Study on the flowability and strength of cement-based materials added with limestone powder and PVA fiber

In order to improve the mechanical properties of cement-based materials, limestone powder (LP) was used as a filler as 0% to 20% cement paste replacement by volume in the mortar. Polyvinyl alcohol (PVA) fiber with 0% to 0.6% by volume were added to produce multiple sets of cement-based materials for basic mechanical properties tests including flowability, compressive strength and tensile strength. The test results indicated that an increase in the amount of PVA fiber and the replacement amount of LP continuously reduce the flowability of cement-based materials. PVA fiber led to the most significant reduction effect. The addition of an appropriate amount of PVA fiber can improve the tensile and compressive strength of cement-based materials, but excessive addition has a negative impact on the compressive strength. When the increase in replacement ratio of LP, the compressive strength of cement-based materials increased. Comparatively, the decrease in tensile strength is not significant.


Introduction
Cement-based materials have the advantages of simple construction, easy molding, and strong practicality, mainly used for filling, connecting, and covering the internal and external surfaces of buildings.However, they also have disadvantages such as poor flexibility, tensile, and shrinkage performance.Therefore, many scholars use high-performance fiber mixtures to make cement-based materials to compensate for the shortcomings of traditional cement-based materials.Among the various widely used fibers, PVA fiber has the characteristics of high strength, high ductility, wear resistance, and corrosion resistance.Compared with other fibers, PVA fiber can produce better adhesion with cement matrix.At present, Mosavinejad et al. [1] found that the content of PVA fiber had a substantial impact on the bending strength by studying the microstructure of UHPC containing PVA short fiber, and the compressive strength of experimental group containing 1.2% PVA fiber increased by 30% compared with the control group.Yang et al. [2] studied that the compressive strength of PVA fiber ultra-high performance concrete modified by nano silica is 10% higher than that of ordinary PVA fiber concrete, and the tensile strength is more than 26% higher.
In recent years, many scholars have mainly focused on exploring the changes in the mechanical properties of PVA fiber cement-based materials or concrete, which has been difficult to meet the needs of developing countries for carbon peaking and carbon neutralization.However, the use of manufactured sand to make PVA fiber cement-based materials, and the use of limestone powder (LP) to replace some cement pastes, can further reduce carbon emissions, which is called for by the most of countries.Chen et al. [3] studied the effect of LP on the hydration of cementitious materials and the performance of concrete, and found that the strength of concrete increases with the increase of LP proportion in the used cementitious materials, while the LP activity index decreases linearly with the increase of water cement ratio.Liu [4] found through research on the effect of lime powder (LP) on the performance of UHPC that the compressive strength of UHPC first increases and then decreases with the replacement rate of limestone powder, and reaches its maximum at a replacement rate of 30%.However, there is a relative lack of research on using LP to replace some cement paste in cementbased materials and adding fibers.As a 'reinforcing' material for cement-based materials, fibers undoubtedly have a significant impact on the improvement in mechanical properties of cement-based materials.
In this study, the flowability, the tensile strength and compressive strength of cement-based materials are tested to evaluate the influence of different PVA fiber content and different LP content on the properties of cement-based strength of materials.

Materials
 Cement: The cement meets the requirements of GB 175-2007 "General Portland Cement".
The main performance indicators are shown in Table

Mix proportions
This experiment was designed with a mass water to adhesive ratio (W/C) of 0.38, 0.32, and 0.26.Because Atahan et al. claimed that changes in fiber volume fraction of 0.5% and 2.0% have a significant impact on the bending and impact resistance of cement materials [5], and based on this, PVA fiber were added at 0%, 0.2%, 0.4%, and 0.6% of the total volume, totaling 12 cement-based materials.The specific mixing ratio details will be detailed in Table 3.In the three sets of cementbased material mixtures, (water+cement+LP) hydrocolloids account for 45% of the total volume, with each set of LP replacing cement paste having volumes of 0%, 10%, and 20%, respectively.PVA fiber account for 0%, 0.2%, 0.4% and 0.6% of the total volume, while the remaining volumes are all fine aggregates.The identification of the mixture follows "X-Y", where the percentage of fiber volume of the mixture is located at position X, and the percentage of lime powder replacing cement paste volume is located at position Y.
In order to ensure a certain workability requirement for cement-based materials during the mixing process, a small amount of SP will be continuously added during the mixing process of cement-based materials.Due to the amount of SP will be affected by the amount of LP and PVA fiber will be different, it is necessary to pass the skip table test of the national standard GB/T2419-2005 "Method for determining the flowability of cement mortar" to make its expansion reach 200-300mm [6] to determine the amount of SP, when the amount of SP has reached 3% of the total masses of cement +LP (the liquid SP content recommended by most manufacturers do not exceed 3%), then stop adding.And the amount of SP added is used as the dosage of SP for subsequent formal mixing.

