and unhydrated particles, and about 70% of the hydration products in UHPC are UHD C-S-H, which indicates that the hydration products of UHPC have much higher mechanical properties than those of conventional concrete whose hydration products are LD C-S-H and/or HD C-SH mainly [48]. In addition, there are few the porosity and LD C-S-H in UHPC implying very denser internal structure of UHPC, and the quantity of HD C-S-H is two or three times as many as that of LD C-S-H. Compared to the proportions of constituent phases of the control specimen obtained from nanoindentation tests, 10% LS improves the hydration degree of the specimen and increases the quantity of C-S-H especially HD C-S-H and UHD C-S-H obviously, and reduces the
Fig. 12. Effect of LS on frequency histogram of elastic modulus of UHPC at 28 days. Table 6 Volume fractions of constituent phases in UHPC matrix (%). Specimen
Porosity
LD C-S-H
HD C-S-H
UHD C-S-H
Unhydrated particle
C-SF C-SF-10LS
3.2 3.3
3.2 4.5
7.4 13.0
36.2 46.6
50.0 32.6
unhydrated particles. LS can consume CH produced by the cement hydration and generate additional C-S-H. It can be inferred that C-S-H generated by the secondary pozzolanic reaction of LS is higher mechanical HD C-S-H and UHD C-S-H mostly in UHPC, which can improve effectively the macroscopic properties of UHPC. Concrete is a composite materials and consists of matrix, aggregate and ITZ. ITZ is considered as a loose phase with higher porosity compared to the bulk matrix, which plays an important role in determining the properties of concrete. Nanoindentation also proves to be a very sensitive and appropriate experimental tool to characterize the properties of ITZ quantitatively [49–51]. Nanoindentation tests of ITZ in specimens began every 10 μm from the aggregate surface until 60 μm, which worked in a direction perpendicular to the aggregate surface. Cementitious materials were heterogeneous, and therefore another direction of nanoindentation tests was every 10 μm along the aggregate surface until 40 μm in order to include all phase results. The properties of ITZ including the effective thickness and mechanical properties vary with the mix proportions, preparation, curing and so on. There is not a clear boundary between ITZ and matrix in concrete. Generally speaking, the elastic modulus shows a stable trend, indicating the test areas are from ITZ to matrix, and therefore the thickness of ITZ between 41
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• The hydration products of UHPC are mainly dominated by UHD C-S-
•
H, and the use of LS improves the hydration degree of the UHPC and increases the quantities of HD C-S-H and UHD C-S-H obviously. ITZ in UHPC has similar compact microstructure and mechanical properties with the matrix, and the use of LS also increases the elastic modulus of ITZ in UHPC. It indicates that UHPC containing LS with the appropriate content has the higher mechanical properties. LS is suitable for use as a SCM to substitute SF for preparation UHPC.
Acknowledgment The authors would like to acknowledge the National Natural Science Foundation of China (Grant Nos. 51602198 and 41427802) for their financial support to the work present in this paper. Fig. 13. Elastic modulus evolution of ITZ in specimens.
References aggregate and matrix is the distance from the aggregate surface to the stable indentation point with the elastic modulus. In order to exclude the effect of unhydrated particles on the ITZ, the results of elastic modulus over 50 GPa obtained from nanoindentation tests are eliminated. Fig. 13 shows the elastic modulus evolution of ITZ in the control specimen and the specimen containing 10% LS at 28 days, and the error bar represents the variation of the obtained data. It can be seen that the elastic modulus across the whole test zone appears to be stable and uniform, and meanwhile there is no distinct trough near the aggregate surface. Zhao and Sun [47] also obtained the similar results when investigating the nano-mechanical behavior of a green UHPC formula which contained high volumes of fly ash and used natural river sand as fine aggregate. It implies ITZ in the UHPC specimens is improved thoroughly and has similar compact microstructure and mechanical properties with the matrix, and there is a tight bond between the aggregate and matrix. These results are corresponding well with the results shown in SEM images. In addition, compared to the control specimen, LS also increases the elastic modulus across the whole test zone of the specimen. This is likely to be attributed to the more additional HD C-S-H and UHD C-S-H with higher mechanical properties generated by the secondary pozzolanic reaction of LS. Xie et al. [52] found that the relationship between porosity and elastic modulus agreed well with empirical formulas stemming mostly from macroscopic tests by comparing the results of backscattered electron and nanoindentation at identical regions, which bridged the microstructure with the mechanical properties of concrete. The denser the microstructure was, the higher the elastic modulus of the same area was and macroscopic mechanical properties of cementitious materials were. Therefore, it indicates that the specimen containing 10% LS with the higher elastic modulus obtained by nanoindentation tests corresponds to the higher mechanical properties including the results shown in Fig. 5.
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4. Conclusions In this paper, the effect of LS to replace SF partially by weight on the compressive strengths and microstructure evolution of UHPC has experimentally been studied by multi-techniques. The following conclusions can be drawn:
• The use of LS to replace SF in UHPC reduces the compressive •
strengths at early ages, and however, the use of 10% LS can increase the compressive strength at later ages surpassing that of the control UHPC containing SF only. SEM images and pore structure analysis show that the use of LS degrades the microstructure of UHPC at early ages, but the use of 10% LS improves the microstructure of UHPC at later ages. This should be attributed to the filling effect and pozzolanic reaction of LS. 42
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