Effect of Redispersible Latex Powder and Fly Ash on Properties of Mortar

Abstract

In this paper, fly ash is mixed into self-flowing cement mortar by the method of equal mass substitution of cement, and the redispersible latex powder is mixed into cement mortar according to the percentage of cementitious material mass, so as to study the influence on the properties and mechanical properties of cement mortar. The test results show that the incorporation of fly ash prolongs the setting time of cement mortar and reduces the flexural strength, compressive strength, and drying shrinkage. With the increase in the content of redispersible latex powder, the setting time of cement mortar increases gradually, the compressive strength and compressive fracture resistance then decrease gradually, and the flexibility and crack resistance of the cement mortar specimen are improved. The flexural strength of the cement mortar specimen first increased and then decreased, and the drying shrinkage first decreased and then increased, indicating that the mixing amount of redispersible latex powder is not the more the better, but that there is a reasonable range to achieve the best comprehensive performance of cement mortar.

1. Introduction

Since the emergence of cement mortar in the 1830s, it has been widely used in various fields of civil engineering. With the continuous improvement of application technologies of cement mortar, it has become the most basic material in the field of civil engineering [1,2,3,4]. With the continuous development of science and technology, the performance of cement mortar has increasingly higher requirements, and the use of cement mortar admixture is a more obvious effect, economic, and reasonable means. Without the admixture of cement mortar, mortar performance will be poor, displaying shortcomings such as isogeneity, shrinkage, a low flexibility and tensile bonding effect, and low freezing resistance. Thus, the development and application of such construction projects have certain limitations.

Fly ash is a powdery substance with very fine particles that can flow in the air. It is a byproduct of coal combustion and has been widely used in concrete worldwide for many years, with economic, environmental protection, and technical benefits [5]. Zhang Lijuan et al. showed that the addition of fly ash could improve the fluidity of cement mortar, but reduce its compressive strength, and this effect would be more significant with the increase in water–binder ratio [6]. Worldwide, coal-fired power plants produce millions of tons of waste fly ash, causing environmental problems and threatening human health. In addition, it is expensive to store and requires a vast amount of warehouse space. Therefore, the recovery and utilization of fly ash into concrete materials can effectively solve the problem of environmental pollution [7]. Research shows that the use of fly ash significantly improves the compressive strength of cement mortar and significantly reduces its water absorption and pores, and that mortar with a fly ash replacement rate of 20% has the highest compressive strength, the lowest water absorption, and the lowest porosity [8]. Fly ash can effectively improve the shear bond and bending strength of cement mortar [9]. Kosior-Kazberuk, Marta, et al. found that the strength of cement mortar decreases with the increase in fly ash content, and the influence on the strength of cement mortar is not obvious in the early curing stage, but mainly increases the strength in the later stage [10]. Fly ash mainly comes from industrial waste, which contains harmful heavy metals and poses a great threat to the atmosphere, water sources, soil, and the public. As for whether the toxicity of fly ash applied to cement-based materials will affect the surrounding environment, Liang Shihua et al. conducted leaching toxicity tests on soil–cement mixed with fly ash, and the results showed that the heavy metal elements chromium, lead, and zinc were far lower than the control values required by the code, and the leaching amount of chromium was reduced by at least 50% and zinc and lead by at least 97%. This is because the hydration reaction in cement can generate a large number of hydrated gels, which have a large specific surface area and surface adsorption energy and play a role in the adsorption and inclusion of heavy metals [11]. However, the incorporation of fly ash into cement mortar also has some defects. For example, fly ash particles are easy to rise and ooze, resulting in early strength reduction in cement mortar and other problems. Polymer material, as a modified material, can effectively improve its bonding properties when added to cement-based materials [12]. Redispersible latex powder is a kind of high polymer powder, including vinyl acetate and ethylene copolymer powder, vinyl chloride and vinyl laurate ternary copolymer powder, and vinyl acetate and ethylene and advanced fatty acid vinyl ester ternary copolymer powder [13,14]. Yaxin Tao et al. found that although the very early mechanical strength of the redispersive polymer powder was slightly reduced, due to the film formed in the limestone-based mixture, it enhanced the mechanical integrity in the hardened state, but enhanced the overall strength of the material [15]. Mashrafi Bin Mobarak et al. found that the interaction between cement and polymer can improve its physical and mechanical properties, such as increasing bonding strength, reducing shrinkage, and reducing water absorption [16]. Teng Zhaohui added redispersible latex powder into cement mortar to study its mechanical properties, and the results showed that the early bond strength and flexure strength of cement mortar were effectively improved after the addition of redispersible latex powder [17]. Liu Jinquan et al. mixed polypropylene fiber, silica ash, and redispersible latex powder into cement slurry for research, and the test results showed that a certain amount of redispersible latex powder could improve the strength of cement slurry and the water conditioning rate [18].

