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Article

Workability and Mechanical Properties of PVA Fiber-Limestone Fine Cementitious Composite

by
Weiliang Xie
1,
Jiajian Chen
1,*,
Tianxiang Chen
1 and
Garfield Xianzhang Guan
2
1
Department of Civil Engineering, Foshan University, Foshan 528200, China
2
Department of Civil Engineering, The University of Hong Kong, Hong Kong 999077, China
*
Author to whom correspondence should be addressed.
Buildings 2024, 14(11), 3679; https://doi.org/10.3390/buildings14113679
Submission received: 25 October 2024 / Revised: 15 November 2024 / Accepted: 15 November 2024 / Published: 19 November 2024
(This article belongs to the Section Building Materials, and Repair & Renovation)
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Abstract

:
Cement-based materials are the most widely used building materials and have two main problems: low flexural/tensile strength and low sustainability. To solve these two problems at the same time, the strategy of the utilization of fillers as cement paste replacement and utilization of fiber was proposed. Mixes with varying PVA fiber and LF were produced for workability and mechanical properties measurement and analysis. The results showed that the addition of PVA fibers reduced the flowability and bonding, while the addition of LF similarly reduced the flowability but enhanced its bonding. Both effects on strength showed an increase and then a decrease. An analysis of microstructure and chemical composition demonstrated that the addition of PVA fiber and/or LF first decreased the porosity, and a further addition increased the porosity. The mixes with 0.2% fiber content showed fracture failure mode, while the mixes with 0.4–0.6% fiber content showed the pulling out of failure mode. A mix with 0.2% fiber content and 10% LF content exhibited concurrently good workability and mechanical properties.

1. Introduction

Since its development, cement mortar-based materials have been used for their excellent compressive strength and durability. With the development of the times and the change in the living environment, the traditional cement mortar is forced to be upgraded and improved; however, simply increasing the amount of cement cannot change the shortcomings of its low flexural performance and ease of cracking, especially after the implementation of China’s carbon reduction and emission reduction policy. Increasing the cement dosage is no longer quite in line with modern economics, and the practice of replacing cement with admixtures has become the direction of research for many scholars in order to better reduce the carbon emissions of cement and improve the efficiency of cement utilization. The experiment proposes the use of limestone powder as an admixture to replace cement paste so as to reduce the amount of cement and realize the purpose of reducing carbon emission and improving the performance of cementitious materials. At the same time, considering the poor flexural performance of cementitious materials and the problem of easy cracking, the experiment also incorporated high-strength PVA short fiber to improve the performance of cementitious materials. Fiber-reinforced cementitious composite has been widely adopted due to its superior durability and impact resistance [1,2,3]. Among the various types of fiber, polyvinyl alcohol (PVA) fiber has excellent mechanical properties, chemical resistance, high-temperature resistance, lightweight, low cost, nice degradability, and high compatibility with cement matrix [4,5,6]. The slump loss of PVA fiber-reinforced concrete is less than that of steel fiber, carbon fiber, and glass fiber [7]. Its application in cement-based materials has attracted a number of studies. Mosavinejad et al. [8] proved that PVA fiber can significantly improve the flexural strength of ultra-high-performance concrete (UHPC) by up to 30%. Huang et al. [9] found that PVA fibers could enhance the mechanical properties of phosphogypsum-based materials; however, the flowability was decreased with the increase in the length and PVA fiber content.
Yew et al. [10] found that the increase in PVA fiber content decreased the flowability of cementitious composites. Cao et al. [11] showed that, at a water/cement ratio of 0.32, mixes containing no more than 0.25% PVA fibers exhibited a small difference in workability compared to those without fibers. Gao and Xu [12] pointed out that the ultimate strain of PVA fiber-reinforced cementitious composite could reach 70 times that of ordinary concrete. Qian et al. [13] found that the smaller the aspect ratio of PVA fiber, the more obvious the flexural strength of high-strength concrete. Experiments proved that although PVA fiber improved the flexural strength of cementitious composites, it increased the porosity of the material and attenuated its strength and stiffness [14,15]. Li et al. [16] found that the tensile ductility of PVA fiber-reinforced cementitious composite was still more than 200 times that of ordinary concrete after a long-term hot and humid environment. Hakan et al. [17] showed that PVA fiber had a significant effect on the flexural properties and impact resistance. Passuello et al. [18] found that PVA fiber better reduced the number and width of cracks in concrete compared with shrinkage-reducing agents. It is concluded that PVA fiber effectively improved the shortcomings of cementitious composites but brought problems, namely, a decrease in flowability and increase in porosity.
To alleviate the above-mentioned two problems caused by PVA fiber, engineers have to increase the water/cement ratio or add mineral admixture with lower fineness. As the water/cement ratio is generally determined by the strength requirement, studies have been devoted to the application of mineral admixture. Özbay et al. [19] found that the addition of blast furnace slag has a positive effect on the workability and mechanical properties of concrete. Lin et al. [20] showed that fly ash improved the strength of cementitious composites. Duval and Kadri [21] found that 10% silica fume as cement replacement did not affect the construction performance of concrete. Zhang et al. [22] found that the increase in fly ash content reduced the chloride ion diffusion rate of concrete. Poon et al. found that after the addition of fly ash and silica fume greatly reduced the interfacial porosity of the mortar. However, blast furnace slag and fly ash lead to the slow development of early strength and are prone to dry shrinkage cracking [23]. Silica fume is expensive, and its use causes a significant increase in the cost of raw materials. Limestone fine (LF), a relatively environmentally friendly and low-cost chemical inert material [24,25,26], can fill the pores in cement-based materials. It can also reduce the shrinkage of cementitious composites and the emission of carbon footprint. As a result, many scholars have carried out relevant research on it. Chen and Kwan [27] found that using LF to replace part of the cement paste can reduce the heat generation and improve the compressive strength. LF as a fine aggregate replacement effectively reduces the porosity and water permeability [28]. Nan et al. [29] studied the rheological properties of ultra-high strength cementitious composites and found that when LF replaced part of cement and silica fume, the yield stress and plastic viscosity of cementitious composites could be significantly reduced, and the flowability of materials could be improved. It has been demonstrated that the addition of LF can greatly reduce the shrinkage strain of concrete and increase its strength [30] and has a significant effect on the permeability and durability of concrete [31]. The utilization of LF as a partial cement paste substitute improved the tensile strength, stiffness, and durability of concrete, and effectively reduced the carbon footprint; however, it led to an increase in the demand for water-reducing agents [32,33]. Burroughs et al. [34] found that LF significantly reduced the production cost and carbon footprint of UHPC without significantly reducing its strength. The above studies show that LF can be used as an ideal mineral admixture. The addition of LF improves the strength but reduces the toughness. The addition of PVA fibers at the same time not only alleviates the problem of reduced toughness but also allows a further increase in flexural strength.
Literature shows that the use of PVA fiber and LF in cement-based material has been separately studied. However, there are few studies on their concurrent use to solve the problems of low flexural strength and low workability. In order to study the effect of their combined use, a total of 12 mortar mixes were produced for testing the workability and compressive strength, flexural strength, splitting strength, and ultrasonic pulse velocity (UPV) in this study. X-ray diffraction (XRD), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP) were also carried out to reveal their influence mechanism.

