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Compactness and hardened properties of machine-made sand mortar with aggregate micro fines
Construction and Building Materials 250 (2020) 118828
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Compactness and hardened properties of machine-made sand mortar with aggregate micro fines Hang He a, Yuli Wang a, Junjie Wang b,⇑ a b
School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454003, China Department of Civil Engineering, Tsinghua University, Beijing 100084, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A compactness model together with
mathematical model was established. Optimum contents of AMF in MS was
obtained. Harden properties of MS mortar was
studied. The mechanism of AMF in MS mortar
was investigated and discussed.
a r t i c l e
i n f o
Article history: Received 20 November 2019 Received in revised form 26 February 2020 Accepted 20 March 2020
Keywords: Packing model Aggregate micro fines Machine-made sand Mortar Compactness
a b s t r a c t Machine-made sand (MS) has become a hot topic in construction materials due to the depletion and scarcity of natural river sand. Aggregate micro fines (AMF) of varied contents were included in MS during the manufacturing process. To quantify the filling role of AMF in MS mortars, a physical packing model together with the related mathematical model of fresh mortar was established by considering the water as a void component in fresh mortar mixture. A total 42 MS mortar mixes with different water-cement (W/ C) ratios (0.45, 0.5, 0.55, 0.6), different sand-cement (S:C) ratios (1:1, 2:1, 3:1) and different contents of AMF (0 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%) were prepared. The effects of AMF on the bulk density and compactness of fresh mortar, flexural and compressive strength, pore structure and microstructure of hardened mortar were investigated. The results show that the changing trend of compactness of fresh mortar is in consistent with the results of the bulk density and mechanical properties of the mortar. The optimal contents of AMF are found to be dependent on the W/C and S:C ratios. Our results show that, when S:C = 2.5:1, the optimal contents of AMF by weight of MS are 5 wt%, 10 wt%, 15 wt%, and 20 wt% for the mixes with the W/C ratios of 0.45, 0.5, 0.55, and 0.6, respectively; when the W/C = 0.5, the optimal AMF contents are 5 wt%, 10 wt%, and 15 wt% for the mixes with the S:C ratios of 1:1, 2:1, and 3:1, respectively. The results of mercury intrusion porosimetry (MIP) and electron backscattered diffraction (EBSD) show that the AMF can modify the pore structure and enhance the interfacial transition zone (ITZ) of mortar. Ó 2020 Elsevier Ltd. All rights reserved.
⇑ Corresponding author. E-mail address: [email protected] (J. Wang). https://doi.org/10.1016/j.conbuildmat.2020.118828 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
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H. He et al. / Construction and Building Materials 250 (2020) 118828
be re-established in order to explore the physical filling role of AMF in mortar or concrete. For this, Wang et al. [24] established a physical packing model and a mathematical model to characterize the physical filling role of the limestone AMF in concrete. A selfcompacting concrete mix with limestone AMF was designed by Diederich et al. [25] through establishing a relationship between the gap of particles and the demanding water. Kwan et al. [26] found that the error of the packing density was less with wetparticle packing than that with dry-particle packing. Kwan et al. [27,28] and Wong et al. [29] established a three-parameter model with a different-size particle packing and a better accuracy. The effect of packing density on the rheology and stability of selfcompacting concrete was studied by Ghoddoousi et al. [30], and their results showed that an accurate mix design of a selfcompacting concrete through wet-particle packing was possible. In summary, the optimal content of AMF in the MS could be different for different mortar or concrete mixtures. Although most experts believe that the physical filling role of AMF is usually the primary effect, but how to quantify this effect of AMF, especially for the fresh mortar and concrete, still needs further investigation. For this, mortars with different water-cement (W/C) ratio and different sand-cement (S:C) ratio were prepared. Based on the theoretical analysis, a physical and a mathematical packing model for the compactness of the fresh mortar were established. The bulk density and the mechanical properties of different mortar samples were studied. The optimum contents of AMF was determined and discussed together with the related mechanisms.
1. Introduction Aggregate micro fines (AMF) are the powders with a particle size usually less than 75 lm, which are produced inevitably during the production of machine-made sand (MS), and the content of AMF in MS can be as high as more than 20 wt%. The mineral and chemical compositions of AMF are the same as the mother rock of MS, which is different from natural sand. The influence of AMF on performance of concrete and mortar has been widely studied. For example, it was reported by Kim [1] that the incorporation of 4 wt% limestone AMF in slag mortar could increase the compressive strength of mortar. The results from Turk et al. [2] showed that the workability of cement-based materials decreased continuously with increase of limestone AMF, but the compressive strengths of 3 and 28 days could be improved depending on the AMF content. Pliya et al. [3] reported that the compressive strength of mortar was improved with 5 wt% AMF, but the flexural strength of mortar was decreased when more than 10 wt% AMF was added. Chong et al. [4] pointed out that AMF could shorten the setting time of cement-based materials and reduce the total hydration heat release, and they suggested the optimum content should not exceed 10 wt%. Mohammadi et al. [5] reported that the physical performance of mortar remained similar when the content of limestone AMF increased from 7.5 to 12 wt%. Khaleel et al. [6] showed that the rheology of fresh mortar and the mechanical performance were improved when AMF content was 10 wt%, and the size of pores inside the sample was obviously reduced. Dave et al. [7] found that the mechanical properties of mortar sample and the chloride penetration resistance were the best when the content of limestone AMF was 7.5–10 wt%. Temiz et al. [8] found that the compressive strength of mortar decreased as the increase of limestone AMF content, but the effect on the setting time was not significant. The results from Senhadji et al. [9] also showed a decrease of the compressive strength as the continuous increase of limestone AMF. Ji et al. [10] showed that the early strength of mortar was increased when the content of limestone AMF was 20 wt%. Rizwan et al. [11] pointed out that the strength of selfcompacting mortar was improved when the content of limestone AMF was 20 wt%, and the size of pores was decreased. AMF was reported to have chemical reaction effect [12–14] and physical filling effect [15–17], and most researchers believed that the primary effect is physical filling effect [18,19]. Sakai et al. [20] found that the rheology of fresh paste was influenced by particle morphology and packing density of AMF. The research from Knop and Peled [21,22] and Knop et al. [23] showed that the hydration rate, packing density and carbonation resistance of cement paste were improved when the cement clinker was mixed with AMF. AMF could change the packing density of mortar or concrete, therefore, and the packing model of fresh mortar or concrete needs
2. Experimental programs 2.1. Materials and mix design Type I ordinary Portland cement was used with a specific surface area of 362 m2/kg, the properties and oxide compositions of cement were shown in Tables 1 and 2. Both AMF and MS were made from limestone. The XRD pattern of AMF was shown in Fig. 1, and the specific surface area was 353 m2/kg. MS had a fineness modulus of 2.93, and its physical properties were shown in Table 3. Fig. 2 shown the particle size distributions of AMF and MS. The MS with 0 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt% and 25 wt% of AMF were obtained by separating the initial AMF from MS through a 75 lm sieve and then adding different AMF contents into MS. Mixing water was clean tap water. The mix design of different mortar samples was shown in Table 4. The W/C was kept constant as 0.5 in the first 18 mixes to study the effects of varied contents of AMF and different S:C ratios. From Mix No. 19 to No. 42, the S:C was kept constant as 2.5:1, and the W/C and AMF contents were varied.
