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Characterization of mechanical behavior and mechanism of calcium carbonate whisker-reinforced cement mortar
Construction and Building Materials 66 (2014) 89–97
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Characterization of mechanical behavior and mechanism of calcium carbonate whisker-reinforced cement mortar Mingli Cao a,⇑, Cong Zhang b, Haifeng Lv b, Ling Xu a a b
Institute of Building Materials, School of Civil Engineering, Dalian University of Technology, Dalian 116000, Liaoning, China Institute of Structural Engineering, School of Civil Engineering, Dalian University of Technology, Dalian 116000, Liaoning, China
h i g h l i g h t s CaCO3 whisker was incorporated into cement mortar. Mechanical behavior and reinforcing mechanism were presented. CaCO3 whisker-reinforced mortar showed satisfied properties. An appropriate weak matrix is very beneficial to CaCO3 whiskers to play their roles.
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Article history: Received 13 March 2014 Received in revised form 24 April 2014 Accepted 18 May 2014 Available online 11 June 2014 Keywords: Calcium carbonate whisker Mortar Fibrous material Reinforcement Mechanical property Reinforcing mechanism
a b s t r a c t In order to reinforce cement mortar, a new kind of micro-fibrous material, calcium carbonate whisker (CaCO3 whisker), was incorporated in this study. Microstructure, mechanical properties and reinforcing mechanism of this composite were characterized. It was found that the addition of CaCO3 whisker improved not only the compressive and flexural strength of cement mortar, but also the load–deflection curves and work of fracture. Further work using mercury intrusion porosimetry tests confirmed the filler effect and the refining of the pore distribution of whiskers in cement mortar. Scanning electron microscopy showed that the microscopic mechanism primarily consists of whisker pullout, crack deflection, whisker-cement coalition pullout, whisker bridging and whisker breakage. These mechanisms are related to the matrix strength. As compared to the strong matrix, the weak matrix that was modified with CaCO3 whisker achieved the highest increase in strength and toughness of the cement mortar. This is likely attributed to the crack deflection mechanism, which is weakened by the strong interfacial bonding between the CaCO3 whisker and cement matrix in the stronger mortar matrix. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Cement based composites are very brittle, and, therefore, are typically reinforced with fibrous materials. A variety of fibrous materials are available commercially and their feasibility in cement based composites has been well documented [1–3]. Recently, there has been considerable interest in the development of nano- and micro-fibrous materials reinforced cement based composites [4]. Extensive research has been conducted in the field of processing technology and towards understanding of the mechanical behavior of the composite. It has been shown that the strength and toughness of brittle cementitious composites can be significantly improved by incorporating nano- or microfibers, which restrict and delay the propagation and coalescence of ⇑ Corresponding author. Tel.: +86 13940849152. E-mail address: [email protected] (M. Cao). http://dx.doi.org/10.1016/j.conbuildmat.2014.05.059 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.
cracks at the microscopic level. Carbon nanotubes and carbon nanofibers have also recently been used in cement based composites, and they have been found to improve not only the mechanical properties but also the intelligentization [5–10]. However, these carbon materials are expensive, which limits their application in cement based composites. In this study, a new type of inexpensive micro-fibrous material, CaCO3 whisker, was incorporated in order to improve the mechanical properties of cement mortar. CaCO3 whisker is a type of inorganic single crystal with a diameter of 0.5–2 lm and an aspect ratio of 20–60. It has excellent mechanical properties as demonstrated by the elastic modulus and the tensile strength being 410–710 GPa and 3–6 GPa, respectively. These basic properties make it suitable to serve as microfibers in cement based composites. Importantly, the production cost of CaCO3 whisker is very low, only about $230 per ton, which is very beneficial to decrease the production cost of microfiber reinforced cement
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based composites. The object of this study was to assess the mechanical properties as well as thoroughly understand the reinforcing mechanisms of CaCO3 whisker reinforced cement mortar. It is thought a better understanding of these materials will lead to significant progress in the characterization and utilization of this new cement based composite. Furthermore, a decrease in the production cost of micro-fibrous material reinforced cement based composites would lead to an increase in their use in large scale construction project. 2. Materials and methods Cement (PC 32.5 and PO 42.5R), ISO 679 standard sand, silica fume (specific surface 17.33 m2/g), fly ash (specific surface 0.41 m2/g, 45 lm sieve residue 23.6%) and CaCO3 whisker (length 20–30 lm, diameter 0.5–2 lm) were the raw materials used in this study. Their chemical components are shown in Table 1. The morphology and XRD pattern of CaCO3 whisker are shown in Fig. 1a–c, along with the TEM image of silica fume used in this study (Fig. 1d). Three types of mortar matrix were designed that had compressive strengths of 30, 40, and 60 MPa. The mix proportion is given in Table 2. Mass fraction (proportion of cement) of 5% (M5), 10% (M10), 15% (M15), and 20% (M20) CaCO3 whisker were employed. The amount of the water reducer (polycarboxylic acid type, ASTM C494 type F, water reducing ratio 24.1%) varied from 0 to 0.5 wt.% of binder content to ensure the mixtures had a similar flow and can be cast easily. Furthermore, an increased mixing time was employed to the mixtures with low water/binder ratio in this study according to Ref. [19], as illustrated in Fig. 2. The flow of the fresh mortar mixtures was evaluated according to ASTM C230 and ASTM C1437-07. Briefly, the diameter of the mortar along the four lines scribed in the table top was measured after the removal of the forming cone and dropping the sample 13 mm, 25 times. The resulting increase in average base diameter of the mortar mass was expressed as a percentage of the original base diameter, as shown in Table 3. The specimens were cast in three lifts, and each was vibrated on a vibration machine for 60 s. After curing for 24 h in a standard curing box of cement, the samples were demoulded and subjected to 20 °C water for 28 days as described by ISO 679-2009. The compressive strength was determined using 70.7 mm 70.7 mm 70.7 mm cubes and a pressure machine at a crosshead speed of 0.5 mm/min according to Chinese standard JGJ/T 70-2009. The loading method for the compression test is shown in Fig. 3a. Samples with a dimension of 40 mm 40 mm 160 mm were used to determine the flexural strength and the work of fracture. A computer controlled electro-hydraulic servo universal tester (WDW-300) was used at a cross-head speed of 0.02 mm/min according to ASTM C348 and ASTM C1609. The loading method used in the flexural test is shown in Fig. 3b. The work of fracture was calculated from the area covered underneath the load–deflection curve divided by twice the fracture surface area of the specimen. The microstructures of the composites were examined using a scanning electron microscopy (SEM, QUANTA 450). A matched energy dispersive spectrometer (EDS) was also used to analyze the elemental composition of the designated location. The pore size distribution of the samples was determined using a mercury intrusion porosimetry (MIP, AutoPore IV 9500).
