Zhou Xinwen 1,2, Liu Jianzhong 2,3, Liu Guangyan 3, Liu Jiaping 1,2
(1. School of materials science and engineering, Southeast University, Nanjing 211189; 2. State Key Laboratory of high performance civil engineering materials, Nanjing 210008; 3. Jiangsu subote new materials Co., Ltd., Nanjing 211103)
Abstract: in order to study the influence of the shape of manufactured sand particles on the rheological properties of mortar, the digital image processing technology (DIP) was used to quantitatively characterize the shape of sand particles. The rheological curve of mortar was measured by design test and the yield stress and apparent viscosity of mortar were calculated. The results show that the aspect ratio (E) and aspect ratio (f) of machine-made sand are 14% and 18% larger than that of natural river sand respectively, which indicates that the rod-shaped and flaky degree of machine-made sand is higher than that of natural river sand; the rheological curve of mortar reflects that the irregular shape of sand particles will increase the flow resistance of mortar, which is more significant under high volume fraction; the yield stress of 40% volume fraction machine-made sand mortar fitted by Chateau model is The results show that the apparent viscosity of machine-made sand mortar is higher than that of natural sand, especially at low shear rate.
Key words: machine made sand; particle shape; rheological property; yield stress; apparent viscosity
Sand is one of the important raw materials of concrete, accounting for about 30% of the total volume of concrete. Its quality and dosage have an important impact on fresh concrete and hardened concrete. At present, high-quality natural river sand resources can not meet the needs of engineering construction in our country, and machine-made sand is widely used as an alternative resource. Therefore, the research on the influence of machine-made sand on concrete performance and its control has become a key problem in the application of machine-made sand resources.
The application of machine-made sand in concrete has become a research hotspot. According to the physical characteristics of machine-made sand, the main focus is on the influence of stone powder, gradation and particle shape on the performance of concrete. It is found that stone powder has influence on workability, mechanical properties and durability of concrete. The content of stone powder has a critical effect on the workability of concrete. The proper content of stone powder can effectively improve the mechanical properties and durability of concrete. Gradation is one of the quality indexes of machine-made sand. It is found that the gradation of machine-made sand affects the workability of concrete. After gradation adjustment, the concrete with workability better than that of natural sand can be prepared. The conclusion that the shape of machine-made sand particles is irregular is widely accepted, but most of them are based on the perceptual judgment of visual observation, and the quantitative characterization of the shape of particles is insufficient. It is pointed out in the literature that the shape of machine-made sand particles has many effects on the workability and mechanical properties of concrete. Generally speaking, the shape of machine-made sand particles affects the workability of concrete. In the existing research, the particle shape is only one of the reasons to explain the change of concrete performance, and the influence of sand particle shape on concrete performance is directly proved by experiments.
In this paper, the shape of sand particles is evaluated by the method of digital image processing (DIP), and the rheological test of mortar with single particle size is designed. The effect of sand particle shape on the flow resistance, yield stress and apparent viscosity of mortar is explored. Combined with the Chateau model, the effect of sand particle shape on the yield stress of mortar is quantitatively revealed, and the effect of sand particle shape on the apparent viscosity of mortar is qualitatively compared In order to provide reference for the material design of machine-made sand concrete.
1 raw materials and test methods
1.1 raw materials
Cement: Onoda cement, grade P · Ⅱ 52.5, performance is shown in Table 1.
Sand: one kind of natural river sand (RS) and three kinds of machine-made sand (MS1, MS2, MS3). In this study, in order to eliminate the influence of stone powder and gradation on the rheological properties of mortar, four kinds of sand with single particle size ranging from 1.18 mm to 2.36 mm were screened for rheological test, and its appearance is shown in Figure 1.
Water reducer: polycarboxylic acid water reducer, solid content is 20%.
1.2 test method
1.2.1 dip method
Use dip method to measure sand length (L), width (W) and thickness (T) as shown in Figure 2, and calculate sand length width ratio (E) and width height ratio (f) according to formula (1) and (2) to describe sand particle shape.
In the formula, e describes the needle rod degree of particles; F describes the flat degree of particles. The axial diameter ratio is defined as (E + F) to describe the three-dimensional shape of particles. The smaller the axial diameter ratio is, the smaller the sum of the aspect ratio and the aspect ratio is, the more regular the particle shape is.
1.2.2 rheological test of mortar
The rst-sst rheometer produced by Brookfield company is used to measure the rheological properties of mortar. The rheological test system is shown in Figure 3. See Table 2 for mortar mix proportion for test.
2 results and discussion
2.1 particle shape characterization
The particle morphology of the sand used in this study was characterized based on dip method, and the results are shown in Figure 4. Taking Fig. 4 (a) as an example, e of particle A is 2.11 and F is 2.61; however, for the particle material with uneven particle shape, the shape of single particle cannot represent its overall shape, that is, particle a cannot represent the overall shape of RS, otherwise it will cause random error, which can also be proved from the standard deviation of RS axial diameter ratio (e-sd and f-sd). Therefore, in this study, the particle shape of each kind of sand is measured with a large sample, and its average value is selected as the result of the characterization of the overall shape of the sand. For example, the e-avg of RS is 1.24, and the f-avg is 1.25, so it is statistically significant to evaluate the results of the sand particle shape.