Table 1 The properties of cement. Fineness %
Stability
Specific surface area (m2/kg)
0.9
Qualified
373
Setting time min
Flexural strength MPa
Compressive strength MPa
Initial
Final
3d
28d
3d
28d
159
224
6.0
10.2
24.8
49.9
Table 2 Oxide compositions of cement (%). CaO
SiO2
Al2O3
Fe2O3
MgO
Na2O
K2O
Loss
60.57
22.99
5.46
3.22
2.31
0.27
0.19
0.59
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H. He et al. / Construction and Building Materials 250 (2020) 118828
Fig. 1. XRD patterns of AMF.
2.2. Test methods The mixing procedure was: the cement and AMF were mixed evenly in dry state and then water was added to form the fresh paste, followed by adding the MS at the last stage to form the fresh mortar. The mass of the fresh paste (mP) was weighed with a 500 ml container, and the bulk density of the paste (qP) was calculated by Eq. (1). The mass of the fresh mortar (mM) was weighed with a 1L container, and the bulk density of the fresh mortar (qM) was determined by Eq. (2). The size of mortar sample was 40 40 160 mm3. The flexural and compressive strength were measured at 7 and 28 days.
qP ¼
mP
ð1Þ
0:5 103
where qP is bulk density of the fresh paste, kg/m3; mP is the mass of fresh paste filled in the 500 ml container, kg.
qM ¼
mM
Fig. 2. Particle size distribution: (a) AMF, (b) MS.
ð2Þ
1 103
where qM is the bulk density of the fresh mortar, kg/m3; mM is the mass of fresh mortar in the 1 L container, kg. XRD was tested with a Smart-lab type X-ray diffractometer made by Rigaku company in Japan, Cu-Ka target, scanning angle 5–70°, scanning rate 10°/min. Mercury intrusion porosimetry (MIP) was tested with a Micromeritics AutoPore Iv 9510 type automated mercury porosimeter. Electron backscattered diffraction (EBSD) was tested with a Merlin Compact field emission scanning electron microscope made by German Zeiss company. 3. Results and analysis 3.1. Packing model (physical and mathematical) of fresh mortar The composition of fresh mortar prepared in this study includes water, cement, AMS and MS, in which cement and water constitutes a pure cement paste, the cement paste together with AMS
formed a mixed binder paste, and the mixed binder paste together with MS constitutes a fresh mortar sample. According to the volume of cement and water, the actual packing modes of cement paste could be divided into three cases, and they were shown as Fig. 3(a), (b), and (c), respectively. The first one is that the water is not enough to occupy the voids between cement particles, which is shown as Fig. 3(a); the second one is that the water is just enough to fill all the voids between cement particles, as is shown in Fig. 3(b); the third one is that the water is too much to fill all the voids between the cement particles, and the cement particles are suspended in the water, which is shown as Fig. 3(c). The first and second cases might not be able to have a good workability, and they will not be analyzed. To characterize the actual void ratio of the third case, water is treated as a void component in the fresh cement paste, and the actual void ratio (eP1) of the paste can thus be obtained by Eq. (3).
Table 3 Properties of MS. Fineness modulus
Loose density (kg/m3)
Apparent density (kg/m3)
Void ratio %
AMF %
2.93
1543
2685
42.5
3.6
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H. He et al. / Construction and Building Materials 250 (2020) 118828
Table 4 Mix design. Groups
AMF (wt. %)
No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No.
0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42
Raw Materials (kg/m3)
S:C
W/C
AMF
OPC
MS
Water
Superplasticizer
0 18 36 54 72 90 0 36 72 108 144 180 0 54 108 162 216 270 0 45 90 135 180 225 0 45 90 135 180 225 0 45 90 135 180 225 0 45 90 135 180 225
360
360 342 324 306 288 270 720 684 648 612 576 540 1080 1026 972 918 864 810 900 855 810 765 720 675 900 855 810 765 720 675 900 855 810 765 720 675 900 855 810 765 720 675
180
1.08
1:1
0.5
180
1.08
2:1
0.5
180
1.08
3:1
0.5
162
1.08
2.5:1
0.45
180
1.08
2.5:1
0.5
198
1.08
2.5:1
0.55
216
1.08
2.5:1
0.6
360
360
360
360
360
360
—cement particles,
(a)
—water
(c)
(b) Fig. 3. Physical packing model of fresh cement paste.
eP1 ¼
mW mW þmC =
qW
qP1
ð3Þ
where, eP1 is the actual void ratio of pure cement paste; mW is the mass of water, kg; qW is the density of mixing water, kg/m3; mC is
the mass of cement, kg; qP1 is the bulk density of pure cement paste, kg/m3. With the consideration of the role of AMF in the blended paste, the packing modes in Fig. 4 are presented. For a given W/C paste, the actual packing of the blended paste could change from Fig. 4
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H. He et al. / Construction and Building Materials 250 (2020) 118828
—cement particles,
—AMF particles,
(a)
—water
(c)
(b) Fig. 4. Physical packing model of fresh cement paste with AMF.
is greater than the void volume between MS particles, and the MS particles are separated by the paste like a suspension, in Fig. 5(c). Obviously, the third fresh mortar has highest flowability. For Fig. 5(c), the fresh mortar can be divided into two different parts: the first one is the fresh blended paste (including water, cement, and AMF), and the second one just is MS. Since the volume of the paste is greater than the voids of the MS particles, the MS particles are suspended in the fresh paste. In this way, to characterize the void ratio of fresh mortar sample, the packing mode of fresh mortar can be treated as that a part of the paste is replaced by the MS particles. In this case, the MS in the fresh mortar is a component with a void ratio of 0, that is to say, the compactness of MS component is 1. The void ratio of the fresh blended paste in the mortar (eP2) can be calculated by Eq. (4), and the void ratio (eM) of the fresh mortar can be obtained as Eq. (5). It can be seen from Eq. (4) that the void ratio (eP2) of the paste component in the fresh mortar gradually decreases as the content of AMF increases, and accordingly, the compactness gradually increases, but at the same time, the volume of MS gradually decreases, which results in the increase of the void ratio (eM) of the fresh mortar.
(a) to Fig. 4(c) with the increase of AMF content. It is seen from Fig. 4(a) that the particles of cement and AMF are suspended in the water; from Fig. 4(b) that the voids between particles of cement and AMF are just occupied by the water; from Fig. 4(c) that the water is too little to fill all the voids between particles of cement and AMF. Similarly, the blended pastes in Fig. 4(b)-(c) could not have a good flowing property, and they are not analyzed. For Fig. 4(a), the actual void ratio of the fresh paste (eP2) can be obtained by Eq. (4). Obviously, it can be seen from Eq. (4) that the eP2 gradually decreases as AMF increases, and accordingly, the compactness gradually increases.
eP2 ¼
mW mW þmC þmAMF =
qW
ð4Þ
qP2
where, eP2 is the actual void ratio of fresh blended paste containing AMF; mAMF is the mass of AMF, kg; qP2 is the bulk density of the fresh blended paste containing AMF, kg/m3. Depending on the volume of the fresh blended paste and MS in the fresh mortar, the actual packing modes of the fresh mortar could be divided as three cases: the first one is that the paste cannot fill the all voids between MS particles, and the MS particles cannot be separated by the paste, which is shown in Fig. 5(a); the second one is that the volume of the paste is just enough to fill the voids of MS particles, but the MS particles are still not separated by the paste, in Fig. 5(b) as the third one, the paste volume
—MS particles,
(a)
eM ¼
^Il ^Il P2 P2 ¼ ð m 1 yS 1 S = mW þmC þmP þmS Þ qS
ð5Þ
qM
where, eM is the void ratio of fresh mortar; yS is the volume fraction of MS particles in the fresh mortar; mS is the mass of the MS, kg; qS
—cement particles,
—AMF particles,
(b) Fig. 5. Physical packing model of fresh mortar with AMF and MS.