At present, there are many examples of research on the performance of cement mortar with redispersible latex powder or fly ash, but few scholars study the performance of cement mortar with redispersible latex powder and fly ash at the same time. This paper will present the following research: (1) the influence of different amounts of redispersible latex powder and fly ash on the setting time of cement mortar; (2) Changes in flexural strength, compressive strength, and compressive fracture resistance of 7d and 28d cement mortar specimens with different amounts of redispersible latex powder and fly ash; (3) The effect of different dosage of redispersible latex powder and fly ash on the shrinkage of cement mortar specimens.

2. Materials and Methods

2.1. Test Materials

The test materials used in this test are as follows: cement; Sand, grade II fly ash, water reducing agent, redispersible latex powder, water.

2.1.1. Cement

The cement is P.O42.5 ordinary Portland cement produced by Zhucheng Yangchun Cement Co., Ltd. (Weifang, China), and its performance index is shown in

Table 1. Technical specifications of cement.

Specific Surface Area (m2/kg)	Setting Time (min)		Density (g/cm3)	Flexural Strength (MPa)		Compressive Strength (MPa)		Stability
	Initial Setting	Final Setting		3d	28d	3d	28d
361	180	260	2.83	5.42	9.24	25.76	50.02	Qualified

.

2.1.2. Sand

The sand is made of Chinese ISO standard sand produced by Xiamen Esou New Standard Sand Co., Ltd. (Xiamen, China). The performance indexes are shown in

Table 2. Basic performance indexes of sand.

Apparent Density (kg/cm3)	Moisture Content (%)	Fineness Modulus	Void Fraction (%)	Particle Size (mm)	Packing Density (kg/cm3)
2570	2.8	2.0~2.5	36.2	<4.75	1426

.

2.1.3. Fly Ash

Class II fly ash produced by Huifeng New Material Co., Ltd. (Jiaxing, China) was used in this test, and its performance and chemical composition are shown in

Table 3. Properties and chemical composition of fly ash.

Density (g/cm3)	Specific Surface Area (m2/kg)	Moisture Content (%)	SiO2	Al2O3	Fe2O3	Na2O	MgO	CaO
2.22	419	0.3	56.20%	26.40%	4.30%	3.20%	2.80%	1.50%

.

2.1.4. Redispersible Latex Powder

Type 5010N redispersible latex powder produced by Wacker Company (Hongkong, China) was used in this test, and its basic performance is shown in

Table 4. Basic properties of redispersible latex powder.

Packing Density (g/L)	PH	Density (g/cm3)	Specific Surface Area (m2/kg)
465	8	1.045	346.7

.

2.2. Mix Ratio Design

All materials were dried in a dryer before testing. A total of 7 groups of fitting ratios were set for comparative analysis, among which Group1 did not add fly ash and redispersible latex powder, Group 2 to Group 7 replaced 20% cement with fly ash by the equal mass replacement method, and redispersible latex powder was gradually increased by 1%. The final mix of cement mortar is shown in

Table 5. Mix ratio of cement mortar.