From Figure 4, it can be seen that the aspect ratio of RS is smaller than MS2 and MS3, almost equal to MS1, which shows that the difference between the maximum and medium axial diameters of RS particles is small, that is, RS particles have fewer rod-shaped particles and smaller narrow length, while MS particles are mostly narrow rod-shaped, and the maximum narrow length of MS3 particles is 1.41, with the highest rod-shaped degree; compared with F of various sands, the aspect ratio of RS is smaller than that of MS This shows that the difference between the medium and minimum axial diameters of RS particles is the smallest, that is to say, RS particles have the least flakiness and the smallest flatness, while MS particles show the most flat flakiness, and MS2 has the largest flatness of 1.47 and the highest flakiness. Therefore, compared with natural sand, manufactured sand is usually irregular in shape, with large narrow length and flatness, and the particles are mostly rod-shaped and flaky, which is basically consistent with the existing research results. This is because the natural river sand particles have been scoured and polished by water for a long time, and the sharp protuberances on the edge of the particles have been abraded, showing a smooth shape similar to the spherical particles, while the machine-made sand particles produced by direct mechanical crushing lack this long-term grinding and shaping process, so they are rod-shaped and flaky irregular shapes.
2.2 analysis of rheological curve of mortar
The rheological curves of mortar with sand volume fraction of 10%, 20% and 35% are selected as representative (see Figure 5) to analyze the shear stress of mortar. Figure 5 (a) shows the rheological curve of 10% volume fraction mortar. It is found that when the shear rate is less than 10 s-1, the shear stress of CP (volume fraction is 0) is greater than rs. A similar conclusion has been found in the literature: when the volume fraction is less than 15%, the yield stress of mortar with 0-0.315 mm particles is less than that of clean mortar. This is because the presence of sand particles in the mortar will break the flocculation structure of the cement paste, reduce the interaction between the cement particles, and thus reduce the shear stress of the mortar. When the sand volume fraction is less than 10%, the shear stress drop effect caused by the sand particles breaking the flocculation structure is greater than the shear stress rise effect caused by the interaction between the clean slurry and the sand particles. When the sand volume fraction is greater than 10%, the shear stress drop effect caused by the sand particles breaking the flocculation structure is less than the thickening effect caused by the sand particles, and the shear stress of the mortar is greater than that of the clean slurry, as shown in Fig. 5 (b) and Fig. 5 (c). Based on the above test results and literature data, it can be explained that the shear stress of mortar is less than that of clean slurry under the condition of low volume fraction and low shear rate. Under the condition of low volume fraction and low shear rate, the existence of sand particles in mortar causes the damage of cement flocculation structure, and the effect of shear stress reduction is dominant, thus the phenomenon of shear stress of mortar is less than that of clean slurry appears; When the volume fraction is high, the interaction (friction and collision) between sand particles increases the resistance of mortar in shear movement. At this time, the effect of the increase of shear stress of mortar caused by the interaction between particles is dominant, so that the shear stress of mortar is greater than that of clean mortar. For high shear rate, the flocculation structure of cement paste has been destroyed by the rapid shear movement. At this time, the sand At the same time, the friction and collision resistance of sand particles still exist in the mortar, so that the shear stress of mortar with any volume fraction under high shear rate is greater than that of clean mortar.
In this study, the rheological curve of mortar with sand volume fraction greater than 10% is similar. Therefore, 20% and 35% volume fraction mortar are selected as the representative of medium volume fraction and large volume fraction mortar for analysis. It can be seen from Figure 5 that under the same shear rate, the shear stress of mortar is greater than that of clean mortar, which further verifies the above discussion. Except for MS1 mortar, the shear stress of mortar increases with the irregularity of particle morphology, namely RS & lt; MS2 & lt; MS3. Therefore, it can be concluded that the more regular the shape of sand particles, the smaller the shear stress, that is to say, the flow resistance of mortar decreases with the regularization of the shape of sand particles. For the case that the axial diameter ratio of MS1 is not consistent with its shear stress, the author thinks that the reason for this phenomenon is that the particle size of MS1 is slightly small and it can not directly compare the shear stress with RS, MS2 and MS3. Through the measurement of particle size by camsizer x2 particle size measuring equipment produced by Retsch technology company, it is found that the particle width (XC) of MS1 is 1.41 mm, while the XC of RS, MS2 and MS3 are 1.51 mm, 1.51 mm and 1.53 mm respectively, and the mesh size range is 1.18-2.36 mm, so it is impossible to distinguish the smaller size of MS1. It has been found that the smaller the size of sand is, the greater the flow resistance of mortar is. For concrete, it can be inferred that the irregular shape of sand particles will increase the flow resistance of concrete and decrease the workability, which is basically consistent with the results of previous research : the irregular shape of machine-made sand particles will cause the slump of concrete to decrease and the workability to deteriorate.
2.3 yield stress analysis of mortar
The yield stress of mortar and the fitting results of chatuae model are shown in Figure 6. It can be found that when the volume fraction of sand is 15%, the yield stress of each group of mortar is almost the same, which is not affected by the shape of sand particles; when the volume fraction reaches 20%, the irregular shape of particles will increase the yield stress of mortar, which is basically consistent with the existing research results, and the increasing effect will increase with the volume fraction of sand And more significantly. In order to reasonably analyze the results, Chateau model is used to analyze the mortar yield stress in this study, and the model fitting results are shown in the solid line in Figure 6. It can be seen from the figure that under the same volume fraction, the yield stress of MS3 mortar is significantly higher than that of MS2 and RS mortar, which indicates that the irregular sand particle shape will lead to the increase of the yield stress of mortar. At the same time, it can be found that when the sand volume fraction is 15%, the yield stress of MS3 mortar is slightly larger than that of MS2 and RS mortar. When the sand volume fraction is 40%, the yield stress of MS3 mortar is about 1.5 times that of RS mortar, which shows that the effect of sand particle shape on the yield stress increases with the increase of volume fraction. Chateau model quantitatively describes the influence of sand particle shape on the yield stress of mortar, and advances the influence of particle shape on the yield stress of mortar from qualitative description to quantitative description, which provides an effective basis for the mix design of machine-made sand concrete.
2.4 analysis of apparent viscosity of mortar