—water
(c)
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H. He et al. / Construction and Building Materials 250 (2020) 118828
is the apparent density of MS, kg/m3; qM is the bulk density of the fresh mortar, kg/m3. The compactness of the fresh mortar can be obtained Eq. (6).
bM ¼ 1 eM
ð6Þ
0.5, 0.55, and 0.6 are 5 wt%, 10 wt%, 15 wt%, and 20 wt%, respectively. It could be indicated that the optimal AMF content for the maximum bulk density increases gradually as the S:C ratio and the W/C ratio increase. A higher bulk density indicates a better the filling effect of the AMF.
where, bM represents the compactness of fresh mortar. 3.2. Results and analysis In order to explore the effects of AMF on the compactness and the bulk density of the fresh mortar, and mechanical properties of hardened mortar, as shown in Table 4, the mortars with different W/C ratio (0.45, 0.5, 0.55, 0.6), S:C ratios (1:1, 2:1, 3:1) and different contents of AMF in MS (0 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%) were prepared and investigated. 3.2.1. Bulk density The effect of AMF on the bulk density of the fresh mortar is shown in Fig. 6. From Fig. 6(a) and (b) that the bulk density of the fresh mortar first increases and then decreases as AMF increases. As is shown in Fig. 6(a), when the fresh mortars had a S:C at 1:1, 2:1, and 3:1, the corresponding optimum contents of AMF for the highest bulk density are 5 wt%, 10 wt%, and 15 wt%, respectively; as shown in Fig. 6(b), the corresponding optimum contents of AMF in the fresh mortars with different W/C of 0.45,
3.2.2. Flexural strength and compressive strength The influence of AMF contents on the flexural and compressive strength of hardened mortar with different S:C ratios is shown in Figs. 7 and 8. It can be found that the changes of the flexural strength and the compressive strength of mortars at the age of 7 and 28 days show the same trend, which both first increase and then decrease. As is shown in Fig. 7(a), the optimum contents of AMF for the highest 7-day flexural strength in the mortar samples with S:C ratio of 1:1, 2:1, 3:1 are 5 wt%, 10 wt%, and 15 wt%, respectively. The changing trend of flexural strength results at 28 days is similar as the results at 7 days. As is shown in Fig. 8 (a), to obtain the highest compressive strength at 7 days, the corresponding optimum contents of AMF in the mortars with S:C ratio of 1:1, 2:1, 3:1 are 5 wt%, 10 wt%, and 15 wt%, respectively. The corresponding compressive strength was increased by 14.9%, by 17.7%, and by 14.2%, respectively, compared with that of the mortar without AMF.
(a) S:C
(a) 7d
(b) W/C
(b) 28d
Fig. 6. Effect of AMF on bulk density results of the fresh mortar.
Fig. 7. Effect of AMF on flexural strength results of mortar with different S:C.
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H. He et al. / Construction and Building Materials 250 (2020) 118828
The influence of AMF on the flexural and compressive strength of mortar samples with different W/C ratios is shown in Figs. 9 and 10. It can be seen that the two types of strength results of the mortars at 7 and 28 days both first increase and then decrease when the W/C ratio increases gradually from 0.45 to 0.6. As is shown in Fig. 9(a), the optimum contents of AMF for the highest flexural strength at 7 days in the mortars with the W/C ratio of 0.45, 0.5, 0.55 and 0.6 are 5 wt%, 10 wt%, 15 wt%, and 20 wt%, respectively. The related flexural strengths are 11.3, 10.2, 9.4 and 9.6 MPa, respectively. In Fig. 9(b), the changing trend of flexural strength at 28 days is similar as that of 7 days. Fig. 10(a) shows that the optimum contents of AMF for the highest 7-day compressive strength of the mortar with different W/C ratios of 0.45, 0.5, 0.55 and 0.6 are 5 wt%, 10 wt%, 15 wt%, and 20 wt%, respectively. The related compressive strengths are 50.4, 44.6, 39.6, and 39.4 MPa, respectively. The result of the compressive strength at 28 days (Fig. 10(b)) is the similar as that of 7 days. In summary, the changing trend of flexural and compressive strength of harden mortars is in consistent with that of bulk density of fresh mortars. Therefore, it can be indicated that a higher bulk density of fresh mortar corresponds to a better packing and compactness of the fresh mortar, and a higher flexural and compressive strength results of the harden mortar.
(a) 7d
(b) 28d Fig. 8. Effect of AMF on compressive strength results of mortar with different S:C.
3.2.3. Compactness The compactness of the fresh mortar mixture was calculated by Eq. (6), and the results are shown in Fig. 11. It can be seen that the compactness of the fresh mortar first increases and then decreases with the increase of AMF content. In Fig. 11(a), the mortar samples with the highest compactness had 5 wt%, 10 wt%, and 15 wt% of AMF when the S:C ratio is 1:1, 2:1, and 3:1, respectively; in Fig. 11(b), the optimum contents of AMF for the highest compactness in the mortars with W/C ratio of 0.45, 0.5, 0.55, and 0.6 are 5 wt%, 10 wt%, 15 wt%, and 20 wt%, respectively. The highest compactness indicates the optimal packing mode, and accordingly, the best physical filling effect of the AMF. This result is also in consistent with the results of strength and bulk density. 3.2.4. MIP and EBSD To analyze the influences of AMF on the pore structure of the paste and interfacial transition zone (ITZ) in mortar samples, MIP and EBSD tests were carried out. The mixes with a constant W/C at 0.5, S:C at 2:1 and different contents of AMF were tested. The effect of AMF on the pore structure of the mortar is shown in Table 2 and Fig. 12. It can be seen from Table 5 that the porosity of the mortar decreases first, and then increases with a continuous increase of
(a) 7d
(b) 28d Fig. 9. Effect of AMF on flexural strength results of mortar samples with different W/C.
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H. He et al. / Construction and Building Materials 250 (2020) 118828
(a) 7d
(a) S:C
(b) 28d
(b) W/C
Fig. 10. Effect of AMF on compressive strength results of mortar samples with different W/C.
