Calcium carbonate (CaCO3) whisker, as a new type of microfibrous material, has been extensively used in the reinforcement of cementitious materials. However, the combined effect of CaCO3 whisker and fly ash on mechanical properties of cementitious materials under high temperatures was still unknown. In this study, the coupling effect of CaCO3 whisker, and fly ash on mechanical properties of the cement was investigated. Two sets of cement mortars were fabricated, including CaCO3 whisker-based mortar which contained 0 wt.%, 5 wt.%, 10 wt.%, 15 wt.%, and 20 wt.% CaCO3 whisker as cement substitution and CaCO3 whisker-based fly ash mortar which contained 30 wt.% fly ash in addition to 0 wt.%, 5 wt.%, 10 wt.%, 15 wt.%, and 20 wt.% CaCO3 whisker as cement substitution. Mass loss, compressive strength, and flexural strength of these two sets of specimens before and after being subjected to high temperatures of 200°C, 400°C, 600°C, 800°C, and 1000°C were measured. Based on the results of the aforementioned tests, load-deflection test was performed on the specimen which exhibited the superior performance to further study its mechanical behavior after exposure to high temperatures. Moreover, microstructural analysis, such as mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM), was conducted to reveal the damage mechanism of high temperature and to illustrate the combined effect of CaCO3 whisker and fly ash on high-temperature resistance of the cement. Results showed that fly ash could improve the high-temperature performance of CaCO3 whisker-based mortar before 600°C and limit the loss of strength after 600°C.
Fire is one of the fatal threats which seriously affect human life, property security, and economic development in the world. Concrete, as a widely used key material in the construction of infrastructural facilities, is noncombustible and performs well when exposed to high temperatures because the components of it, such as cement and aggregates, are chemically inert. However, the physical and chemical deteriorations of cementitious materials due to moisture loss and decomposition of hydrated products at high temperatures may result in the generation of cracks and the loss to mechanical strengths of concrete, finally lead to the structural damage [
As is well known, concrete is a brittle material which has low flexural strength, and in practice, a variety of fibrous materials, such as steel, carbon, polymer, and natural fibers, are often employed to enhance its toughness [
Moreover, as global warming continues, environmental protection should be taken into consideration prior to the construction of concrete structures. However, during the manufacturing of cement, environmental problem associated with huge emissions of greenhouse gases, mainly carbon dioxide (CO2), has raised concern for sustainable development, based on the fact that the production of every tonne of ordinary Portland cement (OPC) releases an equivalent amount of CO2 into the atmosphere [
Fly ash, as one of the most important SCMs, is a by-product of coal power stations and therefore using it as cement substitution not only reduce the production of cement but also provide an efficient method for disposal of waste material compared with other conventional disposal methods. Silicon dioxide (SiO2) and aluminium oxide (Al2O3), as main chemical compositions of fly ash, can react with calcium hydroxide (CH) produced during cement hydration to form calcium silicate hydrate (CSH) and calcium aluminate hydrate (CAH), thereby densifying the paste and achieving higher strength and better durability, which is known as pozzolanic effect of fly ash. Generally, the cement replacement by fly ash is limited to be around 15%–30% by mass of the total binding material. Previous studies have pointed out the superiority of fly ash when it is moderately incorporated into cementitious materials as cement replacement. In particular, the mechanical strength of fly ash concrete after exposure to high temperatures is still comparatively advantageous, although the overall strength of concrete decreases with the increase in maximum temperature. The study of Ibrahim et al. has demonstrated that, after exposure to high temperatures of up to 700°C, mechanical properties of mortars were enhanced when 25% of cement was replaced by fly ash [
To the best of our knowledge, the combined effect of CaCO3 whisker and fly ash on mechanical properties of cementitious materials after exposure to elevated temperatures has not been reported. Better understanding this knowledge could extend the application of CaCO3 whisker and facilitate the utilization of fly ash, thereby lowering the amount of other expensive fibers, reducing the overall cost of concrete projects, and promoting the sustainable development. In the current study, systematic experiments, including mass loss test, compressive strength test, flexural strength test, and load-deflection curve test, were carried out to study the influence of CaCO3 whisker and fly ash on properties of cement mortars before and after exposure to high temperatures of 200°C, 400°C, 600°C, 800°C, and 1000°C. Meanwhile, the effect of CaCO3 whisker on the high-temperature performance of cement mortars was also examined as a reference to see whether fly ash could have a positive influence on the high-temperature performance of CaCO3 whisker-based cement mortar and thus promoting the sustainable development. Furthermore, the deterioration mechanism of high temperatures and the interaction mechanism of CaCO3 whisker and fly ash with the cement were explored by MIP and SEM.
