A research on the preparation and property of a low-cost cement-based self-leveling mortar

HAN Fangfang1,GAO Kuimin2,LIANG Fei1,ZHANG Sen1,CHEN Lingyan1,ZHANG Ming1,JIA Jitang1 (1.Shandong Huawei Yinkai Building Materials Science and Technology Stock Co. Ltd.,Zibo 256410,China; 2.Shandong Taili Mortar Science and Technology Stock Co. Ltd.,Zibo 256410,China)

Abstract:In order to solve problem of the low early strength and large shrinkage of cement-based self-leveling mortar,we of- ten use more aluminate cement,which restrict the mortar application. A low-cost cement-based self-leveling mortar was prepared by using a high early strength polycarboxylate superplasticizer and less aluminate cement. The influences of the amount of alumi- nate cement,sand-binder ratio,coarse whiting content,redispersible emulsion powder and anti-foam agent to mortar were also stud- ied. The optimal mix proportion of materials was determined. The property of the mortar reached the technical requirements of the national standard JC/T 985—2005 “Cement-based self-leveling mortar for ground use”. Compared with the like product in the mar- ket,the cost would decrease 100 yuan per ton.

0 Introduction
Cement-based self-leveling mortar is a kind of cementitious cementitious material, with suitable additives and fine sand mixed. The new flooring material used for leveling or finishing the floor has the advantages of good fluidity and stability, easy construction, low labor intensity, smooth and smooth, high strength, thin leveling thickness and good acid resistance. It is a large supermarket and shopping mall. , Parking lot, factory workshop, warehouse and other ground building ideal materials, is also a development direction of building ground construction at present, the market has great potential.
Cement-based self-leveling mortar technology has been widely used in developed countries such as Europe and the United States, but China’s research on this type started late. At present, it has a large gap compared with foreign developed countries. It mainly has problems of low early strength and large shrinkage. To solve the above problems, domestic manufacturers mainly use aluminate cement, supplemented by portland cement and gypsum. Due to the high price of aluminate cement and the high cost of self-levelling mortar, the cost of large-area use in construction projects is high, which is difficult for users to accept.
Early strength polycarboxylic acid water reducer is a functional water reducer product developed in recent years, which can improve the early strength of the mortar and make the mortar have a lower shrinkage rate.
This article intends to use a common Portland cement, supplemented with aluminate cement and gypsum gelling system and early strength polycarboxylic acid water reducer to develop a kind of high early strength and low shrinkage. Low-cost cement-based self-levelling mortar is of great significance to the development of the domestic mortar industry.

1 Experiment
1.1 Raw materials
Cement: P · O42.5R ordinary portland cement produced by Zibo Baoding Cement Co., Ltd. and CA50 series aluminate cement produced by Shandong Yunhe Cement Co., Ltd .; sand: quartz sand of 40 ~ 120 mesh; heavy calcium carbonate : 200 mesh; gypsum: fluorogypsum; water reducing agent: early strength polycarboxylic acid water reducing agent PCA powder and ordinary polycarboxylic acid water reducing agent powder produced by Shandong Huawei Yinkai Co., Ltd .; defoaming agent: Momentive Chemical 770DD mineral oil dry powder defoamer; redispersible powder: 5044N redispersible powder from WACKER, Germany; cellulose ether: HPMC400 from Hercules; early strength agent: lithium carbonate; retarder: tartaric acid , 200 mesh.
1.2 Mortar preparation process and performance test method
1.2.1 In the preparation process,
first wipe the stirrer and stirring blade with a wet cloth, pour the weighed cement, sand, heavy calcium and other raw materials into the vertical mortar stirrer, and turn on the mixer After mixing for 5 s, add water slowly, add it within 20 to 30 s, stir for 180 s, and stand for 3 min, then stir for 180 s, and you can use.
1.2.2 Performance test method For
flowability, flowability retention, flexural and compressive strength, tensile bond strength, and abrasion resistance of self-leveling mortar, refer to JC / T 985-2005 “Cement-based self-leveling mortar for flooring” For testing, refer to JC / T 1004—2005 “Ceramic Wall and Floor Tile Sealant” for testing.