AMF content, and the porosity of mortar reaches the least when AMF is 10 wt%. Effect of AMF on porosity is in consistent with that of compactness of fresh mortar and the results of the strength of mortar, showing that AMF could modify the porosity and microstructure, and improve mechanical properties of mortar. Fig. 12 shows that the sizes of most pores are less than 200 nm when AMF is 10 wt% and 15 wt%, and AMF significantly optimizes the size distribution of pores. The harmful pores become less when the AMF increases from 0 wt% to 15 wt%, but the harmful pores increase sharply when the content of AMF is more than 15 wt%. In a word, the optimum content of AMF could optimize the pore structure of mortar and decrease the porosity. To investigate the influence of AMF on the microstructure of the paste and interfacial transition zone (ITZ) in mortar samples, EBSD tests were conducted on the samples with the content of AMF 0 wt %, 10 wt% and 25 wt%, as shown in Fig. 13. Any microcracks developed in the sample could facilitate the ingress of harmful substances [31,32]. It can be seen from Fig. 13 that the microstructure and ITZ of hardened mortar is more compact when the content of AMF is 10 wt%, which suggests the enhanced ITZ between aggregates and the paste in mortar is beneficial to the mechanical properties [33,34].
Fig. 11. Effect of AMF on compactness of fresh mortar.
Fig. 12. Effect of AMF on pore size distribution of mortar.
In summary, a suitable or optimum content of AMF can improve the compactness of mortar, and related compactness can be characterized with the established mathematical model in this study. This is to say, the appropriate amount of AMF can optimize the par-
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H. He et al. / Construction and Building Materials 250 (2020) 118828 Table 5 Porosity of mortar with different AMF contents. AMF/wt. %
0
5
10
15
20
25
Porosity/%
22.11
21.99
18.13
18.42
22.81
23.56
Fig. 13. EBSD images of mortar with different AMF contents.
ticle packing, modify the pore structure and ITZ, and improve the physical properties of mortar. 4. Conclusions In order to obtain the optimum design of the MS mortar with AMF, a physical packing model and a mathematical model were established to discuss the role of W/C, S:C and AMF on the compactness of fresh mortar, which were further verified by the experimental work of 42 mixes with different W/C, S:C and AMF contents. The following conclusions could be made: (1) Both the bulk density of the fresh mortar mixture and the mechanical properties at 7 and 28 days of the mortar increased first and then decreased when AMF increased. The optimum contents of AMF in the mix design varied with W/C and S:C ratios. (2) The optimal contents of AMF in the mortars with a constant S:C at 2.5:1 are 5 wt%, 10 wt%, 15 wt%, and 20 wt% when the W/C ratios are 0.45, 0.5, 0.55, and 0.6, respectively. The optimal contents of AMF in the mortars (constant W/C at 0.5) are 5 wt%, 10 wt% and 15 wt% when the ratios of S:C are 1:1, 2:1, and 3:1, respectively. The optimum content of AMF increases if there is an increase of W/C or S:C ratio. (3) The compactness of the fresh mortar mixture can be calculated with the established model, and the results of compactness of the fresh mortar mixture are in consistent with those of bulk density and mechanical properties. The results
of MIP and EBSD showed that the optimum content of AMF could modify the pore structure of the paste and enhance the ITZ in hardened mortar, which further indicates that an appropriate amount of AMF can improve the particle packing of mortar and increase the compactness. CRediT authorship contribution statement Hang He: Data curation, Formal analysis, Methodology, Writing - original draft, Writing - review & editing. Yuli Wang: Conceptualization, Formal analysis, Funding acquisition, Investigation, Supervision, Writing - review & editing. Junjie Wang: Conceptualization, Formal analysis, Investigation, Methodology, Supervision, Writing - original draft, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors appreciate the supports from the National Natural Science Foundation of China (51678220), Program for Innovative Research Team (in Science and Technology) (19IRTSTHN027) and Program for Innovation Scientists and Technicians Troop Construction Projects (CXTD2017088) are appreciated. The authors also
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appreciate the support from Department of Civil Engineering in Tsinghua University. Data availability All data used in this study is available upon requesting from the corresponding author. References [1] H.J. Kim, S. Na, W. Zhang, T. Sagawa, Effect of limestone powder and gypsum on the compressive strength mixture deign of BFS blended cement mortar, J. Adv. Concr. Technol. 15 (2) (2017) 67–80. [2] K. Turk, S. Demirhan, Effect of limestone powder on the rheological, mechanical and durability properties of ECC, Eur. J. Environ. Civ. Eng. 21 (9) (2017) 1–20. [3] P. Pliya, D. Cree, Limestone derived eggshell powder as a replacement in Portland cement mortar, Constr. Build. Mater. 95 (2015) 1–9. [4] L. Chong, C. Shi, J. Yang, H. Jia, Effect of limestone powder on the water stability of magnesium phosphate cement-based materials, Constr. Build. Mater. 148 (2017) 590–598. [5] J. Mohammadi, W. South, Effects of intergrinding 12% limestone with cement on properties of cement and mortar, J. Adv. Concr. Technol. 14 (5) (2016) 215– 228. [6] O.R. Khaleel, H.A. Razak, The effect of powder type on the setting time and self compactability of mortar, Constr. Build. Mater. 36 (36) (2012) 20–26. [7] N. Dave, A.K. Misra, A. Srivastava, S.K. Kaushik, Experimental analysis of strength and durability properties of quaternary cement binder and mortar, Constr. Build. Mater. 107 (2016) 117–124. [8] H. Temiz, F. Kantarcı, Investigation of durability of CEM II B-M mortars and concrete with limestone powder, calcite powder and fly ash, Constr. Build. Mater. 68 (15) (2014) 517–524. [9] Y. Senhadji, G. Escadeillas, M. Mouli, H. Khelafi, Benosman. Influence of natural pozzolan, silica fume and limestone fine on strength, acid resistance and microstructure of mortar, Powder Technol. 254 (254) (2014) 314–323. [10] S.C. Ji, S.W. Kang, W.H. Kim, A study on the strength properties of mortar using limestone powder, Appl. Mech. Mater. 749 (2015) 5. [11] S.A. Rizwan, T.A. Bier, Blends of limestone powder and fly-ash enhance the response of self-compacting mortars, Constr. Build. Mater. 27 (1) (2012) 398– 403. [12] A.M. Poppe, G.D. Schutter, Cement hydration in the presence of high filler contents, Cem. Concr. Res. 35 (12) (2005) 2290–2299. [13] K.D. Weerdt, M.B. Haha, G.L. Saout, K.O. Kjellsen, H. Justnes, B. Lothenbach, Hydration mechanisms of ternary Portland cements containing limestone powder and fly ash, Cem. Concr. Res. 41 (3) (2011) 279–291. [14] T. Matschei, B. Lothenbach, F.P. Glasser, The role of calcium carbonate in cement hydration, Cem. Concr. Res. 37 (4) (2007) 551–558. [15] G. Ye, X. Liu, G.D. Schutter, A.M. Poppe, L. Taerwe, Influence of limestone powder used as filler in SCC on hydration and microstructure of cement pastes, Cem. Concr. Compos. 29 (2) (2007) 94–102.