3. Results and discussion 3.1. Compressive strength The average compressive strength values for all the mortar mixes are shown in Fig. 4. For the composites with a matrix strength of 30 MPa, the compressive strength ranged from 34 MPa to 38 MPa. Additionally, the composites containing 10 wt.% whisker had the highest compressive strength. Similar trends were found for the matrices with a compressive strength of 40 and 60 MPa. The relative increases in compressive strength
caused by whisker loading are shown in Fig. 5. It can be seen that as the maximum boost in compressive strength decreased, the matrix strength increased. The composites with 10 wt.% whisker and a 30 MPa matrix strength had a 13% increase, while the composites with 60 MPa matrix strength only achieved an 8.5% increase. 3.2. Flexural strength The flexural strength of the composites is plotted as a function of the whisker content in Fig. 6. As expected, the flexural strength of all the whisker reinforced mortars was higher than their comparisons. The flexural strength phenotype paralleled that of the compressive strength for the mortars, where it increased gradually and then decreased with an increase in whisker content. The relative increase in flexural strength caused by whisker loading is shown in Fig. 7. For the matrix with 40 MPa compressive strength, the addition of 10% whiskers resulted in the highest level of reinforcement after which the level of reinforcement plateaued. For example, a further increase in matrix strength to 60 MPa did not further enhance the reinforcing effect caused by whisker loading as compared to 40 MPa. The maximum flexural strength increase was approximately 23% in the 40 MPa matrix strength mortar. When these results are compared to carbon nanotube or other expensive microfiber reinforced cementitious composites, the amount of improvement in the mortar properties caused by whisker loading was satisfactory, as illustrated in Table 4. Unlike carbon nanotube or nanofiber, no special dispersion methods (e.g., chemical dispersant, ultrasonic dispersion, etc.) are needed to generate whisker reinforced cementitious composites. 3.3. Load–deflection curves The load–deflection curves for the three matrices, 30 MPa, 40 MPa and 60 MPa, are shown in Figs. 8–10, respectively. The addition of whiskers improved both the peak load and peak deflection of the mortars, but failed to change the brittle nature of cement mortar. Specifically, there was a drastic, yet incomplete, drop in the load after fracture, suggesting semi-stable crack propagation during the test. The peak deflection improved with an increase in whisker content, but decreased with the increase of matrix strength, although the peak loads of the mortars with a strong matrix were higher than those with weak matrix. There was little difference in the slope at the linear stage, suggesting that the deformability of the composite reinforced with whiskers was slightly higher than without whiskers. Nevertheless, the addition of whiskers, most likely due to their microscopic size, did not significantly improve the post-peak behavior. As shown in Table 5, numerous studies have focused on the load–deflection curves of cement based composites reinforced with nano- or micro-fibrous materials. However, within this work, only the peak deflection was compared, because a few other studies failed to obtain the descending or softening branch of the load–deflection curve. This is likely a reflection of a lack in stiffness in the test machine, where more elastic energy is stored in the soft
Table 1 Chemical components of raw materials (wt.%). Composition
CaO
SiO2
Al2O3
Fe2O3
CO2
MgO
K2O
SO3
Na2O
P2O5
MnO
Cement (PC 32.5) Cement (PO 42.5R) Silica fume Fly ash Whisker
52.61 61.13 0.81 6.61 54.93
3.45 21.45 93.47 50.96 0.29
6.95 5.24 0.16 30.61 0.11
2.44 2.89 0.10 5.61 0.07
– 2.37 – – 42.07
4.30 2.08 0.95 0.63 2.14
1.06 0.81 2.89 0.78 –
3.45 2.50 0.84 1.02 0.31
0.16 0.77 0.23 0.17 –
0.05 0.07 0.40 – –
0.08 0.06 0.04 – –
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Fig. 1. (a) macro-morphology, (b) micro-morphology and (c) XRD pattern of CaCO3 whisker (aragonite CaCO3) and (d) TEM image of silica fume.
Table 2 Mix proportion of plain mortar (M0). Matrix strength (MPa)
Cement
Water
Sand
Silica fume
Fly ash
CaCO3 whisker
30 40 60
1 1 1
0.55 0.45 0.3
3 3 3
0 0.05 0.1
0 0 0.2
0–0.2 0–0.2 0–0.2
3.4. Work of fracture
Fig. 2. Mixing procedure for fresh mortar mixtures (⁄ all the mixtures were mixed at least 120 s after adding water and the water reducer. A longer mixing time was applied to the mixtures with low water/binder ratio).
Table 3 Flow percentage of each mortar group (%). Whisker content (wt.%)
0
5
10
15
20
Matrix strength 30 MPa Matrix strength 40 MPa Matrix strength 60 MPa
92 88 87
90 91 91
93 90 89
95 93 92
90 87 92
Work of fracture represents the amount of energy required to create a unit of surface area. Analysis of the work of fracture for the mortars of interest is shown in Fig. 11. For each matrix, the work of fracture had an increasing trend directly related to whisker content, until the whisker content was beyond 10%, beyond which the work of fracture decreased. As compared to other matrices, the composites with moderate matrix strength achieved the highest values, which were reflected in the load–deflection curves. Furthermore, a further increase in the matrix strength would not improve the work of fracture of the whisker reinforced composite. As illustrated in Table 6, the effect of the CaCO3 whisker on the work of fracture results from this study was within previously published ranges. The largest boost in work of fracture was about 90% in CaCO3 whisker reinforced cement mortar. 3.5. Microstructure
load cell and could lead to a destabilization in crack growth. To compensate for this, two additional springs were added to prevent this drastic release of stored energy. It can be observed that the addition of CaCO3 whiskers significantly improved the peak deflection of the mortar composites, despite the absence of a dispersion method.