Raw materials used in this study include cement, fly ash, CaCO3 whisker, and natural river sand. Cement was OPC 42.5 that met the requirements of ASTM C150 specification [
(a) Macroscopic and (b) microscopic images of CaCO3 whisker.
Chemical compositions of raw materials (wt.%).
Oxides | CaO | SiO2 | Al2O3 | Fe2O3 | MgO | SO3 | CO2 | Na2O | K2O | LOI |
---|---|---|---|---|---|---|---|---|---|---|
Cement | 59.42 | 19.93 | 4.88 | 4.31 | 2.02 | 2.80 | — | 1.14 | 0.82 | 4.68 |
Fly ash | 4.86 | 61.43 | 22.60 | 5.45 | 1.04 | 0.16 | — | 0.72 | 1.23 | 2.51 |
Whisker | 54.93 | 0.29 | 0.11 | 0.07 | 2.14 | 0.31 | 42.07 | — | — | — |
In order to investigate the mechanical properties and microstructural mechanism of cement mortars containing CaCO3 whisker and fly ash, different types of specimens were prepared. Cement, natural river sand, and water were used in all the mortar mixtures, along with CaCO3 whisker and fly ash in specific mixtures. To be exact, cement mortars were classified into two sets, which were CaCO3 whisker-based mortar and CaCO3 whisker-based fly ash mortar. The first set was CaCO3 whisker-based mortar which contained OPC and different contents of CaCO3 whisker (e.g., 0 wt.%, 5 wt.%, 10 wt.%, 15 wt.%, and 20 wt.%) as the binder, while the second set was CaCO3 whisker-based fly ash mortar which was fabricated with OPC and 30 wt.% fly ash as a fixed substitution of cement in addition to 0 wt.%, 5 wt.%, 10 wt.%, 15 wt.%, and 20 wt.% CaCO3 whisker. Therefore, a total number of ten mortar mixtures were designed in this study. It should be noted that, for all the mixtures, a constant water-to-binder ratio of 0.4, binder-to-sand ratio of 1 : 2.75, and a total cementitious material content of 500 kg/m3 were used. Details of the mix proportions are listed in Table
Mix proportions of mortars.
Set | Mix proportions |
|
Cement (kg/m3) | CaCO3 whisker (kg/m3) | Fly ash (kg/m3) | Sand (kg/m3) | Water (kg/m3) | Superplasticizer (L/m3) |
---|---|---|---|---|---|---|---|---|
1 | C0 (control) | 0.4 | 500 | 0 | 0 | 1375 | 200 | 0.37 |
C5 | 0.4 | 475 | 25 | 0 | 1375 | 200 | 0.43 | |
C10 | 0.4 | 450 | 50 | 0 | 1375 | 200 | 0.47 | |
C15 | 0.4 | 425 | 75 | 0 | 1375 | 200 | 0.54 | |
C20 | 0.4 | 400 | 100 | 0 | 1375 | 200 | 0.59 | |
|
||||||||
2 | C0F30 | 0.4 | 350 | 0 | 150 | 1375 | 200 | 0.23 |
C5F30 | 0.4 | 325 | 25 | 150 | 1375 | 200 | 0.25 | |
C10F30 | 0.4 | 300 | 50 | 150 | 1375 | 200 | 0.28 | |
C15F30 | 0.4 | 275 | 75 | 150 | 1375 | 200 | 0.31 | |
C20F30 | 0.4 | 250 | 100 | 150 | 1375 | 200 | 0.39 |
Note: C and F represent CaCO3 whisker and fly ash, respectively.
With respect to specimen fabrication, raw materials, such as cement, CaCO3 whisker, fly ash, and sand, according to the corresponding mix proportions, were mixed together for 3 min by an electrically driven mixer before adding the mixing water. Then, 70% of the mixing water was added to the mixture and mixed for another 3 min. Thereafter, the water-reducing agent together with the rest of the mixing water was added, and the mixture was mixed for 1 min to ensure the homogeneity and uniformity of it. The mixture was subsequently placed into stainless steel molds and vibrated on a vibration machine for 2 min to achieve a desirable compactness. And after 24 h of curing, the molded mortar specimens were removed from the molds and left to continue curing for 27 d under a temperature of 20°C and a relative humidity of 95% until the testing day. Cubic specimens of 50 × 50 × 50 mm were prepared for the test of compressive strength and prismatic specimens of 40 × 40 × 160 mm were prepared for the tests of flexural strength and load-deflection curve.