[16] W. Yuli, H. Hang, X. Liu, Influences of aggregate micro fines on the packing of fresh mortar and the performances of mortar, Compos. B Eng. 164 (2019) 493– 498. [17] H. He, Y. Wang, J. Wang, Effects of aggregate micro fines (AMF), aluminum sulfate and polypropylene fiber (PPF) on properties of machine-made sand concrete, Appl. Sci. 9 (11) (2019) 2250. [18] C. Shi, D. Wang, H. Jia, Role of limestone powder and its effect on durability of cement-based materials - a short review, J. Chinese Ceram. Soc. 45 (11) (2017) 1582–1593. [19] Y. Wang, H. He, F. He, Effect of slaked lime and aluminum sulfate on the properties of dry-mixed masonry mortar, Constr. Build. Mater. 180 (2018) 117–123. [20] E. Sakai, K. Masuda, Y. Kakinuma, Y. Aikawa, Effects of shape and packing density of powder particles on the fluidity of cement pastes with limestone powder, J. Adv. Concr. Technol. 7 (3) (2009) 347–354. [21] Y. Knop, A. Peled, Sustainable blended cements—influences of packing density on cement paste chemical efficiency, Materials 11 (4) (2018) 625. [22] Y. Knop, A. Peled, Setting behavior of blended cement with limestone: influence of particle size and content, Mater. Struct. 49 (1) (2015) 1–14. [23] Y. Knop, A. Peled, R. Cohen, Influences of limestone particle size distributions and contents on blended cement properties, Constr. Build. Mater. 71 (71) (2014) 26–34. [24] Y. Wang, Z. Jin, S. Liu, Y. Lei, S. Luo, Physical filling effect of aggregate micro fines in cement concrete, Constr. Build. Mater. 41 (2) (2013) 812–814. [25] P. Diederich, M. Mouret, F. Ponchon, Simple tools for achieving selfcompacting ability of concrete according to the nature of the limestone filler, Constr. Build. Mater. 48 (Complete) (2013) 840–852. [26] A.K.H. Kwan, W.W.S. Fung, Packing density measurement and modelling of fine aggregate and mortar, Cem. Concr. Compos. 31 (6) (2009) 349–357. [27] A.K.H. Kwan, V. Wong, W.W.S. Fung, A 3-parameter packing density model for angular rock aggregate particles, Powder Technol. 274 (2015) 154–162. [28] A.K.H. Kwan, K.W. Chan, V. Wong, A 3-parameter particle packing model incorporating the wedging effect, Powder Technol. 237 (3) (2013) 172–179. [29] V. Wong, A.K.H. Kwan, A 3-parameter model for packing density prediction of ternary mixes of spherical particles, Powder Technol. 268 (1) (2014) 357–367. [30] P. Ghoddousi, A.A.S. Javid, J. Sobhani, Effects of particle packing density on the stability and rheology of self-consolidating concrete containing mineral admixtures, Constr. Build. Mater. 53 (4) (2014) 102–109. [31] J. Wang, Steady-state chloride diffusion coefficient and chloride migration coefficient of cracks in concrete, J. Mater. Civil Eng. 29 (9) (2017), https://doi. org/10.1061/(asce)mt.1943-5533.0001966 04017117. [32] J. Xie, J. Wang, Y. Liu, Y. Wang, Comparison of Three Different Methods for Measuring Chloride Transport in Predamaged Concretes, J. Mater. Civil Eng. 32 (4) (2020), https://doi.org/10.1061/(ASCE)MT.1943-5533.0003108 04020033. [33] J. Xie, J. Zhao, J. Wang, C. Wang, P. Huang, C. Fang, Sulfate resistance of recycled aggregate concrete with GGBS and fly ash-based geopolymer, Materials 12 (8) (2019), https://doi.org/10.3390/ma12081247 1247. [34] J. Xie, W. Chen, J. Wang, C. Fang, B. Zhang, F. Liu, Coupling effects of recycled aggregate and GGBS/metakaolin on physicochemical properties of geopolymer concrete, Constr. Build. Mater. 226 (2019) 345–359, https://doi.org/10.1016/ j.conbuildmat.2019.07.311.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Compactness and hardened properties of machine-made sand mortar with aggregate micro fines Hang He a, Yuli Wang a, Junjie Wang b,⇑ a b
School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454003, China Department of Civil Engineering, Tsinghua University, Beijing 100084, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A compactness model together with
mathematical model was established. Optimum contents of AMF in MS was
obtained. Harden properties of MS mortar was
studied. The mechanism of AMF in MS mortar
was investigated and discussed.
a r t i c l e
i n f o
Article history: Received 20 November 2019 Received in revised form 26 February 2020 Accepted 20 March 2020
Keywords: Packing model Aggregate micro fines Machine-made sand Mortar Compactness
a b s t r a c t Machine-made sand (MS) has become a hot topic in construction materials due to the depletion and scarcity of natural river sand. Aggregate micro fines (AMF) of varied contents were included in MS during the manufacturing process. To quantify the filling role of AMF in MS mortars, a physical packing model together with the related mathematical model of fresh mortar was established by considering the water as a void component in fresh mortar mixture. A total 42 MS mortar mixes with different water-cement (W/ C) ratios (0.45, 0.5, 0.55, 0.6), different sand-cement (S:C) ratios (1:1, 2:1, 3:1) and different contents of AMF (0 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%) were prepared. The effects of AMF on the bulk density and compactness of fresh mortar, flexural and compressive strength, pore structure and microstructure of hardened mortar were investigated. The results show that the changing trend of compactness of fresh mortar is in consistent with the results of the bulk density and mechanical properties of the mortar. The optimal contents of AMF are found to be dependent on the W/C and S:C ratios. Our results show that, when S:C = 2.5:1, the optimal contents of AMF by weight of MS are 5 wt%, 10 wt%, 15 wt%, and 20 wt% for the mixes with the W/C ratios of 0.45, 0.5, 0.55, and 0.6, respectively; when the W/C = 0.5, the optimal AMF contents are 5 wt%, 10 wt%, and 15 wt% for the mixes with the S:C ratios of 1:1, 2:1, and 3:1, respectively. The results of mercury intrusion porosimetry (MIP) and electron backscattered diffraction (EBSD) show that the AMF can modify the pore structure and enhance the interfacial transition zone (ITZ) of mortar. Ó 2020 Elsevier Ltd. All rights reserved.
⇑ Corresponding author. E-mail address: [email protected] (J. Wang). https://doi.org/10.1016/j.conbuildmat.2020.118828 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
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H. He et al. / Construction and Building Materials 250 (2020) 118828
be re-established in order to explore the physical filling role of AMF in mortar or concrete. For this, Wang et al. [24] established a physical packing model and a mathematical model to characterize the physical filling role of the limestone AMF in concrete. A selfcompacting concrete mix with limestone AMF was designed by Diederich et al. [25] through establishing a relationship between the gap of particles and the demanding water. Kwan et al. [26] found that the error of the packing density was less with wetparticle packing than that with dry-particle packing. Kwan et al. [27,28] and Wong et al. [29] established a three-parameter model with a different-size particle packing and a better accuracy. The effect of packing density on the rheology and stability of selfcompacting concrete was studied by Ghoddoousi et al. [30], and their results showed that an accurate mix design of a selfcompacting concrete through wet-particle packing was possible. In summary, the optimal content of AMF in the MS could be different for different mortar or concrete mixtures. Although most experts believe that the physical filling role of AMF is usually the primary effect, but how to quantify this effect of AMF, especially for the fresh mortar and concrete, still needs further investigation. For this, mortars with different water-cement (W/C) ratio and different sand-cement (S:C) ratio were prepared. Based on the theoretical analysis, a physical and a mathematical packing model for the compactness of the fresh mortar were established. The bulk density and the mechanical properties of different mortar samples were studied. The optimum contents of AMF was determined and discussed together with the related mechanisms.