The aforementioned analyses on the addition of CaCO3 whisker to cement mortar concluded that within the studied parameters, a matrix strength of 40 MPa and a whisker content of 10 wt.% resulted in the most efficacious contribution by CaCO3 whisker to cement mortar. In order to explain the enhancement caused by
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Fig. 3. Loading method of (a) compression test and (b) flexural test.
Fig. 4. Compressive strengths of the mortar samples.
Fig. 6. Flexural strength of all mortars.
Fig. 5. Relative increment of compressive strength.
Fig. 7. Relative increment of flexural strength.
whisker loading in cement mortar, the microstructure of the composites was examined. 3.5.1. Pore structure In order to measure the porosity of the different mortars, mercury intrusion porosimetry was performed following 28 days’ curing (Table 7). It was observed that the addition of CaCO3 whisker resulted in a slight increase in the total porosity of the cement mortar. As compared to the weaker matrix, the whisker loading
had a greater influence on the porosity of the strong matrix. This is likely because the composites with a strong matrix had a relatively compact microstructure due to the low water to binder ratio and the use of mineral admixtures, and whisker loading created numerous new interfaces. Generally, a composite with a higher porosity has lower strength. However, based on the pore size distribution presented in Figs. 12–14, it was observed that the addition of whiskers increased the total porosity of all the mortars. However, these
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M. Cao et al. / Construction and Building Materials 66 (2014) 89–97 Table 4 Comparison of the increase in flexural strength in cement based composites reinforced with nano- or micro-reinforcers. Researchers
Reinforcing materials
Composites
Dispersion method
Flexural strength increase
Shah SP (2010) [5] Li GY (2005) [8] Cwirzen A (2009) [11] Li GY (2005) [8] Konsta G (2010) [12] Rashid K (2012) [13] Xu SL (2009) [14] Wang BM (2012) [15] Tyson BM (2011) [16] Cao ML (present work)
Carbon nanotube Carbon nanofiber Carbon nanotube Carbon nanotube Carbon nanotube Carbon nanotube Carbon nanotube Carbon nanotube Carbon nanofiber CaCO3 whisker
Paste Mortar Paste Mortar Paste Paste Mortar Mortar Paste Mortar
Yes Yes Yes Yes Yes Yes Yes Yes Yes No
21–30% 20.5% 10% 24.6% 25% 10–68% 5.4–20.7% 9–40% No positive effect 23%
Fig. 8. Load–deflection curves of the 30 MPa matrix strength composites.
Fig. 9. Load–deflection curves of the 40 MPa matrix strength composites.
pores were primarily less than 50 nm. Therefore, the addition of whiskers may be refining the pore distribution of cement mortar by having a filler effect. This suggests why the strength of whisker reinforced mortars studied was not lower than that of plain mortar. Furthermore, this also explains why the relative boost in compressive strength in the mortars with 30 MPa matrix strength was the highest among the three types of composites. 3.5.2. Microscopic mechanism The microscopic mechanism of reinforcement by CaCO3 whisker consists of whisker pullout, crack deflection, whisker-cement
Fig. 10. Load–deflection curves of the 60 MPa matrix strength composites.
coalition pullout, whisker bridging and whisker breakage, as shown in Figs. 15–17. However, the contribution of each microscopic mechanism to the reinforcing of the composite varies depending on the matrix strength. As reported in Refs. [19–21], the change of the water to binder ratio and the addition of mineral admixtures can change the bonding at the interfaces. Therefore, this observed change in the predominant reinforcement mechanism for each mortar matrix may be due to the changes in the interfacial bonding between the whisker and cement matrix. In the mortars with a matrix strength of 30 MPa, the whisker pullout and crack deflection mechanisms were observed. As shown in Fig. 15a, the whisker pullout in particular predominated, suggesting this is the dominant reinforcement mechanism when the bond strength between the whisker and cement matrix is very low. This was confirmed by examining the EDS patterns of the area of interest as displayed in Figs. 15b and 18a. The EDS pattern shows that the whisker surface had a composition based on calcium, carbon, oxygen, and a little silicon. If there was a strong bonding between the whisker and cement matrix, then the whisker surface would have primarily been composed of silicon. Nevertheless, energy can still be consumed via the friction between the whisker and cement matrix when the whisker is pulled out. Therefore, it is reasonable that the whisker pullout mechanism can toughen the cement mortars. However, whisker pullout is not the optimal mechanism by which to reinforce cement mortar, because the weak bond strength between the whisker and cement matrix is detrimental to the energy consumption by whisker pullout. The whisker-cement coalition pullout, whisker bridging and crack deflection mechanisms were found in the mortars with a 40 MPa matrix strength. Crack deflection in particular was enriched, suggesting that the matrix strength of 40 MPa is conducive to this mechanism. Furthermore, this mechanism
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Table 5 Peak deflection increase of cement based composites reinforced with nano- or micro-reinforcers. Researchers
Reinforcing materials
Composites
Dispersion method
Peak deflection increase
Bryan MT (2011) [17] Xu SL (2009) [14] Bryan MT (2011) [17] Wang BM (2012) [18] Cao ML (present work)
Carbon nanotube Carbon nanotube Carbon nanofiber Carbon nanotube CaCO3 whisker
Paste Mortar Paste Mortar Mortar
Yes Yes Yes Yes No
No positive effect (#) No positive effect () 24.4% (#) 18–97% (#) 43–85% ()
Note: (#) means the deflection in the work is actually the result of cross-head displacement of the test machine; (⁄) means the deflection value is measured by LVDT at the mid-span of the specimen.