High temperatures of 200°C, 400°C, 600°C, 800°C, and 1000°C were selected in this investigation, and these high temperatures were achieved through using an electrically heated furnace which covered a temperature range of 20–1000°C and a constant heating rate of 10°C/min. When the prescribed temperature was reached, the temperature of the furnace was kept constantly for 1h so as to ensure the thermal steady state [
The evaporation of moisture and decomposition of hydration products of cement paste under high temperature will lead to the mass change of cement mortar, which is an important indicator to deteriorations of cement mortar after exposure to high temperature. Therefore, the mass of mortar specimens before and after exposure to various high temperatures was recorded prior to the mechanical property test to calculate the mass loss during the heating process. It should be noted that, before exposure to high temperatures, specimens were put into a drying oven for 24 h at 100°C.
The compressive strength of cubic specimens before and after exposure to different high temperatures was obtained through operating a hydraulic testing machine according to ASTM C109 [
In this investigation, the mortar specimen that exhibited the optimal high-temperature resistance based on the results of the mechanical experiments was selected as the optimal specimen for further microstructural analysis, together with the ordinary specimen C0 which contained only OPC as the binder for comparison.
There is a general agreement that the mechanical properties of a material are closely related to its microstructure [
The microscopic images of the ordinary specimen and the optimal specimen before and after exposure to different high temperatures were also acquired by scanning electron microscopy (SEM) analysis. The specimens should be coated with gold to be conductive prior to the test.
When exposed to various high temperatures, the mass of a substance will be altered due to different thermal stability of its components. As a result, the mass variation is a direct index of the deterioration of the material after exposure to high temperature. Previous investigations have indicated that the mass loss of the cementitious material after exposure to high temperatures is mainly due to the evaporation of physically bounded water (80–150°C), the dehydration of C-S-H, AFm, and AFt (≤350°C), the decomposition of CH (400–550°C), and the decarbonation of CaCO3 (≥600°C) [
Mass loss of (a) CaCO3 whisker-based specimens and (b) CaCO3 whisker-based fly ash specimens after exposure to various temperatures.
With respect to CaCO3 whisker-based mortar specimens, as shown in Figure
Regarding CaCO3 whisker-based fly ash specimens, as shown in Figure
In general, exposure to high temperature will lead to the moisture loss of the mortar specimen which is mainly due to the evaporation of bound water and the dehydration of hydration products, and the mass loss increased with the increase in temperature. In addition, a larger CaCO3 whisker content could lead to a higher mass loss when the temperature was between 600 and 1000°C.
After measuring the mass of CaCO3 whisker-based specimens before and after exposure to high temperatures, the mechanical properties of those specimens were immediately evaluated. The compressive strength and flexural strength of CaCO3 whisker-based specimens are displayed in Figures
Compressive strength of (a) CaCO3 whisker-based specimens and (b) CaCO3 whisker-based fly ash specimens before and after exposure to various temperatures.
Flexural strength of (a) CaCO3 whisker-based specimens and (b) CaCO3 whisker-based fly ash specimens before and after exposure to various temperatures.
Before exposure to high temperatures, both the compressive strength and flexural strength of CaCO3 whisker-based specimen were improved rapidly with the increase in CaCO3 whisker content until this content was up to 10 wt.%; thereafter, the strength enhancement was negatively correlated with the increase in CaCO3 whisker content, as shown in Figures
After exposure to 200°C, the compressive strength of CaCO3 whisker-based specimen was approximately the same with the strength at 20°C, while for CaCO3 whisker-based fly ash specimen, the compressive strength showed a slight increase. Furthermore, an evident improvement was observed in the compressive strength of all the specimens when exposed to 400°C, and C10F30 had the highest compressive strength at 400°C, with the strength increased by 23.8% compared with the unheated C10F30. Relatively lower improvements were found in the compressive strength of C0 and C10 at 400°C, which were 16.2% and 18.4%, respectively, indicating that the presence of fly ash had a positive effect on the compressive strength of CaCO3 whisker-based specimens. The improvement in strength is likely due to the acceleration in cement hydration at high temperatures. Cement hydration process will proceed at a higher speed with the increase in temperature, and meanwhile, high temperature will promote more generation of high-density C-S-H which has excellent high-temperature resistance [
When the temperature further increased to 600°C, both the compressive strength and flexural strength of CaCO3 whisker-based specimens showed a remarkable decrease, and the reason may be that the combined effect of moisture loss and decomposition of CH increases the porosity of the specimen and makes the microstructure of the specimen coarser and at the same time, weakens the bond between whiskers and cement paste; therefore, the strength of CaCO3 whisker-based specimens declined accordingly. It is worth noting that, although both the compressive strength and flexural strength of all the CaCO3 whisker-based specimens reduced significantly at 600°C, the strength of fly ash specimens still had comparative advantage in high-temperature resistance, which indicated that fly ash could improve the high-temperature resistance of CaCO3 whisker-based specimens.