1. Introduction Aggregate micro fines (AMF) are the powders with a particle size usually less than 75 lm, which are produced inevitably during the production of machine-made sand (MS), and the content of AMF in MS can be as high as more than 20 wt%. The mineral and chemical compositions of AMF are the same as the mother rock of MS, which is different from natural sand. The influence of AMF on performance of concrete and mortar has been widely studied. For example, it was reported by Kim [1] that the incorporation of 4 wt% limestone AMF in slag mortar could increase the compressive strength of mortar. The results from Turk et al. [2] showed that the workability of cement-based materials decreased continuously with increase of limestone AMF, but the compressive strengths of 3 and 28 days could be improved depending on the AMF content. Pliya et al. [3] reported that the compressive strength of mortar was improved with 5 wt% AMF, but the flexural strength of mortar was decreased when more than 10 wt% AMF was added. Chong et al. [4] pointed out that AMF could shorten the setting time of cement-based materials and reduce the total hydration heat release, and they suggested the optimum content should not exceed 10 wt%. Mohammadi et al. [5] reported that the physical performance of mortar remained similar when the content of limestone AMF increased from 7.5 to 12 wt%. Khaleel et al. [6] showed that the rheology of fresh mortar and the mechanical performance were improved when AMF content was 10 wt%, and the size of pores inside the sample was obviously reduced. Dave et al. [7] found that the mechanical properties of mortar sample and the chloride penetration resistance were the best when the content of limestone AMF was 7.5–10 wt%. Temiz et al. [8] found that the compressive strength of mortar decreased as the increase of limestone AMF content, but the effect on the setting time was not significant. The results from Senhadji et al. [9] also showed a decrease of the compressive strength as the continuous increase of limestone AMF. Ji et al. [10] showed that the early strength of mortar was increased when the content of limestone AMF was 20 wt%. Rizwan et al. [11] pointed out that the strength of selfcompacting mortar was improved when the content of limestone AMF was 20 wt%, and the size of pores was decreased. AMF was reported to have chemical reaction effect [12–14] and physical filling effect [15–17], and most researchers believed that the primary effect is physical filling effect [18,19]. Sakai et al. [20] found that the rheology of fresh paste was influenced by particle morphology and packing density of AMF. The research from Knop and Peled [21,22] and Knop et al. [23] showed that the hydration rate, packing density and carbonation resistance of cement paste were improved when the cement clinker was mixed with AMF. AMF could change the packing density of mortar or concrete, therefore, and the packing model of fresh mortar or concrete needs
2. Experimental programs 2.1. Materials and mix design Type I ordinary Portland cement was used with a specific surface area of 362 m2/kg, the properties and oxide compositions of cement were shown in Tables 1 and 2. Both AMF and MS were made from limestone. The XRD pattern of AMF was shown in Fig. 1, and the specific surface area was 353 m2/kg. MS had a fineness modulus of 2.93, and its physical properties were shown in Table 3. Fig. 2 shown the particle size distributions of AMF and MS. The MS with 0 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt% and 25 wt% of AMF were obtained by separating the initial AMF from MS through a 75 lm sieve and then adding different AMF contents into MS. Mixing water was clean tap water. The mix design of different mortar samples was shown in Table 4. The W/C was kept constant as 0.5 in the first 18 mixes to study the effects of varied contents of AMF and different S:C ratios. From Mix No. 19 to No. 42, the S:C was kept constant as 2.5:1, and the W/C and AMF contents were varied.
Table 1 The properties of cement. Fineness %
Stability
Specific surface area (m2/kg)
0.9
Qualified
373
Setting time min
Flexural strength MPa
Compressive strength MPa
Initial
Final
3d
28d
3d
28d
159
224
6.0
10.2
24.8
49.9
Table 2 Oxide compositions of cement (%). CaO
SiO2
Al2O3
Fe2O3
MgO
Na2O
K2O
Loss
60.57
22.99
5.46
3.22
2.31
0.27
0.19
0.59
3
H. He et al. / Construction and Building Materials 250 (2020) 118828
Fig. 1. XRD patterns of AMF.
2.2. Test methods The mixing procedure was: the cement and AMF were mixed evenly in dry state and then water was added to form the fresh paste, followed by adding the MS at the last stage to form the fresh mortar. The mass of the fresh paste (mP) was weighed with a 500 ml container, and the bulk density of the paste (qP) was calculated by Eq. (1). The mass of the fresh mortar (mM) was weighed with a 1L container, and the bulk density of the fresh mortar (qM) was determined by Eq. (2). The size of mortar sample was 40 40 160 mm3. The flexural and compressive strength were measured at 7 and 28 days.
qP ¼
mP
ð1Þ
0:5 103
where qP is bulk density of the fresh paste, kg/m3; mP is the mass of fresh paste filled in the 500 ml container, kg.
qM ¼
mM
Fig. 2. Particle size distribution: (a) AMF, (b) MS.
ð2Þ
1 103
where qM is the bulk density of the fresh mortar, kg/m3; mM is the mass of fresh mortar in the 1 L container, kg. XRD was tested with a Smart-lab type X-ray diffractometer made by Rigaku company in Japan, Cu-Ka target, scanning angle 5–70°, scanning rate 10°/min. Mercury intrusion porosimetry (MIP) was tested with a Micromeritics AutoPore Iv 9510 type automated mercury porosimeter. Electron backscattered diffraction (EBSD) was tested with a Merlin Compact field emission scanning electron microscope made by German Zeiss company. 3. Results and analysis 3.1. Packing model (physical and mathematical) of fresh mortar The composition of fresh mortar prepared in this study includes water, cement, AMS and MS, in which cement and water constitutes a pure cement paste, the cement paste together with AMS
formed a mixed binder paste, and the mixed binder paste together with MS constitutes a fresh mortar sample. According to the volume of cement and water, the actual packing modes of cement paste could be divided into three cases, and they were shown as Fig. 3(a), (b), and (c), respectively. The first one is that the water is not enough to occupy the voids between cement particles, which is shown as Fig. 3(a); the second one is that the water is just enough to fill all the voids between cement particles, as is shown in Fig. 3(b); the third one is that the water is too much to fill all the voids between the cement particles, and the cement particles are suspended in the water, which is shown as Fig. 3(c). The first and second cases might not be able to have a good workability, and they will not be analyzed. To characterize the actual void ratio of the third case, water is treated as a void component in the fresh cement paste, and the actual void ratio (eP1) of the paste can thus be obtained by Eq. (3).
Table 3 Properties of MS. Fineness modulus
Loose density (kg/m3)
Apparent density (kg/m3)
Void ratio %
AMF %
2.93
1543
2685
42.5
3.6
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H. He et al. / Construction and Building Materials 250 (2020) 118828
Table 4 Mix design. Groups
AMF (wt. %)
No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No.