Table 7 Porosity of mortars as measured by mercury intrusion porosimetry (%). Whisker content (wt.%)
0
5
10
15
20
Matrix strength 30 MPa Matrix strength 40 MPa Matrix strength 60 MPa
18.35 13.48 10.23
18.62 14.12 12.11
19.66 14.93 12.97
20.03 16.22 14.34
20.11 16.85 15.08
Fig. 11. Results of work of fracture.
assumes that the crack is initiated in the cement matrix and then expands around the whiskers. As the crack changes its direction, its orientation relative to the applied stress changes and, thus, will require more energy to extend further, as shown in Fig. 16c and d. Here, a crack can be seen extending from the cement matrix towards a whisker interface. This crack was then diverted along the interface and turned back towards the cement matrix, incurring energy consumption due to the change of the orientation of the stress and creation of a new interface. In this study, the ability of the whiskers in the 40 MPa mortars to strengthen and toughen may be mainly due to the crack deflection mechanism, since this mechanism was easily found within the composites. According to Evans’s model [22], the bond strength between the whisker and cement matrix will be appropriate weak if the crack deflection mechanism is lacking, but should be higher than when the whisker pullout is the predominant mechanism. This may also explain why crack deflection is the primary mechanism in the mortars with a 40 MPa matrix strength. While whisker-cement coalition pullout (Fig. 16a) and whisker bridging (Fig. 16b) are not the predominant reinforcing mechanisms in the mortars tested, they do contribute to strengthening and toughening of the mortars. For e example, the energy consumption by whisker-cement coalition pullout is directly related to the amount of friction generated. This friction is the result of
Fig. 12. Pore size distribution for the composites with a matrix strength of 30 MPa.
Fig. 13. Pore size distribution for the composites with a matrix strength of 40 MPa. Table 6 Comparison of studies on the effect of the addition of nano- or micro-reinforcers on the work of fracture. Researchers
Reinforcing materials
Composites
Dispersion method
Work of fracture increase
Bryan MT (2011) [17] Xu SL (2009) [14] Bryan MT (2011) [17] Wang BM (2012) [18] Cao ML (present work)
Carbon nanotube Carbon nanotube Carbon nanofiber Carbon nanotube CaCO3 whisker
Paste Mortar Paste Mortar Mortar
Yes Yes Yes Yes No
No positive effect No positive effect 53.7% 32.8–94% 36.7–90%
M. Cao et al. / Construction and Building Materials 66 (2014) 89–97
Fig. 14. Pore size distribution for the composites with a matrix strength of 60 MPa.
the relative motion of the cement matrix itself, because the cement encased around the whisker will be broken away from the cement matrix by overcoming the ultimate shear strength of cement
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matrix itself. This hypothesis is supported by the EDS patterns in the area of interest depicted in Figs. 16a and 18b. The EDS pattern shows that the whisker surface had a composition mainly based on calcium and silicon, suggesting that the whisker was encased by cement and there was strong bonding between the whisker and cement matrix. Bridging is a very common mechanism for fibrous materials to reinforce composites. Despite this, only low amounts of whisker bridging were found within the mortars with 40 MPa matrix strength. According to Chol’s explanation [23], the bonding at the interface determines the toughening contributed by whisker bridging if the whisker and matrix conditions are stated. A weak interface is conducive to whisker bridging, because partial debonding of the whisker along the interface can occur. While whisker bridging is a high-efficiency strengthening and toughening mechanism, the conditions that encourage whisker bridging in cement mortar remain to be elucidated. In the mortars with 60 MPa matrix strength, only two mechanisms of reinforcement, whisker-cement coalition pullout and whisker breakage, were found (Fig. 17). This type of whisker breakage implies very strong bonding between the whisker and cement matrix. The whisker prohibited and blunted the crack tip and propagation process when the cracks tried to cross the
Fig. 15. Mechanisms of reinforcement by CaCO3 whisker in the 30 MPa matrix, including (a) whisker pullout and (b) crack deflection.
Fig. 16. The reinforcing mechanisms of CaCO3 whisker in the 40 MPa matrix, including (a) whisker-cement coalition pullout, (b) whisker bridging and (c and d) crack deflection.
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Fig. 17. Reinforcing mechanisms of CaCO3 whisker in the matrix of 60 MPa, including (a) whisker-cement coalition pullout and (b) whisker breakage.
Fig. 18. EDS pattern of the areas of interest from (a) Fig. 16b and (b) Fig. 17a.
whisker. However, the stress concentration created by the crack tip may lead to the breakage of whiskers, and then the cracks would continue propagating. In this scenario, although the whisker breakage is via brittle fracturing, some energy is still consumed. This may explain why in the mortars with 60 MPa of matrix strength, the peak load is higher, but the peak deflection is smaller than weaker matrices (Figs. 8–10). Therefore, it can be concluded that suitably weak interfacial bonding is very necessary to achieve the largest beneficial effect by whisker loading on the strength and toughness of cement mortar. 4. Conclusions A new kind of less expensive fibrous material, calcium carbonate whisker, was incorporated into cement mortar to improve its mechanical properties. Conclusions drawn from this work are as follows: (1) The inclusion of whiskers only slightly improves the compressive strength of cement mortar, but significantly improves the flexural strength, load–deflection curves and work of fracture. Compared to other nano- or micro-fibrous materials reinforced cementitious composites, the whisker reinforced ones obtain a relatively satisfactory performance. (2) The optimal amount of whiskers in cement mortar is 10 wt.%, beyond which the mechanical properties will be weakened. The efficiency of whisker loading in a high strength matrix cannot be further improved, although the interfacial bonding between whisker and cement matrix is stronger than that of low strength matrix. This is likely
attributed to the crack deflection mechanism, which is weakened by strong interfacial bonding between whisker and cement matrix in a strong mortar matrix. (3) The addition of whiskers can refine the pore distribution in cement mortar. Although the addition of whiskers increased the total porosity of all the tested mortars, this was primarily an increase in pores with a size less than 50 nm. The filler effect and refining of pore distribution by whiskers account for the enhancement of the microstructure in cement mortar. (4) The microscopic mechanisms of reinforcement mainly consist of whisker pullout, crack deflection, whisker-cement coalition pullout, whisker bridging and whisker breakage. The occurrence of each mechanism is related to the matrix strength. When compared to the strong matrix, a weak matrix receives the most benefit from calcium carbonate whisker in terms of strengthening and toughening the cement mortar.