2. Experimental Program

2.1. Materials

The raw materials used in this study included ordinary Portland cement, limestone fine (LF), manufactured sand, PVA fiber, water, and water-reducing agents. The cement was of 42.5 grade. Its density was measured to be 3100 kg/m3. Its chemical composition and strength performances are shown in Table 1 and Table 2, respectively. According to its supplier, it met the Chinese standard GB 175-2003 [35]. The manufactured sand was made from granite using mechanical crushing. Its density and fineness modulus were measured to be 2380 kg/m3 and 2.70, respectively. The density of LF was measured to be 2800 kg/m3. Its chemical composition is listed in Table 3, which shows that the LF is mainly composed of CaCO3. The mixing water was tap water from the laboratory. The PVA fiber was light yellow in color. Its physical properties are listed in Table 4. According to its supplier, it met the Chinese standard GB/T 14462-93 [36]. The water-reducing agent was the polycarboxylic type with a water-reducing efficiency of ≥25%, which met the Chinese standard GB 50119-2013 [37].

2.2. Mix Proportions

The powder paste volume and water/cement ratio by volume were set constant to 0.45 and 1.2 for all the mixes in this study. To maintain a constant aggregate volume, LF was added to partially replace cement paste (cement + water) with volumes of 0%, 10%, and 20%, respectively. The PVA fiber varied among 0%, 0.2%, 0.4%, and 0.6% by volume of the whole mix. The water/cement ratio was set constant at 37% by mass for all the mixes. To maintain similar workability for practical use, the dosage of the water-reducing agent was not predetermined but added bit by bit to the mix until achieving a 250 ± 50 mm flow spread or reaching 3% by mass of the powder. The mix proportions are detailed in Table 5. The mixes are identified as “X-Y”, where X indicates PVA fiber content and Y indicates LF content.
The workability, compressive strength, flexural strength, splitting strength, and UPV of the mortar were measured. The microstructure and composition were evaluated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP). To maintain constant workability, the dosage of the water-reducing agent was not predetermined but added little by little to arrive at a 200–300 mm flow spread in the jumping table test [38]. As the supplier of water-reducing agents does not recommend a dosage higher than 3.0% of the cement by mass, the dosage was kept at 3.0%; however, even targeted flow spread was not reached.