Exposure to 800°C led to a sharp decrease in both compressive strength and flexural strength of specimens, which is mainly due to the decomposition of CaCO3 and CSH at about 800°C [
To sum up, the incorporation of CaCO3 whisker could improve both compressive strength and flexural strength of mortar specimens when the temperature was less than 600°C, and the optimal dosage of it was 10 wt.%. At 800°C and 1000°C, the strength of specimens rapidly decreased with the increase in CaCO3 whisker content; therefore, the specimens with CaCO3 whisker exhibited lower strength than C0. The presence of fly ash enhanced the high-temperature performance of CaCO3 whisker-based specimens before 600°C and limited the loss of strength after 600°C. According to the results, the ordinary mortar (C0) and the mortar containing 10 wt.% CaCO3 whisker and 30 wt.% fly ash (C10F30) were selected to perform the load-deflection test and the microscopic tests to explore the mechanism behind.
Load-deflection curves of the ordinary mortar and the mortar containing 10 wt.% CaCO3 whisker and 30 wt.% fly ash before and after exposure to different high temperatures were measured through the three-point bending test, and the results are presented in Figure
Load-deflection curves of (a) C0 and (b) C10F30 before and after exposure to various temperatures.
It is examined that the maximum load of both C0 and C10F30 increased with the increase in temperature until a maximal value was reached at 400°C, while the maximum load of the two specimens dropped rapidly after being exposed to 600–1000°C. Although the maximum load of the two specimens decreased at 600°C, the maximum load of C10F30 was still higher than that of C0, which is mainly due to the fact that CaCO3 whisker can not only reinforce the specimen but also bridge microcracks in the specimen. Besides, the slope of the load-deflection curve of C10F30 in the linear stage before 600°C was a little bigger than that of C0, indicating that the flexural stiffness of C10F30 was higher than that of C0. Meanwhile, it is also worthy to note that the maximum deflection of C10F30 was a little larger than that of C0 before 600°C, demonstrating that the addition of CaCO3 whisker and fly ash could improve the deformability and toughness of the specimen. This may be attributed to the fact that CaCO3 whisker itself has high tensile strength, and the incorporation of CaCO3 whisker can improve the flexural behavior of the specimen. Nevertheless, the improvement in flexural toughness was not remarkable, which can be attributed to the tiny size of whiskers, and this may be the reason to encourage the combined use of CaCO3 whiskers with other large-sized fibers, so as to improve the multiscale cracking resistance of cementitious materials.
The significant decline in the maximum load of C10F30 and C0 at 800°C pointed out that the decomposition of CaCO3 and CSH at this temperature range had a negative effect on the strength of specimens, and microstructural analysis will make an explanation about the change in mechanical performance. With regards to the slope of the load-deflection curve in the linear stage at 800°C, it can be examined that the slope of C10F30 was almost the same with that of C0, suggesting that the two specimens had the same flexural stiffness after exposure to 800°C. The reason may be that the addition of fly ash can enable the generation of high-density CSH which is high-temperature resistant since the decomposition of CaCO3 takes place at this temperature. Similarly, the flexural stiffness of the two specimens at 1000°C experienced a further decline.
The pore size distribution of the ordinary mortar and the mortar containing 10 wt.% CaCO3 whisker and 30 wt.% fly ash before and after exposure to different high temperatures were obtained through utilizing MIP method, as presented in Figure
Pore size distribution of (a) C0 and (b) C10F30 before and after exposure to various temperatures.
When exposed to 200°C, C0 kept approximately the same porosity compared with the specimen unheated, while the porosity of C10F30 experienced a slight reduction. This phenomenon can be attributed to the fact that the increase in temperature had a positive influence on the rehydration of cement, and the pozzolanic reaction of fly ash was also stimulated by the high temperature, which enables CH to react with the main components of fly ash, such as SiO2 and Al2O3, to form C-S-H and C-A-H. Therefore, the overall porosity of the specimens showed a decrease.
Similarly, the porosity of both C0 and C10F30 showed a further reduction after exposure to 400°C. High temperature may induce the generation of a large number of microcracks due to different thermal expansion coefficients of hydration products, while the presence of microfibrous CaCO3 whisker may bridge these cracks, thus refining the pore size distribution and enhance the toughness of the specimen. Furthermore, the filler effect of CaCO3 whisker can also densify the microstructure of the mortar due to the good compatibility between them. This may be the reason why C10F30 exhibited a superior high-temperature resistance in terms of mechanical strength compared with C0, since the mechanical strength of the specimen is closely linked with its porosity and pore size distribution.