Acknowledgement The authors acknowledge the support of this work by The Natural Science Foundation of China under Grant No. 51102035. References [1] Eethar TD, Mahyuddin R. High strength characteristics of cement mortar reinforced with hybrid fibres. Constr Build Mater 2011;25:2240–7. [2] Eethar TD, Mahyuddin R. Development of high strength flowable mortar with hybrid fiber. Constr Build Mater 2010;24:1043–50.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Characterization of mechanical behavior and mechanism of calcium carbonate whisker-reinforced cement mortar Mingli Cao a,⇑, Cong Zhang b, Haifeng Lv b, Ling Xu a a b
Institute of Building Materials, School of Civil Engineering, Dalian University of Technology, Dalian 116000, Liaoning, China Institute of Structural Engineering, School of Civil Engineering, Dalian University of Technology, Dalian 116000, Liaoning, China
h i g h l i g h t s CaCO3 whisker was incorporated into cement mortar. Mechanical behavior and reinforcing mechanism were presented. CaCO3 whisker-reinforced mortar showed satisfied properties. An appropriate weak matrix is very beneficial to CaCO3 whiskers to play their roles.
a r t i c l e
i n f o
Article history: Received 13 March 2014 Received in revised form 24 April 2014 Accepted 18 May 2014 Available online 11 June 2014 Keywords: Calcium carbonate whisker Mortar Fibrous material Reinforcement Mechanical property Reinforcing mechanism
a b s t r a c t In order to reinforce cement mortar, a new kind of micro-fibrous material, calcium carbonate whisker (CaCO3 whisker), was incorporated in this study. Microstructure, mechanical properties and reinforcing mechanism of this composite were characterized. It was found that the addition of CaCO3 whisker improved not only the compressive and flexural strength of cement mortar, but also the load–deflection curves and work of fracture. Further work using mercury intrusion porosimetry tests confirmed the filler effect and the refining of the pore distribution of whiskers in cement mortar. Scanning electron microscopy showed that the microscopic mechanism primarily consists of whisker pullout, crack deflection, whisker-cement coalition pullout, whisker bridging and whisker breakage. These mechanisms are related to the matrix strength. As compared to the strong matrix, the weak matrix that was modified with CaCO3 whisker achieved the highest increase in strength and toughness of the cement mortar. This is likely attributed to the crack deflection mechanism, which is weakened by the strong interfacial bonding between the CaCO3 whisker and cement matrix in the stronger mortar matrix. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Cement based composites are very brittle, and, therefore, are typically reinforced with fibrous materials. A variety of fibrous materials are available commercially and their feasibility in cement based composites has been well documented [1–3]. Recently, there has been considerable interest in the development of nano- and micro-fibrous materials reinforced cement based composites [4]. Extensive research has been conducted in the field of processing technology and towards understanding of the mechanical behavior of the composite. It has been shown that the strength and toughness of brittle cementitious composites can be significantly improved by incorporating nano- or microfibers, which restrict and delay the propagation and coalescence of ⇑ Corresponding author. Tel.: +86 13940849152. E-mail address: [email protected] (M. Cao). http://dx.doi.org/10.1016/j.conbuildmat.2014.05.059 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.
cracks at the microscopic level. Carbon nanotubes and carbon nanofibers have also recently been used in cement based composites, and they have been found to improve not only the mechanical properties but also the intelligentization [5–10]. However, these carbon materials are expensive, which limits their application in cement based composites. In this study, a new type of inexpensive micro-fibrous material, CaCO3 whisker, was incorporated in order to improve the mechanical properties of cement mortar. CaCO3 whisker is a type of inorganic single crystal with a diameter of 0.5–2 lm and an aspect ratio of 20–60. It has excellent mechanical properties as demonstrated by the elastic modulus and the tensile strength being 410–710 GPa and 3–6 GPa, respectively. These basic properties make it suitable to serve as microfibers in cement based composites. Importantly, the production cost of CaCO3 whisker is very low, only about $230 per ton, which is very beneficial to decrease the production cost of microfiber reinforced cement
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based composites. The object of this study was to assess the mechanical properties as well as thoroughly understand the reinforcing mechanisms of CaCO3 whisker reinforced cement mortar. It is thought a better understanding of these materials will lead to significant progress in the characterization and utilization of this new cement based composite. Furthermore, a decrease in the production cost of micro-fibrous material reinforced cement based composites would lead to an increase in their use in large scale construction project. 2. Materials and methods Cement (PC 32.5 and PO 42.5R), ISO 679 standard sand, silica fume (specific surface 17.33 m2/g), fly ash (specific surface 0.41 m2/g, 45 lm sieve residue 23.6%) and CaCO3 whisker (length 20–30 lm, diameter 0.5–2 lm) were the raw materials used in this study. Their chemical components are shown in Table 1. The morphology and XRD pattern of CaCO3 whisker are shown in Fig. 1a–c, along with the TEM image of silica fume used in this study (Fig. 1d). Three types of mortar matrix were designed that had compressive strengths of 30, 40, and 60 MPa. The mix proportion is given in Table 2. Mass fraction (proportion of cement) of 5% (M5), 10% (M10), 15% (M15), and 20% (M20) CaCO3 whisker were employed. The amount of the water reducer (polycarboxylic acid type, ASTM C494 type F, water reducing ratio 24.1%) varied from 0 to 0.5 wt.% of binder content to ensure the mixtures had a similar flow and can be cast easily. Furthermore, an increased mixing time was employed to the mixtures with low water/binder ratio in this study according to Ref. [19], as illustrated in Fig. 2. The flow of the fresh mortar mixtures was evaluated according to ASTM C230 and ASTM C1437-07. Briefly, the diameter of the mortar along the four lines scribed in the table top was measured after the removal of the forming cone and dropping the sample 13 mm, 25 times. The resulting increase in average base diameter of the mortar mass was expressed as a percentage of the original base diameter, as shown in Table 3. The specimens were cast in three lifts, and each was vibrated on a vibration machine for 60 s. After curing for 24 h in a standard curing box of cement, the samples were demoulded and subjected to 20 °C water for 28 days as described by ISO 679-2009. The compressive strength was determined using 70.7 mm 70.7 mm 70.7 mm cubes and a pressure machine at a crosshead speed of 0.5 mm/min according to Chinese standard JGJ/T 70-2009. The loading method for the compression test is shown in Fig. 3a. Samples with a dimension of 40 mm 40 mm 160 mm were used to determine the flexural strength and the work of fracture. A computer controlled electro-hydraulic servo universal tester (WDW-300) was used at a cross-head speed of 0.02 mm/min according to ASTM C348 and ASTM C1609. The loading method used in the flexural test is shown in Fig. 3b. The work of fracture was calculated from the area covered underneath the load–deflection curve divided by twice the fracture surface area of the specimen. The microstructures of the composites were examined using a scanning electron microscopy (SEM, QUANTA 450). A matched energy dispersive spectrometer (EDS) was also used to analyze the elemental composition of the designated location. The pore size distribution of the samples was determined using a mercury intrusion porosimetry (MIP, AutoPore IV 9500).