3. Testing Method

3.1. Workability

3.1.1. Flow Spread

The flow spread was measured according to the Chinese standard GB/T 2419-2005 [39]. The mean value of the two orthogonal diameters was taken as the result, as shown in Figure 1.

3.1.2. Adhesiveness

The mass of the stone bar (diameter: 25 mm, length: 200 mm, shown in Figure 2a) or ribbed steel bar (diameter: 15 mm, length: 200 mm, rib depth: 1.0 mm, shown in Figure 2b) was measured to be mi. The surface area of the bar was calculated to be S. To carry out the test, the bar was slowly inserted into the cementitious composite for a depth of 50 mm and slowly lifted after 10 s. The mass of the bar with adhered cementitious composite was measured to be mf. The adhesiveness A was calculated as the mass of adhered slurry on the unit surface area of the measured object by [40]:
A = m f − m i S ,

3.1.3. Rheological Parameter

The flow spread test can only be used for the flow characteristics of the material under static conditions. In order to more scientifically study the rheological properties and shear deformation behavior of the material under dynamic conditions, it is necessary to use a rheometer to test the rheological curve. To scientifically express the rheology, the yield stress, and plastic viscosity were measured. The GY-2 rheometer produced by Tianjin Sansi Test Instrument Manufacturing Company (Tianjin, China) was used, as shown in Figure 3. It is applicable to both concrete and mortar with a maximum mixing particle size of 40 mm [41]. Both the varying shear rate mode and the constant shear rate mode were applied. Varying shear rate mode, which plots the complete rheological profile at multiple shear rates, studies the overall flow performance and viscosity characteristics of a mix; the constant shear rate model concentrates on the stress change and thixotropic behavior of mixes at a static or low flow rate. Their rheometer test settings are tabulated in Table 6.
In the varying shear rate mode, the rheological model is fitted according to the test results. The Bingham model is usually suitable for describing the rheological behavior of the material showing a linear change of shear stress with shear rate during the shear deformation process. However, most cementitious composites show nonlinear changes. Therefore, in order to describe the rheological behavior of cementitious composites more reasonably, it is more suitable to use the Herschel–Bulkley model to calculate and fit the test results of the rheological curve [42]. The Herschel–Bulkley model formula is as follows:
τ = τ 0 + K γ n ,
where τ is the shear stress (Pa), τ0 is the yield stress (Pa), K is the apparent viscosity (Pa sn), γ is the shear rate (s−1), n is the flow index (dimensionless). For each mix, the best-fitting curve is obtained by regression analysis based on the model. The value of τ0 is used as a measure of required stress to start the shear, the value of K is used as a measure of the variation of shear stress with the change in shear stress, and the value of n is used as a measure of the shear behavior of the material (n > 1.0 indicates shear thickening, n < 1.0 indicates shear thinning).
In the constant shear rate mode, τmax is the maximum shear stress. Since the shear stress of each group of ratios tends to stabilize after 15 s from the peak value, the average value of shear stress in the time period of 15~30 s is taken as the shear stress stabilization value τstable.

3.2. Strength

The compressive, flexural, and splitting strength tests were carried out as per Chinese standard GB-T 17671-2021 [43] at the ages of 7 days, 14 days, and 28 days, respectively. The debris of samples after the test was collected and dried in a 100 °C oven to constant weight for later SEM, MIP, and XRD microscopic tests.

3.3. Ultrasonic Pulse Velocity

UPV test measures the velocity of ultrasonic waves passing through the targeted material based on acoustic theory as per China Engineering Construction Standardization Association standard T/CECS 02-2020 [44].

3.4. Microstructure Characterization

3.4.1. SEM Images

A scanning electron microscope (SEM) (model Hitachi Regulus 8100) was applied. The shooting multiple ranged from 100 to 20,000. The equipment was supplied by Hitachi High Tech Co., Ltd. (Tokyo, Japan) and is shown in Figure 4.