2 Results and discussion
Most cement self-levelling mortars are fast-drying and early-strength systems. The fast-drying and early-strength properties are mainly achieved by adjusting the mix ratio of ordinary portland cement and aluminate cement. Ordinary Portland cement has a moderate setting speed, with low early strength and significant strength growth in the later period; aluminate cement has large heat of hydration, rapid heat release and concentration, fast setting speed, high early strength, and compact structure, impermeability, Excellent corrosion resistance, but the strength increase in the later period is not obvious. The two types of cement can be used in combination to achieve complementary advantages.
See Table 1 for common cement-based self-leveling mortar foundation formulas.

Basic formula of cement-based self-leveling mortar

2.1 Early-strength polycarboxylic acid water reducing agent PCA and the effect of the proportion of ordinary portland cement and high alumina cement on the performance of
self-leveling mortar. Water reducing agent is the most important additive in self-leveling mortar. It is currently widely used in China. It is a common type polycarboxylic acid water reducing agent. The early-strength polycarboxylic acid water reducing agent PCA used in this test can promote the hydration of cement, thereby improving the early strength of self-leveling mortar. According to Table 1, keep the other components and dosages unchanged, change the type of water reducing agent and the ratio of ordinary Portland cement to aluminate cement, and study its influence on the performance of self-leveling mortar. .

Effect of Superplasticizer and Aluminate Cement Amount on Self-leveling Mortar Performance

It can be seen from Table 2 that under the condition that the ordinary portland cement and aluminate cement have the same ratio, compared with the ordinary formula using ordinary polycarboxylic acid water reducing agent self-leveling mortar, the early strength polycarboxylic acid is used. The compressive and flexural strength of the water-reducing agent PCA’s self-levelling mortar increased by 30% and 64%, respectively, and the compressive and flexural strength and tensile bond strength increased slightly, and the shrinkage decreased by 23% on 28 days.
When PCA, an early-strength superplasticizer, is used, the compressive and flexural strength of self-leveling mortar gradually decreases with the decrease of aluminate cement content, but the compressive and flexural strength and tensile strength of 28 d are reduced gradually. The bond strength and shrinkage have increased slightly, and the performance is still better than the self-leveling mortar of the basic formula. The price of aluminate cement is about 5 times that of ordinary portland cement. In this experiment, the amount of aluminate cement is selected as 10%, which can reduce the cost of the self-leveling mortar of the basic formula by 100 yuan / t. In subsequent experiments, PCA, an early-strength polycarboxylic acid water-reducing agent, was selected. Ordinary portland cement accounted for 25% and aluminate cement accounted for 10%.
2.2 The effect of mortar-binder ratio on the performance of self-leveling mortar
According to the optimal ratio of 2.1 and keeping other components unchanged, the effect of the ratio of mortar-binder on the performance of self-leveling mortar was studied. The results are shown in Table 3.

Effect of mortar-binder ratio on properties of self-leveling mortar

It can be seen from Table 3 that with the increase of the sand-to-rubber ratio, the water consumption of the mortar increases, and the strength at 1 d and 28 d gradually decreases. When the sand-rubber ratio is 0.9 ~ 1.0, although the mortar has higher strength at 1 d and 28 d, its workability is average, and its shrinkage is larger. When the mortar-rubber ratio reaches 1.1 to 1.2, the strength of self-levelling mortar can meet the requirements for use for 1 d and 28 d, and the mortar has good workability, the surface of the hardened specimen is smooth, and the shrinkage rate is also low; continue to increase the mortar-rubber ratio At 1.3, a small amount of bleeding occurred on the surface of the mortar, and the surface of the hardened test specimen was peeled and frosted, and the strength was low. From the aspects of mortar workability, strength and cost, the sand-rubber ratio was selected to be 1.1 in subsequent experiments.
2.3 Effect of the amount of heavy calcium on the properties of self-leveling mortar (see Table 4)
Based on the 2.2 preferred ratio and keeping other components unchanged, the effect of the amount of heavy calcium on the properties of self-leveling mortar was studied. The results are shown in Table 4.