3. Results and discussion 3.1. Compressive strength The average compressive strength values for all the mortar mixes are shown in Fig. 4. For the composites with a matrix strength of 30 MPa, the compressive strength ranged from 34 MPa to 38 MPa. Additionally, the composites containing 10 wt.% whisker had the highest compressive strength. Similar trends were found for the matrices with a compressive strength of 40 and 60 MPa. The relative increases in compressive strength
caused by whisker loading are shown in Fig. 5. It can be seen that as the maximum boost in compressive strength decreased, the matrix strength increased. The composites with 10 wt.% whisker and a 30 MPa matrix strength had a 13% increase, while the composites with 60 MPa matrix strength only achieved an 8.5% increase. 3.2. Flexural strength The flexural strength of the composites is plotted as a function of the whisker content in Fig. 6. As expected, the flexural strength of all the whisker reinforced mortars was higher than their comparisons. The flexural strength phenotype paralleled that of the compressive strength for the mortars, where it increased gradually and then decreased with an increase in whisker content. The relative increase in flexural strength caused by whisker loading is shown in Fig. 7. For the matrix with 40 MPa compressive strength, the addition of 10% whiskers resulted in the highest level of reinforcement after which the level of reinforcement plateaued. For example, a further increase in matrix strength to 60 MPa did not further enhance the reinforcing effect caused by whisker loading as compared to 40 MPa. The maximum flexural strength increase was approximately 23% in the 40 MPa matrix strength mortar. When these results are compared to carbon nanotube or other expensive microfiber reinforced cementitious composites, the amount of improvement in the mortar properties caused by whisker loading was satisfactory, as illustrated in Table 4. Unlike carbon nanotube or nanofiber, no special dispersion methods (e.g., chemical dispersant, ultrasonic dispersion, etc.) are needed to generate whisker reinforced cementitious composites. 3.3. Load–deflection curves The load–deflection curves for the three matrices, 30 MPa, 40 MPa and 60 MPa, are shown in Figs. 8–10, respectively. The addition of whiskers improved both the peak load and peak deflection of the mortars, but failed to change the brittle nature of cement mortar. Specifically, there was a drastic, yet incomplete, drop in the load after fracture, suggesting semi-stable crack propagation during the test. The peak deflection improved with an increase in whisker content, but decreased with the increase of matrix strength, although the peak loads of the mortars with a strong matrix were higher than those with weak matrix. There was little difference in the slope at the linear stage, suggesting that the deformability of the composite reinforced with whiskers was slightly higher than without whiskers. Nevertheless, the addition of whiskers, most likely due to their microscopic size, did not significantly improve the post-peak behavior. As shown in Table 5, numerous studies have focused on the load–deflection curves of cement based composites reinforced with nano- or micro-fibrous materials. However, within this work, only the peak deflection was compared, because a few other studies failed to obtain the descending or softening branch of the load–deflection curve. This is likely a reflection of a lack in stiffness in the test machine, where more elastic energy is stored in the soft
Table 1 Chemical components of raw materials (wt.%). Composition
CaO
SiO2
Al2O3
Fe2O3
CO2
MgO
K2O
SO3
Na2O
P2O5
MnO
Cement (PC 32.5) Cement (PO 42.5R) Silica fume Fly ash Whisker
52.61 61.13 0.81 6.61 54.93
3.45 21.45 93.47 50.96 0.29
6.95 5.24 0.16 30.61 0.11
2.44 2.89 0.10 5.61 0.07
– 2.37 – – 42.07
4.30 2.08 0.95 0.63 2.14
1.06 0.81 2.89 0.78 –
3.45 2.50 0.84 1.02 0.31
0.16 0.77 0.23 0.17 –
0.05 0.07 0.40 – –
0.08 0.06 0.04 – –
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Fig. 1. (a) macro-morphology, (b) micro-morphology and (c) XRD pattern of CaCO3 whisker (aragonite CaCO3) and (d) TEM image of silica fume.
Table 2 Mix proportion of plain mortar (M0). Matrix strength (MPa)
Cement
Water
Sand
Silica fume
Fly ash
CaCO3 whisker
30 40 60
1 1 1
0.55 0.45 0.3
3 3 3
0 0.05 0.1
0 0 0.2
0–0.2 0–0.2 0–0.2
3.4. Work of fracture
Fig. 2. Mixing procedure for fresh mortar mixtures (⁄ all the mixtures were mixed at least 120 s after adding water and the water reducer. A longer mixing time was applied to the mixtures with low water/binder ratio).
Table 3 Flow percentage of each mortar group (%). Whisker content (wt.%)
0
5
10
15
20
Matrix strength 30 MPa Matrix strength 40 MPa Matrix strength 60 MPa
92 88 87
90 91 91
93 90 89
95 93 92
90 87 92
Work of fracture represents the amount of energy required to create a unit of surface area. Analysis of the work of fracture for the mortars of interest is shown in Fig. 11. For each matrix, the work of fracture had an increasing trend directly related to whisker content, until the whisker content was beyond 10%, beyond which the work of fracture decreased. As compared to other matrices, the composites with moderate matrix strength achieved the highest values, which were reflected in the load–deflection curves. Furthermore, a further increase in the matrix strength would not improve the work of fracture of the whisker reinforced composite. As illustrated in Table 6, the effect of the CaCO3 whisker on the work of fracture results from this study was within previously published ranges. The largest boost in work of fracture was about 90% in CaCO3 whisker reinforced cement mortar. 3.5. Microstructure
load cell and could lead to a destabilization in crack growth. To compensate for this, two additional springs were added to prevent this drastic release of stored energy. It can be observed that the addition of CaCO3 whiskers significantly improved the peak deflection of the mortar composites, despite the absence of a dispersion method.