3.4.2. XRD Analysis

The Bruker D8 Discover diffractometer was used to perform X-ray diffraction to obtain the diffraction pattern. The test range was 5~50°, and the scanning time was 5 min. The equipment was supplied by Bruker AXS Co., Ltd. (Karlsruhe, Germany) and is shown in Figure 5.

3.4.3. Mercury Intrusion Porosimetry

The Auto Pore Iv 9500 high-performance automatic mercury porosimeter was used to obtain the pore size distribution of the samples. It can offer low-pressure and high-pressure up to 60,000 psi (414 MPa) and cover, which covers most of the pores related to the recognized characteristics. The equipment was supplied by Micromeritics Co., Ltd. (Norcross, GA, USA) and is shown in Figure 6.

4. Results

4.1. Workability

4.1.1. Flow Spread

Figure 7 shows that the addition of LF and PVA fiber led to an increase in water-reducing agent content and a decrease in flow spread. Because PVA fiber has certain hygroscopicity [45] and hydrophilicity [46], it would be wrapped by water molecules to form a film on the surface, resulting in a reduced amount of water for cement hydration. At the same time, the fibers form a network structure in the mix, making the mix easier to agglomerate. The fiber surface has a strong adsorption capacity, which will significantly attenuate the dispersion effect of the water-reducing agent [47]. The 0.6% PVA fiber pairs of mixes showed the greatest decrease in extensibility compared to the control group, with a decrease of 46%.
For a given amount of PVA fiber content, LF as cement paste replacement reduced the flowability; however, this decrease in flowability could be restored by adding SP. The addition of 10% LF had no obvious effect on the demand for SP, which was attributed to its lubrication and filling effect of LF [48,49], while 20% LF leads to a sharp increase in the water-reducing agent demand, expansion dropped by 2%. This is mainly due to the increase in solid surface area [31].

4.1.2. Adhesiveness

Figure 8 shows that the steel bar has better adhesiveness to the cementitious composite than the stone bar due to the rough and uneven surface. This was expected because ribbed steel bars and relatively smooth stone bars were adopted for the adhesiveness test. At a given LF content, the addition of PVA fiber led to a decrease in both the stone and steel adhesiveness. This is mainly because the cementitious composite agglomerated due to the adsorption capacity of the fibers and became difficult to adhere to the stone and steel [9]. The maximum reduction in adhesiveness was 71% for stone when 0.6% PVA fiber was added at 0% LF content. This dramatic decrease in adhesiveness was because the mixes with high PVA fiber became rather dry and much more difficult to adhere to the smooth surface of the stone rod. The surface wettability was also an important indicator of the durability such as initial sorptivity [50,51]. This adverse effect of PVA fiber on adhesiveness should be taken into consideration in mix design to avoid the detrimental effect of poor bonding of the paste-aggregate and paste-reinforcing bar. At the same PVA fiber content, the addition of LF increased the stone adhesiveness. This was mainly due to the filling effect of LF [49].

4.1.3. Rheological Parameters

The shear stress–shear rate curves of all the mixes adopting varying shear rate modes are shown in Figure 9 and the rheological parameter calculation results are tabulated in Table 7. All the correlation coefficient R2 is higher than 0.94, which demonstrates the high acceptability of the regression. Regardless of the LF content, higher PVA fiber content resulted in higher yield stress and apparent viscosity. The maximum increase in yield stress was 226% by the addition of 0.6% PVA at 0% LF content, and the maximum increase in apparent viscosity was 270% by the addition of 0.6% PVA at 10% LF content. Therefore, the excessive addition of PVA fiber would affect pumpability because the cementitious composites undergo a high shear rate during pumping.
At the same as the PVA fiber content, the addition of LF causes τ0 to show a tendency to first decrease and then increase. For instance, 10% LF could lower the τ0 from 75.91 to 71.88 MPa. This was due to the lubricating effect of the LF, which makes the particles in the cementitious composite easier to start flowing. Different from the effect on yield stress, the addition of LF always increased in apparent viscosity. This is mainly because the addition of LF as a cement paste replacement increased the amount of solid particles, which required higher torque to move them at the same shear rate. The flow index n value results show that all the mixes except 0.0-10 exhibited shear-thinning behavior, which was intensified by the addition of PVA fiber. It is interesting that the addition of LA may increase the n value at low PVA fiber content, which could turn the shear-thinning behavior into shear-thickening behavior.
In the constant velocity mode, the addition of PVA fiber and/or LF always increased the τmax and τstable. It is evident that the addition of PVA fibers lengthened the time cost from the peak to the plateau and decreased the smoothness of the curves after τmax. In other words, it decreased the thixotropy (the ease of recovering the structure after being damaged by shearing) [52].