Production process.
Each batch of cement-based materials is mixed using a fully automatic single shaft mixer, and the mixing process is as follows: 1) First, add fine aggregate, cement, LP, and PVA fiber (while stirring) and stir for 120 seconds.
2) Mix for another 120 seconds, adding water and a small amount of SP (starting at 0.5% of the total masses of cement+LP) while stirring until the specified expansion is reached or the SP dosage is increased to 3%. 3) After meeting the requirements, pour the cement-based material sample into a size of 40mm × 40mm × In a 160mm mold, after leveling and standing for 24 ± 2 hours, remove the mold, and then place the sample in a curing box with a temperature of 20 ± 2 ℃ and a relative humidity of over 90% for curing 28d.

Flowability
Table 4 and figure 3 list the amount of SP required for various cement-based material mixtures to reach the specified workability.It is observed from the table that with the constant water-cement ratio and the increase in fiber, the amount of SP required for mixing cement-based materials also increases.
When ceteris paribus remain unchanged, the fiber content reaches 0.6%, even if the amount of SP used for three groups reaches 3%, the expansion of mixed cement-based materials cannot meet the specified requirements.These phenomena are reasonable [2] and the possible reasons are: firstly, PVA fiber have a certain degree of water absorption, which also reduces the water molecules involved in cement reactions.Secondly, due to the hydrophilic nature of PVA fiber, water molecules form a thin film layer on the fiber surface after absorbing a certain amount of water.Furthermore, due to the presence of many (-OH) hydrocarbon groups on the surface of PVA fiber that can bond well with the cement matrix [7], its surface area needs to be partially encapsulated by cement paste, thereby further reducing the amount of cement paste that surrounds the aggregate.The more fibers there are, the more significant the viscosity effect [8].
In addition, under the condition of constant fiber content, the amount of SP increased with the amount of cement paste replaced by LP.When the replacement ratio is 10%, the change in SP amount is not significant, but when the replacement ratio is 20%, there is a significant increase.This is caused by the following reasons.Firstly, the addition of LP leads to a decrease in the water cement ratio (water/(cement+LP)), resulting in a decrease in the amount of water used, resulting in less water being encapsulated by cement particles.Therefore, more water reducing agents are needed to disperse them in the cement matrix to ensure fluidity.
Secondly, the fineness of the LP used is smaller than that of cement, which means its specific surface area is larger than that of cement.With the same water consumption, wetting its surface will require more water, which will inevitably affect the decrease in fluidity of cement-based materials [9].Finally, LP has a filling effect, as its particles are small and done not react with water.Therefore, under the action of SP, it is filled into the pores between cement and fine aggregate particles, forcing the filled water in the pores to be forced out, thereby improving the fluidity of cement-based materials [10,11].Therefore, when the LP replacement amount is 10%, the increase in SP usage and the loss of expansion are not significant.When the replacement amount is 20%, due to the excessive increase in LP and the further decrease in water usage, LP needs more SP to disperse in order to play its filling role.