0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42
Raw Materials (kg/m3)
S:C
W/C
AMF
OPC
MS
Water
Superplasticizer
0 18 36 54 72 90 0 36 72 108 144 180 0 54 108 162 216 270 0 45 90 135 180 225 0 45 90 135 180 225 0 45 90 135 180 225 0 45 90 135 180 225
360
360 342 324 306 288 270 720 684 648 612 576 540 1080 1026 972 918 864 810 900 855 810 765 720 675 900 855 810 765 720 675 900 855 810 765 720 675 900 855 810 765 720 675
180
1.08
1:1
0.5
180
1.08
2:1
0.5
180
1.08
3:1
0.5
162
1.08
2.5:1
0.45
180
1.08
2.5:1
0.5
198
1.08
2.5:1
0.55
216
1.08
2.5:1
0.6
360
360
360
360
360
360
—cement particles,
(a)
—water
(c)
(b) Fig. 3. Physical packing model of fresh cement paste.
eP1 ¼
mW mW þmC =
qW
qP1
ð3Þ
where, eP1 is the actual void ratio of pure cement paste; mW is the mass of water, kg; qW is the density of mixing water, kg/m3; mC is
the mass of cement, kg; qP1 is the bulk density of pure cement paste, kg/m3. With the consideration of the role of AMF in the blended paste, the packing modes in Fig. 4 are presented. For a given W/C paste, the actual packing of the blended paste could change from Fig. 4
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H. He et al. / Construction and Building Materials 250 (2020) 118828
—cement particles,
—AMF particles,
(a)
—water
(c)
(b) Fig. 4. Physical packing model of fresh cement paste with AMF.
is greater than the void volume between MS particles, and the MS particles are separated by the paste like a suspension, in Fig. 5(c). Obviously, the third fresh mortar has highest flowability. For Fig. 5(c), the fresh mortar can be divided into two different parts: the first one is the fresh blended paste (including water, cement, and AMF), and the second one just is MS. Since the volume of the paste is greater than the voids of the MS particles, the MS particles are suspended in the fresh paste. In this way, to characterize the void ratio of fresh mortar sample, the packing mode of fresh mortar can be treated as that a part of the paste is replaced by the MS particles. In this case, the MS in the fresh mortar is a component with a void ratio of 0, that is to say, the compactness of MS component is 1. The void ratio of the fresh blended paste in the mortar (eP2) can be calculated by Eq. (4), and the void ratio (eM) of the fresh mortar can be obtained as Eq. (5). It can be seen from Eq. (4) that the void ratio (eP2) of the paste component in the fresh mortar gradually decreases as the content of AMF increases, and accordingly, the compactness gradually increases, but at the same time, the volume of MS gradually decreases, which results in the increase of the void ratio (eM) of the fresh mortar.
(a) to Fig. 4(c) with the increase of AMF content. It is seen from Fig. 4(a) that the particles of cement and AMF are suspended in the water; from Fig. 4(b) that the voids between particles of cement and AMF are just occupied by the water; from Fig. 4(c) that the water is too little to fill all the voids between particles of cement and AMF. Similarly, the blended pastes in Fig. 4(b)-(c) could not have a good flowing property, and they are not analyzed. For Fig. 4(a), the actual void ratio of the fresh paste (eP2) can be obtained by Eq. (4). Obviously, it can be seen from Eq. (4) that the eP2 gradually decreases as AMF increases, and accordingly, the compactness gradually increases.
eP2 ¼
mW mW þmC þmAMF =
qW
ð4Þ
qP2
where, eP2 is the actual void ratio of fresh blended paste containing AMF; mAMF is the mass of AMF, kg; qP2 is the bulk density of the fresh blended paste containing AMF, kg/m3. Depending on the volume of the fresh blended paste and MS in the fresh mortar, the actual packing modes of the fresh mortar could be divided as three cases: the first one is that the paste cannot fill the all voids between MS particles, and the MS particles cannot be separated by the paste, which is shown in Fig. 5(a); the second one is that the volume of the paste is just enough to fill the voids of MS particles, but the MS particles are still not separated by the paste, in Fig. 5(b) as the third one, the paste volume
—MS particles,
(a)
eM ¼
^Il ^Il P2 P2 ¼ ð m 1 yS 1 S = mW þmC þmP þmS Þ qS
ð5Þ
qM
where, eM is the void ratio of fresh mortar; yS is the volume fraction of MS particles in the fresh mortar; mS is the mass of the MS, kg; qS
—cement particles,
—AMF particles,
(b) Fig. 5. Physical packing model of fresh mortar with AMF and MS.
—water
(c)
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H. He et al. / Construction and Building Materials 250 (2020) 118828
is the apparent density of MS, kg/m3; qM is the bulk density of the fresh mortar, kg/m3. The compactness of the fresh mortar can be obtained Eq. (6).
bM ¼ 1 eM
ð6Þ
0.5, 0.55, and 0.6 are 5 wt%, 10 wt%, 15 wt%, and 20 wt%, respectively. It could be indicated that the optimal AMF content for the maximum bulk density increases gradually as the S:C ratio and the W/C ratio increase. A higher bulk density indicates a better the filling effect of the AMF.
where, bM represents the compactness of fresh mortar. 3.2. Results and analysis In order to explore the effects of AMF on the compactness and the bulk density of the fresh mortar, and mechanical properties of hardened mortar, as shown in Table 4, the mortars with different W/C ratio (0.45, 0.5, 0.55, 0.6), S:C ratios (1:1, 2:1, 3:1) and different contents of AMF in MS (0 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%) were prepared and investigated. 3.2.1. Bulk density The effect of AMF on the bulk density of the fresh mortar is shown in Fig. 6. From Fig. 6(a) and (b) that the bulk density of the fresh mortar first increases and then decreases as AMF increases. As is shown in Fig. 6(a), when the fresh mortars had a S:C at 1:1, 2:1, and 3:1, the corresponding optimum contents of AMF for the highest bulk density are 5 wt%, 10 wt%, and 15 wt%, respectively; as shown in Fig. 6(b), the corresponding optimum contents of AMF in the fresh mortars with different W/C of 0.45,
3.2.2. Flexural strength and compressive strength The influence of AMF contents on the flexural and compressive strength of hardened mortar with different S:C ratios is shown in Figs. 7 and 8. It can be found that the changes of the flexural strength and the compressive strength of mortars at the age of 7 and 28 days show the same trend, which both first increase and then decrease. As is shown in Fig. 7(a), the optimum contents of AMF for the highest 7-day flexural strength in the mortar samples with S:C ratio of 1:1, 2:1, 3:1 are 5 wt%, 10 wt%, and 15 wt%, respectively. The changing trend of flexural strength results at 28 days is similar as the results at 7 days. As is shown in Fig. 8 (a), to obtain the highest compressive strength at 7 days, the corresponding optimum contents of AMF in the mortars with S:C ratio of 1:1, 2:1, 3:1 are 5 wt%, 10 wt%, and 15 wt%, respectively. The corresponding compressive strength was increased by 14.9%, by 17.7%, and by 14.2%, respectively, compared with that of the mortar without AMF.
(a) S:C
(a) 7d
(b) W/C
(b) 28d
Fig. 6. Effect of AMF on bulk density results of the fresh mortar.
Fig. 7. Effect of AMF on flexural strength results of mortar with different S:C.