4.2. Strength

4.2.1. Compressive Strength

The compressive strength testing results are graphically displayed in Figure 10. The highest compressive strength was 80.76 MPa and occurred at mix 0.2–10. As expected, the compressive strength of all the mixes increased with age regardless of the PVA fiber content and LF content. It was noticed that the increase in strength with age was proportionally higher at a higher PVA fiber content. This was because the bonding of hardened cement paste to PVA fiber improved with age, as illustrated in the yellow circle area of the SEM images in Figure 11. Regardless of the PVA fiber content, the incorporation of LF first has a positive effect on the compressive strength. Moreover, 10% LF best increased the compressive strength by 40%. The LF offered a nucleation effect that promoted the growth and precipitation of the hydration products of the cement [53]. A further addition of LF beyond 10% turned to significantly decrease the compressive strength. This was due to the dilution effect of LF that reduced the cement content and hydration product [54].
At the same LF content and age, 0.2% PVA fiber had little effect on the compressive strength, while a further addition of PVA fiber turned to significantly decreased the compressive strength. The slight improvement in compressive strength at a low fiber content could be due to the bridging effect that limited the propagation of cracks [54], while the decrease in compressive strength at a high fiber content could be due to the increase in voids when excessive fiber was embedded into the cement paste.

4.2.2. Flexural Strength

The flexural strength testing results are graphically displayed in Figure 12. The results show that the highest flexural strength occurred at mix 0.4–10. Similar to the compressive strength, the flexural strength of all the mixes increased with age. At the same PVA fiber content, LF improved the flexural strength. At 0.6% PVA fiber content, 10% LF best increased the flexural strength by 32.3%, and it has a strength of 17.62 MPa. Further addition of LF beyond 20% turned to significantly decrease the flexural strength due to the dilution effect of LF to reduce the hydration product [54].
At the same LF content, PVA fiber improved the flexural strength, albeit in the increase in the porosity inside the cementitious composite due to the binding and bridging effect of the fiber [55]. Different from the effect on compressive strength, 0.6% rather than 0.4% was the optimum PVA fiber content for the highest flexural strength. At 10% LF content, 0.4% PVA fiber increased the flexural strength by 25.4%. Results show that further addition of PVA fiber beyond 0.4% turned to decrease the flexural strength. This was because a high fiber content weakened its bonding with hardened paste [56].

4.2.3. Splitting Tensile Strength

The splitting tensile testing results are displayed in Figure 13. The same as flexural strengths, the highest splitting tensile strength occurred at mix 0.4-10. At 0.4% PVA fiber content, 10% LF best increased the splitting tensile strength by 29%. Since the effect and influence mechanism of PVA fiber and/or LF on splitting tensile testing were similar to the effect on flexural strength, no more discussion is repeated.

4.3. Ultrasonic Pulse Velocity Testing

The UPV results are displayed in Figure 14. As expected, the UPV of all the mixes increased with age. The highest UPV occurred at mix 0.0-10. Regardless of the PVA fiber content, the addition of 10% LF increased the UPV, while its further addition turned to a decrease in the UPV. At the same LF content, the addition of PVA fiber always decreased the UPV. It is concluded that the effect of PVA fiber and/or LF on the UPV is consistent with the effect on compressive strength. This demonstrated the influence mechanism on PVA fiber and/or LF on UPV and compressive strength is similar.

4.4. Microstructure Analysis

4.4.1. SEM Images

SEM images were obtained to disclose the changes in internal structure and mechanical properties. As shown in Figure 15a,b mixes 0.0-0 and 0.0-10 owned smooth and flat surfaces. The nucleation effect of LF in mix 0.0-10 promoted the formation of calcium–silicate–hydrate (C-S-H) gel and reduced the voids. Compared to mix 0.0-0, which exhibited granular crystals and microcracks, mix 0.2-10 had improved strength. It can be observed that mix 0.6-20 showed huge voids and palpable defects. It demonstrated that LF as an excessive cement paste replacement decreased the amount of hydration products.
Figure 15c–e illustrates that PVA fibers were well embedded into the hardened cement paste. This good bonding between PVA fiber and cementitious matrix was helpful in improving the strength, especially the flexural strength and splitting tensile strength. The 0.2-10 and 0.4-10 mixes exhibited smooth surface. It is worth pointing out that most of the fibers were broken instead of pulled out, which hinted at a fracture failure mode [57]. Compared to the pull-out failure mode, this fracture failure mode demonstrated a good bonding between the fiber and the hardened cement paste, which resulted in improved ultimate tensile stress but inferior ultimate tensile strain [58].
Figure 15d illustrates the fiber overlap in mix 0.4-10. This high amount of fiber improved the flexural and splitting tensile strength, which can be proven by the increase in 28d flexural strength from 14.31 MPa for the 0-10 mix to 17.62 MPa for the 0.4-10 mix. The SEM image of the 0.4-10 mix showed that a high fiber content would result in more voids and defects.
The SEM image of the 0.6-10 mix in Figure 15e shows that although the fibers can be embedded into the cement matrix, the extracted part shows that the fiber surface attachment is not as much as that of the 0.2-10 mix, and the part where the fibers are bonded to the cement matrix is looser, and more voids are found compared with that of the 0.2-10 mix. From Figure 15d,e, it is not difficult to find that when the fiber content is ≥0.4%, more obvious voids and defects are likely to appear at the intersection of PVA fibers and cement matrix.