Group	Cement (g)	Sand (g)	Water (g)	Water Reducing Agent	Fly Ash (g)	Redispersible Latex Powder
1	500	1300	260	0.10%	0	0%
2	400	1300	260	0.10%	100	0%
3	400	1300	260	0.10%	100	1%
4	400	1300	260	0.10%	100	2%
5	400	1300	260	0.10%	100	3%
6	400	1300	260	0.10%	100	4%
7	400	1300	260	0.10%	100	5%

.

2.3. Test Methods

2.3.1. Setting Time Test

The setting time test refers to JGJ/T70-2009 “Standard Test Method for Basic Performance of Building Mortar”, and the penetration resistance method is used to determine the setting time of mortar mix. Place the configured mortar mix into the container, and the mortar should be 10 cm lower than the upper mouth of the container. Measure the penetration resistance value with ZKS-100 mortar setting time tester at 20 ± 2 °C. Contact the surface of the mortar with a penetration test needle with a section of 30 mm², and slowly and evenly press it vertically into the inside of the mortar at a depth of 25 mm within 10 s. Record the meter reading N_p at each insertion. The penetration value of mortar should be calculated according to Equation (1).

$$f_p=\frac{N_p}{A_p},$$

(1)

In the equation:

$f_p$—Penetration resistance (MPa), accurate to 0.01 MPa;

$N_p$—Static pressure when penetration depth is 25 mm (N);

$A_p$—The cross-sectional area of the penetration needle is 30 mm².

The time and corresponding penetration resistance values were recorded from the time of adding water and stirring, and the relation diagram between penetration resistance value and time was drawn. The time corresponding to the penetration resistance value of 0.5 MPa was the measured value of the setting time of mortar.

Figure 1 shows the setting test of cement mortar.

2.3.2. Flexural Strength Test

The flexural strength test refers to GB/T17671-1999 “National Standard of the People’s Republic of China” cement mortar strength test method (ISO method), using 40 mm × 40 mm × 160 mm prismatic specimen cement compressive strength and flexural strength determination method. The newly prepared specimens are cured with film in moisture for 24 h, and then demolded and cured in water until strength test. When the age reaches 7d, 14d, and 28d, the specimen is taken out of the water, and one side of the specimen is placed on the supporting cylinder of the DKZ-5000 flexural testing machine. The long axis of the specimen is perpendicular to the supporting cylinder, and the load is evenly applied vertically to the relative side of the prism at a rate of 50 ± 10 N/s through the loaded cylinder until it is broken.

The flexural strength R_f is expressed in N/mm² (MPa) and calculated according to Equation (2):

$$R_f=\frac{1.5F_{f}L}{b^3},$$

(2)

In the equation:

$F_f$—The load applied to the middle of the prism at the time of fracture, N;

$L$—Distance between supporting columns, mm;

$b$—Edge length of prismatic square section, mm.

Figure 2 shows the flexural test of cement mortar.

2.3.3. Compressive Strength Test

According to the GB/T17671-1999 “National Standard of the People’s Republic of China” cement mortar strength test method (ISO method), using a WE-30 hydraulic universal testing machine and compressive strength testing machine fixture, on the side of the broken half prism.

Figure 3 shows the compressive test of cement mortar.

2.3.4. Compressive Fracture Resistance Than

The compressive fracture resistance can reflect the flexibility of the specimen. The smaller the compression and folding ratio, the better the flexibility of the specimen and the better the resistance to cracking.

2.3.5. Drying Shrinkage Test

The drying shrinkage rate test was carried out according to JGJ/T70-2009 “Standard Test Method for Basic Properties of Building Mortar” to determine the natural drying shrinkage value of mortar. The mixed mortar was loaded into 40 mm × 40 mm × 160 mm prismatic test mold, vibrated, and placed in the room at 20 ± 5 °C. Then, the surface of the mortar was wiped flat after 4 h, the mold was removed after curing for 7d under standard conditions, and the mold was numbered to indicate the testing direction. The shrinkage value of mortar during natural drying is calculated according to Equation (3):

$${\epsilon }_{at}=\frac{L_0-L_t}{L-L_d},$$

(3)

In the equation:

${\epsilon }_{at}$—The corresponding value is the natural drying shrinkage value of mortar specimen at day t (7d, 14d, 28d), mm;

$L_0$—The length of the specimen 7 days after forming, mm;

$L$—The length of the specimen, is 160 mm;

$L_d$—The sum of the length of two shrinkage heads embedded in the mortar, is 20 ± 2 mm;

$L_t$—The corresponding is the measured length of the specimen at day t (7d, 14d, 28d), mm.