Strength
From figure 4, it can be seen that, with the same LP content, the compressive strength of cement-based materials at 28 days has been improved to some extent when the PVA fiber content is 0.2%.At 0.4%, the compressive strength reaches its maximum, while at 0.6%, it actually decreases.The increasing trend of compressive strength between 0% and 0.4% is due to the addition of a certain amount of PVA fiber filling the local voids and microcracks inside the cement-based material, causing the internal particles to be in a state of multiple connections, which to some extent hinders the propagation of cracks and bears a certain amount of tensile stress.When the dosage exceeds 0.4%, the voids and microcracks inside the cement matrix will increase.Firstly, the hydrophilicity of the fibers' leads to a decrease in the effective participation of water in the cements reaction.Secondly, the increase in fiber content leads to an increase in the cement paste that wraps the fibers, making the coarse aggregate in the cement-based material unable to be fully wrapped, resulting in an increase in voids and a decrease in overall strength.However, it can also be seen from the chart that, within the replacement ratio of 0-20%, with the increase of LP replacement amount, the overall compressive strength of cement-based materials continues to rise.This is because of the filling effect of LP.Although LP does not have the Volcanic ash effect, its filling effect can improve the particle grading of aggregates and reduce the porosity; Moreover, the chemical reaction between it and the aluminum phase in the cement inhibits the formation of AFm (Calcium aluminates), and on the contrary, produces a more solid and larger Calcium aluminates, reducing the size of internal pores.LP will also enhance and accelerate the hydration of cement in silicate mineral due to the role of crystal nucleus, promote the growth and precipitation of early hydration products of cement [12], and further improve the strength of cementbased materials.Under the combined action of LP and PVA fiber, the compressive strength of cementbased materials reached the highest of 74.48MPa, which increased by 155% compared to the control group.From figure 5, it can be seen that when the LP replacement amount remains unchanged and the fiber content is 0%~0.6%, the tensile strength of cement-based materials continuously increases with the increase of PVA fiber content, with an increase range of 19%~21%, reaching a maximum of 14.83MPa.Due to the bridging effect of fiber binding [13], the cement matrix and aggregate in cement-based materials are interconnected, forming a bridge like structure, filling and connecting cracks, enhancing the cohesion of cement-based materials, and thereby increasing the overall continuity of cement-based materials.Moreover, due to the high tensile strength and toughness of PVA fiber, they can effectively resist the tensile stress of cement-based materials, reduce the degree of stress concentration, and effectively alleviate and prevent the formation and expansion of cracks.Furthermore, the addition of PVA fiber is fully encapsulated by the cement paste in the cement-based material, which not only enables it to be more fully linked to the aggregate, but also improves the compactness and tensile strength.Among them, when the amount of fiber added increases from 0.2% to 0.4%, the maximum increase in tensile strength and compressive strength of cement-based materials are also reached.This may be because when the fiber content reaches 0.4%, the bonding degree between PVA fiber and cement paste reaches the best, and the connection between them is the most effective.
However, under the condition of constant fiber content, when the LP replacement amount is between 0% and 20%, the tensile strength gradually decreases with the increase of LP replacement amount, with a decrease range of 0.6% to 1.1%.This is due to the dilution effect of LP, which reduces the hydration products per unit volume of cement-based materials [13], resulting in PVA fiber not being fully encapsulated by cement paste, resulting in ineffective connection with internal cementbased materials, and even causing PVA fiber to aggregate in cement-based materials.

Conclusions
This study investigated the effects of PVA fiber content ranging from 0% to 0.6% and LP content ranging from 0% to 20% on the flowability, tensile strength and compressive strength of cement-based materials.Based on the experimental results, the following conclusions are drawn: 1) After adding PVA fiber, the expansion of cement-based materials decreases, and the demand for SP increases.When the fiber content is 0.6%, even if the SP content reaches 3%, it cannot achieve the specified degree of expansion.When the amount of LP added is 0-20%, there is no significant change in the expansion of cement-based materials.The amount of SP will increase with the increase of LP, especially when the LP content increases from 10% to 20%.
2) The addition of PVA fiber and LP can significantly improve the compressive strength of cement-based materials.When the fiber content is less than 0.4%, the compressive strength increases with the increase of LP and PVA fiber content.However, when the PVA fiber content reaches 0.6%, the excessive fiber addition leads to a decrease in compressive strength.When the PVA fiber content is 0.4% and LP content is 20%, the 28d compressive strength of cement-based materials increases by a maximum of 155%.
3) The tensile strength of cement-based materials increases with the increase of PVA fiber content.However, it gradually decreases with the increase of LP dosage.
To conclude, the dosage of PVA fiber and LP should be reasonably controlled according to the performance requirement for practical use.

Table 1 .
.  Fine aggregate: Fine aggregate used locally produced limestone sand and granite sand in Foshan.It complies with the sand standard for Zone II with a fineness modulus of 3.0 and 2.7.Performance indicators of cement.

Table 2 .
Physical properties of PVA fiber.
Use the GB/T2419-2005 "Method for Determining the Flowability of Cement Mortar" to mix the evenly mixed cement-based material (for the specified small slump cylinder and circular jump table, first slightly wet the instrument with water, then pour the cement-based material into the slump cylinder twice, compact it, and level the surface.Remove the slump cylinder, start the jump table, and after 25 jumps, measure the maximum diameter and vertical diameter of the cementbased material.)The jumping table test method is used for scalability testing, and details during the testing process can refer to this specification.The specific operating methods and test results are based on the "Testing Method for Cement Mortar Strength (ISO Method)"(GB-T 17671-2021).
2.4.1.Flowability.2.4.2.Strength.The strength testing of cement-based materials includes 28d compressive strength testing and 28d tensile strength testing.The experimental steps include placing the 28d cured cementbased material test block into the WAW-2000 microcomputer controlled electro-hydraulic servo universal testing machine and a standardized testing fixture for compressive and three-point tensile strength testing.