4.4.2. XRD Analysis

To reveal the effect of LF content on cementitious composites, Figure 16 shows the XRD pattern of mixes at 7-day and 28-day age, in which the intensity peaks of ettringite (AFt), hemicarboaluminate (Hc), monocarboaluminate (Mc), calcium carbonate (CaCO3), and Portlandite (CH) are presented. Since fiber did not affect the hydration of the cement, the XRD pattern of mixes with varying PVA fiber content was not carried out. As can be observed from the results, ettringite was detected for all the mixes. Its peak value of the 0.0-0 mix at 7d was higher than that at 28d, which was due to the reaction between the AFt and aluminum phase in cement to form calcium monosulphoaluminate (AFm). However, the peak of AFt of mixes 0.0-10 changed little from the 7d age to the 28d age, and the peak of the AFt of mixes 0.0-20 became slightly higher from the 7d age to the 28d age. This is because CaCO3 in LF reacted with the aluminum phase in the cement to form Hc and Mc, thus inhibiting the formation of AFm [59,60]. This explanation was echoed by the decrease in Hc and increase in Mc with age.
Furthermore, in the diffraction spectra of 7d, the Mc peak at about 12.5° is lower than the Hc peak at 11.5°, and, conversely, in 28d, the Mc peak at 12.5° is instead higher than the Hc peak at 11.5°. This is because the chemical property of Mc was more stable than Hc, which is consistent with the findings by Matschei et al. and Thongsanitgarn et al. [59,60]. Relevant studies [61,62] show that the molar volume of AFm is smaller than that of AFt, the conversion of AFt to AFm in a mix of 0.0-0 would cause shrinkage cracking and porosity. The AFt is larger and harder than AFm [59,63], reducing the porosity and improving the strength.
In addition, the CH peak was detected near 18°, 34°, and 47°. LF as cement paste replacement reduced the formation of hydration products such as CH. This explained why the peak value of carboaluminates (Hc and Mc) of mix 0.0-20 was higher than mix 0.0-10. The small amount of free carbonate ions in LF would react with CH to form calcium carbonate precipitation in pores or surfaces. It reduced the CH content and improved strength [64].

4.4.3. Pore Conditions

Figure 17 shows the pore size distribution of hardened cementitious composites at 28-day age. According to the pore size distribution in Figure 17a and the pore volume of different fractions in Figure 17c, the pore size mainly ranged between 10 nm and 100 nm. According to Wu et al. [65], the pores can be categorized as highly harmful (≥200 mm), harmful (50~200 mm), slightly harmful (20~50 mm), and innocuous (≤20 mm) according to their sizes. As can be seen from the cumulative pore volume in Figure 16b, the cumulative pores volume of the mixes with LF were lower than the counterpart without LF at 28d age. This can be attributed to the smaller particle size of LF, which fills into the highly harmful pores (≥200 mm) of the material. Some highly harmful pores (≥200 mm) and harmful pores (50~200 mm) were transformed into slightly harmful pores (20~50 mm) and innocuous pores (≤20 mm).
However, compared with the 10% LF mix, the 20% LF mix caused an increase in the overall porosity due to the increase in highly harmful and slightly harmful pores. This was because although LF could fill the small pores, it could not compensate for the large pores caused by the reduction in cement paste at a high cement paste replacement. Since 10% LF caused little harmful effect on flowability, the optimal percentage of LF as cement paste replacement for concurrent flowability-pore volume was 10%. The mixes with 10% LF had the lowest porosity, which resulted in the best compressive strength. This was in agreement with the UPV results in Figure 14, the compressive, flexural, and splitting tensile strength results in Figure 10, Figure 12 and Figure 13, and the findings of Li et al. [66].
On the other hand, the increase in PVA fiber content increased the porosity of the mix. This was due to the viscous effect of PVA fiber that made the LF particles adhere to its surface, and, consequently, the reduced amount of LF particles for pore filling [67]. This unfavorable viscous effect was not significant at a low PVA fiber content. For instance, compared to the 0.0-10 mix without PVA fiber, the 0.2-10 mix with 0.2% PVA fiber only slightly increased the porosity of the harmful pores from 3.61% to 4.18%.