Figure 4 shows the shrinkage test of cement mortar.

3. Results

3.1. Setting Time Test

Figure 5 shows the influence of fly ash and redispersible latex powder with different dosages on the setting time of cement mortar.

Figure 5 shows that the setting time of cement mortar can be increased by adding fly ash or redispersible latex powder, indicating that both fly ash and redispersible latex powder can increase the setting time of cement mortar. The setting time of cement mortar increased by 8.6% from 302 to 328 min after adding 20% fly ash. With the content of redispersable emulsion powder gradually increasing from 0% to 6%, the setting time of mortar also increased from 328 to 429 min, increasing by 30.8%. The setting time of cement mortar increases linearly with the increase of polymer content of rubber powder, and the growth rate is the same. The retarding effect of latex powder on cement mortar is more obvious than that of fly ash, while fly ash only weakly increases the setting time of cement mortar.

This is because fly ash is a potential active material. When mixing mortar mix, the activity of fly ash is stimulated, which reduces the water consumption of standard cement consistency and prolongs the setting time of cement mortar mix [19]. The tiny latex particles in the redispersible latex powder are adsorbed on the surface of the crystallization of cement hydration product Ca(OH)₂, forming ionic bonds between the carboxylate ions of the polymer and Ca²⁺ in the hydration product, accelerating the precipitation of Ca(OH)₂ crystallization, affecting the cement hydration, and thus affecting the setting time of mortar [20,21].

3.2. Flexural Strength Test

Figure 6 shows the influence of fly ash and different dosage of redispersible latex powder on the flexural strength of cement mortar specimens.

Figure 6 shows that the flexural strength of cement mortar increases with the growth of age. By comparing the data of the first group and the second group, it can be found that the flexural strength of ordinary cement mortar decreases significantly after the addition of fly ash, among which the specimens of 7 days decrease by 12.8% from 6.32 to 5.51 MPa. Specimens at 14 days decreased from 7.11 to 6.07 MPa, a decrease of 14.6%. The 28d specimen decreased from 7.78 to 6.55 MPa, which decreased by 15.8%. With the increase in the content of redispersible emulsion powder, the flexural strength of the cement mortar specimens at the age of 7, 14, and 28 days all showed a trend of increasing first and then decreasing, and the compressive strength of the specimens reached the peak at the content of 5%. The peak value of specimens aged 7 days is 5.69 MPa, which is 3.3% higher than that of the second group. The peak value of the specimen at the age of 14 days is 6.44 MPa, which increases by 6.1%. The peak value of the specimen at the age of 28 days is 6.75 MPa, an increase of 3.2%. It shows that the bending strength of cement mortar mixed with fly ash will be reduced, and the bending strength will be increased after the addition of redispersible emulsion powder, but too much will make the bending strength decreased. The reasons for this phenomenon are as follows: The hydration reaction of fly ash mainly depends on the activation of Ca(OH)₂ generated by cement hydration. After the fly ash replaces part of the cement, the proportion of cement in the cement–fly ash system decreases, and the effective water–cement ratio controlling the hydration rate of cement increases relatively, which reduces the concentration of Ca²⁺ ions in the solution, resulting in a slow hydration rate of the whole system. The generated hydration product particles are not tightly connected with each other, thus reducing the flexural strength of cement mortar [22]. After the emulsion powder is mixed with mortar, it is formed into a polymer emulsion and evenly dispersed in the mortar. During the hydration and hardening process, the emulsion loses water and forms polymer film, which fills the vacancy and defect in the mortar and forms the anchoring effect. Therefore, within a certain range, with the increase in the content of latex powder, the defects in mortar become reduced and its bending strength increases. However, when the latex powder exceeds a certain amount, due to the air entraining effect of latex powder and the property of large viscosity after emulsification, the bubbles in mortar increase and the large bubbles do not easily overflow and form harmful pores, thus leading to the decline in the mortar’s bending strength [23].