The aforementioned analyses on the addition of CaCO3 whisker to cement mortar concluded that within the studied parameters, a matrix strength of 40 MPa and a whisker content of 10 wt.% resulted in the most efficacious contribution by CaCO3 whisker to cement mortar. In order to explain the enhancement caused by
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Fig. 3. Loading method of (a) compression test and (b) flexural test.
Fig. 4. Compressive strengths of the mortar samples.
Fig. 6. Flexural strength of all mortars.
Fig. 5. Relative increment of compressive strength.
Fig. 7. Relative increment of flexural strength.
whisker loading in cement mortar, the microstructure of the composites was examined. 3.5.1. Pore structure In order to measure the porosity of the different mortars, mercury intrusion porosimetry was performed following 28 days’ curing (Table 7). It was observed that the addition of CaCO3 whisker resulted in a slight increase in the total porosity of the cement mortar. As compared to the weaker matrix, the whisker loading
had a greater influence on the porosity of the strong matrix. This is likely because the composites with a strong matrix had a relatively compact microstructure due to the low water to binder ratio and the use of mineral admixtures, and whisker loading created numerous new interfaces. Generally, a composite with a higher porosity has lower strength. However, based on the pore size distribution presented in Figs. 12–14, it was observed that the addition of whiskers increased the total porosity of all the mortars. However, these
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M. Cao et al. / Construction and Building Materials 66 (2014) 89–97 Table 4 Comparison of the increase in flexural strength in cement based composites reinforced with nano- or micro-reinforcers. Researchers
Reinforcing materials
Composites
Dispersion method
Flexural strength increase
Shah SP (2010) [5] Li GY (2005) [8] Cwirzen A (2009) [11] Li GY (2005) [8] Konsta G (2010) [12] Rashid K (2012) [13] Xu SL (2009) [14] Wang BM (2012) [15] Tyson BM (2011) [16] Cao ML (present work)
Carbon nanotube Carbon nanofiber Carbon nanotube Carbon nanotube Carbon nanotube Carbon nanotube Carbon nanotube Carbon nanotube Carbon nanofiber CaCO3 whisker
Paste Mortar Paste Mortar Paste Paste Mortar Mortar Paste Mortar
Yes Yes Yes Yes Yes Yes Yes Yes Yes No
21–30% 20.5% 10% 24.6% 25% 10–68% 5.4–20.7% 9–40% No positive effect 23%
Fig. 8. Load–deflection curves of the 30 MPa matrix strength composites.
Fig. 9. Load–deflection curves of the 40 MPa matrix strength composites.
pores were primarily less than 50 nm. Therefore, the addition of whiskers may be refining the pore distribution of cement mortar by having a filler effect. This suggests why the strength of whisker reinforced mortars studied was not lower than that of plain mortar. Furthermore, this also explains why the relative boost in compressive strength in the mortars with 30 MPa matrix strength was the highest among the three types of composites. 3.5.2. Microscopic mechanism The microscopic mechanism of reinforcement by CaCO3 whisker consists of whisker pullout, crack deflection, whisker-cement
Fig. 10. Load–deflection curves of the 60 MPa matrix strength composites.
coalition pullout, whisker bridging and whisker breakage, as shown in Figs. 15–17. However, the contribution of each microscopic mechanism to the reinforcing of the composite varies depending on the matrix strength. As reported in Refs. [19–21], the change of the water to binder ratio and the addition of mineral admixtures can change the bonding at the interfaces. Therefore, this observed change in the predominant reinforcement mechanism for each mortar matrix may be due to the changes in the interfacial bonding between the whisker and cement matrix. In the mortars with a matrix strength of 30 MPa, the whisker pullout and crack deflection mechanisms were observed. As shown in Fig. 15a, the whisker pullout in particular predominated, suggesting this is the dominant reinforcement mechanism when the bond strength between the whisker and cement matrix is very low. This was confirmed by examining the EDS patterns of the area of interest as displayed in Figs. 15b and 18a. The EDS pattern shows that the whisker surface had a composition based on calcium, carbon, oxygen, and a little silicon. If there was a strong bonding between the whisker and cement matrix, then the whisker surface would have primarily been composed of silicon. Nevertheless, energy can still be consumed via the friction between the whisker and cement matrix when the whisker is pulled out. Therefore, it is reasonable that the whisker pullout mechanism can toughen the cement mortars. However, whisker pullout is not the optimal mechanism by which to reinforce cement mortar, because the weak bond strength between the whisker and cement matrix is detrimental to the energy consumption by whisker pullout. The whisker-cement coalition pullout, whisker bridging and crack deflection mechanisms were found in the mortars with a 40 MPa matrix strength. Crack deflection in particular was enriched, suggesting that the matrix strength of 40 MPa is conducive to this mechanism. Furthermore, this mechanism
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Table 5 Peak deflection increase of cement based composites reinforced with nano- or micro-reinforcers. Researchers
Reinforcing materials
Composites
Dispersion method
Peak deflection increase
Bryan MT (2011) [17] Xu SL (2009) [14] Bryan MT (2011) [17] Wang BM (2012) [18] Cao ML (present work)
Carbon nanotube Carbon nanotube Carbon nanofiber Carbon nanotube CaCO3 whisker
Paste Mortar Paste Mortar Mortar
Yes Yes Yes Yes No
No positive effect (#) No positive effect () 24.4% (#) 18–97% (#) 43–85% ()
Note: (#) means the deflection in the work is actually the result of cross-head displacement of the test machine; (⁄) means the deflection value is measured by LVDT at the mid-span of the specimen.