Effect of the amount of heavy calcium on the fluidity of self-leveling mortar over time

It can be known from Table 4 that under the condition of reaching the same fluidity, the water-gel ratio decreases first and then increases with the increase of the amount of heavy calcium. The water-gel ratio is the smallest when the amount of heavy calcium is 17.5%. The shape of heavy calcium particles is irregular, and the particle size distribution is wide. When the amount of heavy calcium is small, it is not enough to completely fill the pores between the sand. It requires a large amount of water and high hydrogel; but when the amount is appropriate, heavy calcium It can tightly fill the pores between sand particles, make the aggregate skeleton structure compact, reduce the amount of water required, and reduce the water-to-gel ratio. When the amount of heavy calcium is too high, the specific surface area increases, the water demand increases, and the water-to-gel ratio will increase. The addition of heavy calcium can significantly reduce the loss of mortar over time, and the loss of mortar is minimal when its dosage is 17.5%.
2.4 The influence of the amount of redispersible emulsion powder on the performance of the
self-leveling mortar. Based on the optimal mix ratio of 2.3 self-leveling mortar (that is, the amount of heavy calcium is 17.5%, the water-to-gel ratio is 0.43, and the other components remain unchanged). The effect of the amount of redispersible emulsion powder on the properties of the mortar is shown in Table 5.

Effect of the amount of redispersible emulsion powder on the properties of self-leveling mortar

It can be seen from Table 5 that the greater the amount of redispersible emulsion powder, the more flexural and tensile strength, air content, abrasion resistance, and setting time of the mortar will increase, while the shrinkage will decrease. After the amount of compressive strength reached 2.4%, the amount of redispersible polymer powder continued to increase, and the compressive strength decreased greatly. Under the condition of ensuring that the fluidity, compressive strength and gas content meet the requirements of JC / T 985-2005, the amount of redispersible emulsion powder is selected to be 2.4%.
2.5 Effect of defoaming agent on the apparent state of mortar
Cement-based self-leveling mortar has high standards for fluidity, apparent state, shrinkage and strength. It requires that the surface of the mortar after hardening should be flat, free of pinholes, and have good properties. Homogeneity and high strength. The defoamer can eliminate the air bubbles entrained or generated during the mixing and construction of the mortar, improve the compressive strength and improve the surface condition. Based on the 2.4 optimal formula, the effect of the amount of defoamer on the properties of the mortar was studied. The results are shown in Table 6.

Effect of the amount of defoaming agent on the properties of self-leveling mortar

It can be seen from Table 6 that within the experimental dosage range, the incorporation of a defoamer helps to improve the compressive strength of the self-levelling mortar, and the surface flatness and smoothness are greatly improved. The optimal amount of defoamer selected in the experiment was 0.15%.
2.6 Performance testing
The optimal mix ratio of the self-leveling mortar determined by the above experiments is shown in Table 7, and the performance test results are shown in Table 8.

Optimal ratio of cement-based self-leveling mortar, performance of self-leveling mortar prepared by optimal ratio

Table 8 shows that the cement-based self-leveling mortar mixed with early-strength polycarboxylic acid water-reducing agent PCA meets the performance indicators such as flowability, strength, shrinkage rate, and setting time on the premise of reducing the amount of aluminate cement. JC / T 985-2005 requirements.

3 Conclusions
(1) When using early-strength polycarboxylic acid water reducing agent PCA, as the amount of aluminate cement decreases, the 1-day compressive and flexural strength of the self-levelling mortar gradually decreases, but the 28-day compressive and The flexural strength, tensile bond strength, and shrinkage increased slightly.
(2) With the increase of the sand-to-rubber ratio, the water consumption of the mortar increased, and the strength gradually decreased at 1 d and 28 d.
(3) The addition of heavy calcium can significantly reduce the loss of fluidity of the mortar over time; as the amount of redispersible polymer powder increases, the flexural and tensile strength, air content, abrasion resistance and setting time of the mortar increase. Larger, and the shrinkage rate will decrease accordingly; adding an appropriate amount of defoamer will help improve the compressive strength of the self-levelling mortar, and the surface flatness and smoothness will be greatly improved.
(4) The optimal mixing ratio of cement-based self-leveling mortar is determined through experiments: m (ordinary portland cement): m (aluminate cement): m (quartz sand): m (heavy calcium): m (fluorine) Gypsum): m (defoamer): m (early strength polycarboxylic acid water reducer PCA): m (redispersible latex powder): m (retarder): m (early strength agent): m ( Cellulose ether) = 250: 100: 385: 174: 60: 1.5: 3: 24: 1.7: 0.6: 0.6.
(5) The performance of the cement-based self-levelling mortar prepared according to the optimal ratio is in compliance with the requirements of JC / T985-2005 standard. Due to the use of early-strength polycarboxylic acid water-reducing agent PCA, the amount of aluminate cement is reduced, so that the cost of self-levelling mortar is reduced by 100 yuan / t compared with commercially available products.

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