3.3. Compressive Strength Test

Figure 7 shows the influence of fly ash and different dosages of redispersible latex powder on the compressive strength of cement mortar specimens.

Figure 7 shows that adding fly ash and redispersible emulsion powder to cement mortar mix will lead to the decline of its early compressive strength. According to the data of the first and second sets, the compressive strength of ordinary cement mortar decreases after the addition of fly ash, among which the specimens at 7 days decreased from 15.76 to 13.35 MPa, a decrease of 15.3%. Specimens at 14 days decreased from 17.57 to 14.86 MPa, a decrease of 15.4%. The specimens at 28 days decreased from 19.53 to 17.42 MPa, a decrease of 10.8%. With the increase in the content of redispersible emulsion powder, the compressive strength of cement mortar specimens is still decreasing, but the decreasing range is obviously decreasing. When the content of rubber powder increases from 0% to 6%, the compressive strength of specimens aged 7 days decreases by 18.1% from 13.35 to 10.93 MPa. The compressive strength of specimens at the age of 14 days decreases from 14.86 to 11.24 MPa by 24.4%. The compressive strength of the specimens at the age of 28 days decreases by 26.8% from 17.42 to 12.76 MPa. The reasons for this phenomenon are as follows: Fly ash is a potential active material with low activity. After replacing cement of equal quality with fly ash, cement clinker minerals in cement mortar are reduced, hydration ability is weakened, hydration products are generated in small quantity, and the connection between products is not close enough. The total hydration rate in the cementing–fly ash system slows down, thus reducing the strength of the cement mortar [24]. However, the addition of adhesive powder will reduce its density, and the affinity between adhesive powder and mortar is weak, which makes the structure of the cement mortar specimen loose, thus leading to the reduction in its compressive strength [25].

3.4. Compressive Fracture Resistance Than

Figure 8 shows the influence of fly ash and different dosages of redispersible latex powder on the compressive fracture resistance of cement mortar specimens.

The results in Figure 8 show that after the incorporation of fly ash into ordinary cement mortar, the compressive fracture resistance of the 28d specimen increases from 2.51 to 2.66, an increase of 6.0%, while the 7d and 14d specimens basically have no change, indicating that fly ash has basically no effect on the compressive fracture resistance than of the early cement mortar specimen, but increases the compressive fracture resistance than of the later specimen. With the increase of the mixing amount of redispersible latex powder, the compressive fracture resistance than of cement mortar specimens gradually decreases, but the rate of reduction gradually becomes slow. When the mixing amount of rubber powder exceeds 5%, the compressive fracture resistance than basically does not change. When the dosage of rubber powder increases from 0% to 6%, the compressive fracture resistance than of 7d specimens decreases by 19.8% from 2.42 to 1.94. The compressive fracture resistance than of 14d specimens decreased by 27.8% from 2.45 to 1.77. The compressive fracture resistance than of 28d specimens decreased by 28.2% from 2.66 to 1.91. It shows that the effect of redispersible latex powder on the compressive fracture resistance of cement mortar specimens in the late stage is greater than that in the early stage.

3.5. Drying Shrinkage Test

Figure 9 shows the influence of fly ash and different dosages of redispersible latex powder on the drying shrinkage of cement mortar specimens.