5. Conclusions

This study measured and analyzed the workability and mechanical properties of PVA fiber-limestone fine cementitious composite with 0–0.6% PVA of the mix and 0–20% LF as a cement paste replacement. The major findings are concluded as follows:
(1)
Addition of PVA fiber and/or LF as cement paste replacement decreased the flowability. The addition of LF increased the adhesiveness, but the addition of PVA fiber decreased the adhesiveness.
(2)
Addition of PVA fiber first increased and then decreased the strength. The optimum PVA fiber content was 0.2% for compressive strength and 0.4% for flexural strength and splitting tensile strength.
(3)
Addition of PVA fiber increased the porosity, as demonstrated by the UPV test. The optimum PVA fiber and the optimum LF content for the lowest porosity were 0.2% and 10%, respectively.
(4)
Microscopic observations and pore condition results demonstrated that the LF particles filled into the pores of the hardened cementitious composites. This pore-filling effect explained the increase in strength.
(5)
A PVA fiber and hardened cement paste of 0.2% can produce a better connection effect; however, 0.4~0.6% PVA fiber may lead to larger pores at the junction of fiber and paste.
(6)
The fibers in mixes with 0.2% PVA fiber showed fracture failure mode, while the fibers in mix with 0.4–0.6% PVA fiber showed pulling out failure mode.
Finally, it is noteworthy to point out that, although the findings were obtained through given raw materials, i.e., PVA fiber and LF, the strategy of the utilization of fillers as cement paste replacement and the utilization of fiber has been demonstrated to be able to solve the low flexural/tensile strength and low sustainability problems of the cement-based materials. In other words, different types of fiber and mineral admixture can be adopted following this strategy. Further study along this line is recommended.

Author Contributions

Conceptualization, W.X. and J.C.; methodology, J.C.; validation, J.C.; formal analysis, J.C. and W.X.; investigation, J.C.; data curation, W.X., T.C. and G.X.G.; writing—original draft, J.C. and W.X.; writing—review and editing, J.C. and W.X.; visualization, J.C.; supervision, J.C.; project administration, J.C. and W.X.; funding acquisition, J.C.; software, G.X.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Natural Science Foundation of Guangdong Province of China [2022A1515010404 and 2024A1515011894].