7
H. He et al. / Construction and Building Materials 250 (2020) 118828
The influence of AMF on the flexural and compressive strength of mortar samples with different W/C ratios is shown in Figs. 9 and 10. It can be seen that the two types of strength results of the mortars at 7 and 28 days both first increase and then decrease when the W/C ratio increases gradually from 0.45 to 0.6. As is shown in Fig. 9(a), the optimum contents of AMF for the highest flexural strength at 7 days in the mortars with the W/C ratio of 0.45, 0.5, 0.55 and 0.6 are 5 wt%, 10 wt%, 15 wt%, and 20 wt%, respectively. The related flexural strengths are 11.3, 10.2, 9.4 and 9.6 MPa, respectively. In Fig. 9(b), the changing trend of flexural strength at 28 days is similar as that of 7 days. Fig. 10(a) shows that the optimum contents of AMF for the highest 7-day compressive strength of the mortar with different W/C ratios of 0.45, 0.5, 0.55 and 0.6 are 5 wt%, 10 wt%, 15 wt%, and 20 wt%, respectively. The related compressive strengths are 50.4, 44.6, 39.6, and 39.4 MPa, respectively. The result of the compressive strength at 28 days (Fig. 10(b)) is the similar as that of 7 days. In summary, the changing trend of flexural and compressive strength of harden mortars is in consistent with that of bulk density of fresh mortars. Therefore, it can be indicated that a higher bulk density of fresh mortar corresponds to a better packing and compactness of the fresh mortar, and a higher flexural and compressive strength results of the harden mortar.
(a) 7d
(b) 28d Fig. 8. Effect of AMF on compressive strength results of mortar with different S:C.
3.2.3. Compactness The compactness of the fresh mortar mixture was calculated by Eq. (6), and the results are shown in Fig. 11. It can be seen that the compactness of the fresh mortar first increases and then decreases with the increase of AMF content. In Fig. 11(a), the mortar samples with the highest compactness had 5 wt%, 10 wt%, and 15 wt% of AMF when the S:C ratio is 1:1, 2:1, and 3:1, respectively; in Fig. 11(b), the optimum contents of AMF for the highest compactness in the mortars with W/C ratio of 0.45, 0.5, 0.55, and 0.6 are 5 wt%, 10 wt%, 15 wt%, and 20 wt%, respectively. The highest compactness indicates the optimal packing mode, and accordingly, the best physical filling effect of the AMF. This result is also in consistent with the results of strength and bulk density. 3.2.4. MIP and EBSD To analyze the influences of AMF on the pore structure of the paste and interfacial transition zone (ITZ) in mortar samples, MIP and EBSD tests were carried out. The mixes with a constant W/C at 0.5, S:C at 2:1 and different contents of AMF were tested. The effect of AMF on the pore structure of the mortar is shown in Table 2 and Fig. 12. It can be seen from Table 5 that the porosity of the mortar decreases first, and then increases with a continuous increase of
(a) 7d
(b) 28d Fig. 9. Effect of AMF on flexural strength results of mortar samples with different W/C.
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H. He et al. / Construction and Building Materials 250 (2020) 118828
(a) 7d
(a) S:C
(b) 28d
(b) W/C
Fig. 10. Effect of AMF on compressive strength results of mortar samples with different W/C.
AMF content, and the porosity of mortar reaches the least when AMF is 10 wt%. Effect of AMF on porosity is in consistent with that of compactness of fresh mortar and the results of the strength of mortar, showing that AMF could modify the porosity and microstructure, and improve mechanical properties of mortar. Fig. 12 shows that the sizes of most pores are less than 200 nm when AMF is 10 wt% and 15 wt%, and AMF significantly optimizes the size distribution of pores. The harmful pores become less when the AMF increases from 0 wt% to 15 wt%, but the harmful pores increase sharply when the content of AMF is more than 15 wt%. In a word, the optimum content of AMF could optimize the pore structure of mortar and decrease the porosity. To investigate the influence of AMF on the microstructure of the paste and interfacial transition zone (ITZ) in mortar samples, EBSD tests were conducted on the samples with the content of AMF 0 wt %, 10 wt% and 25 wt%, as shown in Fig. 13. Any microcracks developed in the sample could facilitate the ingress of harmful substances [31,32]. It can be seen from Fig. 13 that the microstructure and ITZ of hardened mortar is more compact when the content of AMF is 10 wt%, which suggests the enhanced ITZ between aggregates and the paste in mortar is beneficial to the mechanical properties [33,34].
Fig. 11. Effect of AMF on compactness of fresh mortar.
Fig. 12. Effect of AMF on pore size distribution of mortar.
In summary, a suitable or optimum content of AMF can improve the compactness of mortar, and related compactness can be characterized with the established mathematical model in this study. This is to say, the appropriate amount of AMF can optimize the par-
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H. He et al. / Construction and Building Materials 250 (2020) 118828 Table 5 Porosity of mortar with different AMF contents. AMF/wt. %
0
5
10
15
20
25
Porosity/%
22.11
21.99
18.13
18.42
22.81
23.56
Fig. 13. EBSD images of mortar with different AMF contents.
ticle packing, modify the pore structure and ITZ, and improve the physical properties of mortar. 4. Conclusions In order to obtain the optimum design of the MS mortar with AMF, a physical packing model and a mathematical model were established to discuss the role of W/C, S:C and AMF on the compactness of fresh mortar, which were further verified by the experimental work of 42 mixes with different W/C, S:C and AMF contents. The following conclusions could be made: (1) Both the bulk density of the fresh mortar mixture and the mechanical properties at 7 and 28 days of the mortar increased first and then decreased when AMF increased. The optimum contents of AMF in the mix design varied with W/C and S:C ratios. (2) The optimal contents of AMF in the mortars with a constant S:C at 2.5:1 are 5 wt%, 10 wt%, 15 wt%, and 20 wt% when the W/C ratios are 0.45, 0.5, 0.55, and 0.6, respectively. The optimal contents of AMF in the mortars (constant W/C at 0.5) are 5 wt%, 10 wt% and 15 wt% when the ratios of S:C are 1:1, 2:1, and 3:1, respectively. The optimum content of AMF increases if there is an increase of W/C or S:C ratio. (3) The compactness of the fresh mortar mixture can be calculated with the established model, and the results of compactness of the fresh mortar mixture are in consistent with those of bulk density and mechanical properties. The results
of MIP and EBSD showed that the optimum content of AMF could modify the pore structure of the paste and enhance the ITZ in hardened mortar, which further indicates that an appropriate amount of AMF can improve the particle packing of mortar and increase the compactness. CRediT authorship contribution statement Hang He: Data curation, Formal analysis, Methodology, Writing - original draft, Writing - review & editing. Yuli Wang: Conceptualization, Formal analysis, Funding acquisition, Investigation, Supervision, Writing - review & editing. Junjie Wang: Conceptualization, Formal analysis, Investigation, Methodology, Supervision, Writing - original draft, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors appreciate the supports from the National Natural Science Foundation of China (51678220), Program for Innovative Research Team (in Science and Technology) (19IRTSTHN027) and Program for Innovation Scientists and Technicians Troop Construction Projects (CXTD2017088) are appreciated. The authors also
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