Figure 9 Test results show that the dry shrinkage rate of cement mortar is significantly reduced after fly ash is added into the cement mortar, indicating that fly ash has a significant effect on improving the shrinkage performance of cement mortar mix. With the increase in the content of redispersible latex powder, the drying shrinkage rate of cement mortar specimens decreases first and then increases. The inflection point occurs when the content of rubber powder is 4%, indicating that the drying shrinkage rate of the specimen is smaller with the increase in the content of redispersible latex powder, but that there is an optimal content. Compared with 0% cement powder content test group, the drying shrinkage rate of the cement mortar sample with 4% cement powder yield decreases from 0.032 mm to 0.27 mm, which decreases by 15.6%. The drying shrinkage of specimens at 14 days decreased by 7.0% from 0.043 to 0.04 mm. The drying shrinkage of the specimen at 28 days decreased by 8.0% from 0.5 to 0.046 mm. The reasons for this phenomenon are as follows: Redispersible latex powder has a very strong hydrophilicity, and thus mortar-mixing it can absorb a large number of water molecules attached to the surface of the sand. With the hydration of cement, hydration products and polymer mesh structures formed by the interpenetrating mesh structure become more and more dense, gradually offseting a part of the mortar internal shrinkage force. A small amount of mixed adhesive powder shows that mortar shrinkage deformation decreases; however, as the content of rubber powder continues to increase, the degree of cement hydration increases, and the hydration products gradually fill the internal space of the mortar. This means the volume of the chemical reaction between polymer and cement hydration products is smaller than the sum of the volumes of polymer, cement, and water, so the shrinkage of mortar increases with the increase in the content of rubber powder [26,27].

4. Conclusions and Prospects

4.1. Conclusions

In this paper, 20% cement is replaced by fly ash by the equal mass substitution method, and on this basis, the redispersible latex powder is added in 1% increments to study the effects of fly ash and redispersible latex powder with different dosages on the setting time, flexural strength, compressive strength, compressive fracture resistance, and drying shrinkage of cement mortar. The conclusions are as follows:

After 20% cement is replaced by fly ash by the equal mass substitution method, the setting time of the cement mortar specimen is prolonged, and the flexural strength, compressive strength, and drying shrinkage of cement mortar specimen are reduced due to the activity and dispersion of fly ash.

On the basis of adding 20% fly ash, with the increase in the content of redispersible latex powder, the setting time of cement mortar specimen gradually increases, and the compressive strength and compression folding ratio gradually decreases. When the yield of rubber powder reaches 6%, the setting time of the specimen increases by 30.8%. The compressive strength of 7d, 14d, and 28d specimens decreased by 18.1%, 24.4%, and 26.8%, respectively. The compressive fracture resistance decreased by 19.8%, 27.8%, and 28.2%, respectively, indicating that the redispersible latex powder can improve the flexibility and crack resistance of cement mortar specimens.

The apparent flexural strength of redispersible latex powder increases with the addition of fly ash-modified cement mortar, the setting time increases with the increase of the content, the compression ratio and shrinkage rate also improve, but the compressive strength decreases.

The bending strength of cement mortar increases first and then decreases with the increase in the content of rubber powder, and the compression ratio and shrinkage rate decrease first and then increase. It shows that the dosage of rubber powder is not simply the greater the better, but that there is a reasonable range. When the content of redispersible emulsion powder is 4%, the flexural strength of cement mortar is the maximum. When the dosage is 4%~5%, the folding ratio is the smallest. When the dosage is 3%, the shrinkage rate is the smallest.

4.2. Prospects

With the development of concrete technology and the enhancement of people’s awareness of environmental protection, concrete with waste materials as the main component—as a kind of sustainable green concrete—will become a development direction of concrete in the future and will also become a new way to solve the problem of construction waste.

In this paper, fly ash and redispersible latex powder are added into cement mortar. On one hand, the construction cost can be reduced, and the environmental pressure can be alleviated. On the other hand, the performance of cement mortar can be improved. Through the analysis of the results of this experiment, it is found that there are still some deficiencies to be studied and improved:

(1) The porosity, water absorption, and aging properties of the new material need to be further studied.

(2) The influence mechanism of fly ash and redispersible emulsion powder on the properties of mortar can be further analyzed by combining the microscopic properties with the macroscopic properties.

(3) Whether other waste materials, such as waste bricks and straw, can be used in cement materials.