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Buildings 14 03679 g001
Figure 1. Photo showing flow spread test.
Figure 1. Photo showing flow spread test.
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Figure 2. Photo showing adhesiveness tests.
Figure 2. Photo showing adhesiveness tests.
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Figure 3. Photo showing rheological parameter test.
Figure 3. Photo showing rheological parameter test.
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Figure 4. Photo showing SEM image observation.
Figure 4. Photo showing SEM image observation.
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Figure 5. Photo showing XRD analysis.
Figure 5. Photo showing XRD analysis.
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Figure 6. Photo showing mercury intrusion porosimetry analysis.
Figure 6. Photo showing mercury intrusion porosimetry analysis.
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Figure 7. Flow spread result.
Figure 7. Flow spread result.
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Figure 8. Adhesive results.
Figure 8. Adhesive results.
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Figure 9. Shear stress–shear rate curves.
Figure 9. Shear stress–shear rate curves.
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Figure 10. Compressive strength.
Figure 10. Compressive strength.
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Figure 11. The bonding of PVA fiber and hardened cement paste at different ages.
Figure 11. The bonding of PVA fiber and hardened cement paste at different ages.
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Figure 12. Flexural strength.
Figure 12. Flexural strength.
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Figure 13. Splitting tensile strength.
Figure 13. Splitting tensile strength.
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Figure 14. Ultrasonic pulse velocity.
Figure 14. Ultrasonic pulse velocity.
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Figure 15. SEM images at 28d age.
Figure 15. SEM images at 28d age.
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Figure 16. XRD pattern for mixes with varying LF content.
Figure 16. XRD pattern for mixes with varying LF content.
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Figure 17. Pore conditions.
Figure 17. Pore conditions.
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Table 1. The chemical composition of cement.
Table 1. The chemical composition of cement.
CompositionCaOSiO2MgOAl2O3Fe2O3K2ONa2OElse
(%)11.5250.851.5617.2113.141.320.523.88
Table 2. Strength performances of cement.
Table 2. Strength performances of cement.
Time of Setting (min)Compressive Strength (MPa)Tensile Strength (MPa)
Initial settingFinal setting3d28d3d28d
45600≥23.0≥42.5≥3.5≥6.5
Table 3. The chemical composition of fine limestone.
Table 3. The chemical composition of fine limestone.
CompositionCaCO3SiO2FeAl2O3MgOElse
(%)98.50.020.00030.010.111.26
Table 4. Physical properties of PVA fiber.
Table 4. Physical properties of PVA fiber.
Fiber Length
(mm)		Fiber Diameter
(μm)		Specific GravityTensile Strength
(MPa)		Elastic Modulus
(GPa)		Dry Fracture Elongation
(%)
6 or 9311.30180037.017 ± 3.0
Table 5. Mix proportion of PVA fiber-limestone fine cementitious composite.
Table 5. Mix proportion of PVA fiber-limestone fine cementitious composite.
Mix No.Water
(kg/m3)		Cement
(kg/m3)		Fine Aggregate
(kg/m3)		LF
(kg/m3)		PVA Fiber
(kg/m3)		Water Reducing Agent
(%)
0.0-0245.5639.61309.00.00.00.60
0.2-0245.5639.61304.20.02.61.12
0.4-0245.5639.61229.50.05.21.90
0.6-0245.5639.61294.70.07.83.00
0.0-10220.9575.71309.0126.00.00.72
0.2-10220.9575.71304.2126.02.61.20
0.4-10220.9575.71229.5126.05.22.00
0.6-10220.9575.71294.7126.07.83.00
0.0-20196.4511.71309.0252.02.61.40
0.2-20196.4511.71304.2252.02.62.00
0.4-20196.4511.71229.5252.05.23.00
0.6-20196.4511.71294.7252.07.83.00
Table 6. Rheometer test settings.
Table 6. Rheometer test settings.
Test ModePre-Stirring Time
/s		Pre-Stirring Rotation Rate
/r/s		Testing Time
/s		Number of Testing PointsInitial Rotation Rate
/r/s		Final Rotation Rate
/r/s
Varying shear rate mode200.5125250.50.05
Constant shear rate mode--302400.0250.025
Table 7. Rheometer testing results.
Table 7. Rheometer testing results.
Mix No.Varying Shear Rate ModeConstant Shear Rate Mode
Yield Stress (τ0)
/Pa		Apparent Viscosity (K)
/Pa sn		Correlation
Coefficient
(R2)		Flow Index (n)τmax
/Pa		τstable
/Pa
0.0-075.9159.840.9960.9590.3581.76
0.2-0157.2958.510.9970.93140.44126.71
0.4-0200.3169.200.9970.92235.76199.64
0.6-0248.03104.350.9950.77299.33267.67
0.0-1071.8875.890.9991.10108.4693.49
0.2-10145.1187.300.9980.96166.07142.71
0.4-10187.20146.130.9980.76260.45214.84
0.6-10211.52280.990.9980.40428.81351.03
0.0-20121.26227.510.9960.92187.22131.04
0.2-20157.80264.830.9860.76211.25161.55
0.4-20208.66304.500.9790.70295.40251.02
0.6-20256.13332.420.9430.20519.70412.45
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Xie, W.; Chen, J.; Chen, T.; Guan, G.X. Workability and Mechanical Properties of PVA Fiber-Limestone Fine Cementitious Composite. Buildings 2024, 14, 3679. https://doi.org/10.3390/buildings14113679

AMA Style

Xie W, Chen J, Chen T, Guan GX. Workability and Mechanical Properties of PVA Fiber-Limestone Fine Cementitious Composite. Buildings. 2024; 14(11):3679. https://doi.org/10.3390/buildings14113679

Chicago/Turabian Style

Xie, Weiliang, Jiajian Chen, Tianxiang Chen, and Garfield Xianzhang Guan. 2024. "Workability and Mechanical Properties of PVA Fiber-Limestone Fine Cementitious Composite" Buildings 14, no. 11: 3679. https://doi.org/10.3390/buildings14113679

APA Style

Xie, W., Chen, J., Chen, T., & Guan, G. X. (2024). Workability and Mechanical Properties of PVA Fiber-Limestone Fine Cementitious Composite. Buildings, 14(11), 3679. https://doi.org/10.3390/buildings14113679

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