The overall porosity of both C0 and C10F30 increased after being subjected to 600°C, with the reduction in volume of harmless pores and the increase in volume of harmful pores. The decomposition of CH, together with loss of moisture in this temperature range, increased the porosity of the two specimens, and the reason why C10F30 still maintain the superior mechanical strength compared with C0 is most likely due to the combined effect of CaCO3 whisker to bridge the microcracks and fly ash to proceed the pozzolanic reaction with CH and free lime [
The porosity of C10F30 and C0 increased dramatically after exposure to 800°C and 1000°C, because more hydration products, such as CaCO3 and CSH, will be disintegrated, resulting in a much coarser microstructure of the specimen. Meanwhile, although CaCO3 whisker decomposed in this temperature range, the porosity of C10F30 was almost equal to C0, the reason may be that fly ash enables the generation of thermostable hydration products [
Microscopic images of the ordinary mortar and the mortar containing 10 wt.% CaCO3 whisker and 30 wt.% fly ash before and after exposure to different high temperatures are presented in Figures
Microscopic images of C0 before and after exposure to different temperatures: (a) 20°C, (b) 200°C, (c) 400°C, (d) 600°C, (e) 800°C, and (f) 1000°C.
Microscopic images of C10F30 before and after exposure to different temperatures (CaCO3 whiskers are marked in the red circle): (a) 20°C, (b) 200°C, (c) 400°C, (d) 600°C, (e) 800°C, and (f) 1000°C.
The microstructural images of C0 before and after exposure to high temperatures of 200°C, 400°C, 600°C, 800°C, and 1000°C are shown in Figure
Figure
The objective of the current study is to investigate the combined effect of CaCO3 whisker and fly ash on cement mortars before and after exposure to varying high temperatures and evaluate the high-temperature resistance of different types of mortars. Conclusions drawn from the experimental results were listed as follows: High-temperature exposure caused the increase in mass loss for all the CaCO3 whisker-based mortars, regardless of without or with fly ash, and the mass loss increased with the increase in maximum temperature. CaCO3 whisker-based fly ash mortars exhibited a slight increase in mass loss in comparison with their corresponding mortars without fly ash. Before exposure to high temperatures, the mechanical strength of CaCO3 whisker-based mortars was enhanced with the increasing dosage of CaCO3 whisker, while the mechanical strength showed a downward trend when the CaCO3 whisker content was more than 10 wt.%. The incorporation of fly ash led to a slight decrease in mechanical strength of mortars. With respect to CaCO3 whisker-based specimens, they reached their maximum strength after exposure to 400°C, while the strength of them decreased notably when exposed to 600°C. At 800°C and 1000°C, the strength of these specimens exhibited a further decrease, and due to the decomposition of CaCO3 whisker, specimens with more CaCO3 whisker had the lower strength. With respect to CaCO3 whisker-based fly ash specimens, in spite of a slight decrease in the strength at 20°C, the strength of them at 200°C and 400°C was even higher than that of corresponding specimens without fly ash. And after exposure to 600°C, the residual strength was still higher, indicating that the high-temperature resistance of CaCO3 whisker-based mortar was improved due to the presence of fly ash. Although the decomposition of CaCO3 whisker took place at 800°C and 1000°C, the strength of fly ash specimens was almost the same with C0. Compared with C0, C10F30 exhibited superior mechanical properties after exposure to different temperatures. With the incorporation of CaCO3 whisker and fly ash, the flexural stiffness and flexural toughness of the specimen were improved before 600°C. Although the total porosity of C10F30 at 20°C increased, the overall pore distribution was refined compared with C0. There was an evident decline in pore volume of mortars when exposed to 400°C, but exposure to temperatures higher than 600°C resulted in a significant increase in pore volume. Microscopic images showed that the filler effect and crack-bridging effect of CaCO3 whisker, together with the acceleration in pozzolanic reaction of fly ash at high temperatures, made C10F30 possess preferable high-temperature resistance capacity, which could be reflected in the results of mechanical tests.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
This research was sponsored by the Fundamental Research Funds for the Central Universities of Chang’an University (no. 300102218523), First-Class Discipline and First-Class Professional Construction of Chang’an University (no. 0021/300203110004), and the Outstanding Doctoral Dissertation Cultivation Subsidy Project of Chang’an University (no. 300102219715). The authors acknowledged the support.