Table 7 Porosity of mortars as measured by mercury intrusion porosimetry (%). Whisker content (wt.%)
0
5
10
15
20
Matrix strength 30 MPa Matrix strength 40 MPa Matrix strength 60 MPa
18.35 13.48 10.23
18.62 14.12 12.11
19.66 14.93 12.97
20.03 16.22 14.34
20.11 16.85 15.08
Fig. 11. Results of work of fracture.
assumes that the crack is initiated in the cement matrix and then expands around the whiskers. As the crack changes its direction, its orientation relative to the applied stress changes and, thus, will require more energy to extend further, as shown in Fig. 16c and d. Here, a crack can be seen extending from the cement matrix towards a whisker interface. This crack was then diverted along the interface and turned back towards the cement matrix, incurring energy consumption due to the change of the orientation of the stress and creation of a new interface. In this study, the ability of the whiskers in the 40 MPa mortars to strengthen and toughen may be mainly due to the crack deflection mechanism, since this mechanism was easily found within the composites. According to Evans’s model [22], the bond strength between the whisker and cement matrix will be appropriate weak if the crack deflection mechanism is lacking, but should be higher than when the whisker pullout is the predominant mechanism. This may also explain why crack deflection is the primary mechanism in the mortars with a 40 MPa matrix strength. While whisker-cement coalition pullout (Fig. 16a) and whisker bridging (Fig. 16b) are not the predominant reinforcing mechanisms in the mortars tested, they do contribute to strengthening and toughening of the mortars. For e example, the energy consumption by whisker-cement coalition pullout is directly related to the amount of friction generated. This friction is the result of
Fig. 12. Pore size distribution for the composites with a matrix strength of 30 MPa.
Fig. 13. Pore size distribution for the composites with a matrix strength of 40 MPa. Table 6 Comparison of studies on the effect of the addition of nano- or micro-reinforcers on the work of fracture. Researchers
Reinforcing materials
Composites
Dispersion method
Work of fracture increase
Bryan MT (2011) [17] Xu SL (2009) [14] Bryan MT (2011) [17] Wang BM (2012) [18] Cao ML (present work)
Carbon nanotube Carbon nanotube Carbon nanofiber Carbon nanotube CaCO3 whisker
Paste Mortar Paste Mortar Mortar
Yes Yes Yes Yes No
No positive effect No positive effect 53.7% 32.8–94% 36.7–90%
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Fig. 14. Pore size distribution for the composites with a matrix strength of 60 MPa.
the relative motion of the cement matrix itself, because the cement encased around the whisker will be broken away from the cement matrix by overcoming the ultimate shear strength of cement
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matrix itself. This hypothesis is supported by the EDS patterns in the area of interest depicted in Figs. 16a and 18b. The EDS pattern shows that the whisker surface had a composition mainly based on calcium and silicon, suggesting that the whisker was encased by cement and there was strong bonding between the whisker and cement matrix. Bridging is a very common mechanism for fibrous materials to reinforce composites. Despite this, only low amounts of whisker bridging were found within the mortars with 40 MPa matrix strength. According to Chol’s explanation [23], the bonding at the interface determines the toughening contributed by whisker bridging if the whisker and matrix conditions are stated. A weak interface is conducive to whisker bridging, because partial debonding of the whisker along the interface can occur. While whisker bridging is a high-efficiency strengthening and toughening mechanism, the conditions that encourage whisker bridging in cement mortar remain to be elucidated. In the mortars with 60 MPa matrix strength, only two mechanisms of reinforcement, whisker-cement coalition pullout and whisker breakage, were found (Fig. 17). This type of whisker breakage implies very strong bonding between the whisker and cement matrix. The whisker prohibited and blunted the crack tip and propagation process when the cracks tried to cross the
Fig. 15. Mechanisms of reinforcement by CaCO3 whisker in the 30 MPa matrix, including (a) whisker pullout and (b) crack deflection.
Fig. 16. The reinforcing mechanisms of CaCO3 whisker in the 40 MPa matrix, including (a) whisker-cement coalition pullout, (b) whisker bridging and (c and d) crack deflection.
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Fig. 17. Reinforcing mechanisms of CaCO3 whisker in the matrix of 60 MPa, including (a) whisker-cement coalition pullout and (b) whisker breakage.
Fig. 18. EDS pattern of the areas of interest from (a) Fig. 16b and (b) Fig. 17a.
whisker. However, the stress concentration created by the crack tip may lead to the breakage of whiskers, and then the cracks would continue propagating. In this scenario, although the whisker breakage is via brittle fracturing, some energy is still consumed. This may explain why in the mortars with 60 MPa of matrix strength, the peak load is higher, but the peak deflection is smaller than weaker matrices (Figs. 8–10). Therefore, it can be concluded that suitably weak interfacial bonding is very necessary to achieve the largest beneficial effect by whisker loading on the strength and toughness of cement mortar. 4. Conclusions A new kind of less expensive fibrous material, calcium carbonate whisker, was incorporated into cement mortar to improve its mechanical properties. Conclusions drawn from this work are as follows: (1) The inclusion of whiskers only slightly improves the compressive strength of cement mortar, but significantly improves the flexural strength, load–deflection curves and work of fracture. Compared to other nano- or micro-fibrous materials reinforced cementitious composites, the whisker reinforced ones obtain a relatively satisfactory performance. (2) The optimal amount of whiskers in cement mortar is 10 wt.%, beyond which the mechanical properties will be weakened. The efficiency of whisker loading in a high strength matrix cannot be further improved, although the interfacial bonding between whisker and cement matrix is stronger than that of low strength matrix. This is likely
attributed to the crack deflection mechanism, which is weakened by strong interfacial bonding between whisker and cement matrix in a strong mortar matrix. (3) The addition of whiskers can refine the pore distribution in cement mortar. Although the addition of whiskers increased the total porosity of all the tested mortars, this was primarily an increase in pores with a size less than 50 nm. The filler effect and refining of pore distribution by whiskers account for the enhancement of the microstructure in cement mortar. (4) The microscopic mechanisms of reinforcement mainly consist of whisker pullout, crack deflection, whisker-cement coalition pullout, whisker bridging and whisker breakage. The occurrence of each mechanism is related to the matrix strength. When compared to the strong matrix, a weak matrix receives the most benefit from calcium carbonate whisker in terms of strengthening and toughening the cement mortar.
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