Study on the effect of double mineral admixtures on the properties of high water materials

In this paper, different contents of fly ash, silica fume, slag, and Nano-SiO2 were added to the high water material. Then, fly ash-silica fume, slag-silica fume, and Nano SiO2-silica fume are mixed into the high water material. Through the strength test of two groups of specimens, the influence of different types of mineral admixtures and blending ratios on the physical and mechanical properties of high-water materials was studied. The mechanical parameters of high water doped materials were measured by ultrasonic method, and the internal pore structure of high water doped materials was analyzed. In the case of double mixing, the optimal mixing amount is 4% fly ash and 6% silica fume, and the 7 d compressive strength of this group of specimens is increased by 15.85% compared with the original high water material. Through ultrasonic detection, the first wave is smooth and stable, the longitudinal wave amplitude curve is flat, the peak difference is slight, and the periodic amplitude is uniform, indicating that the structure of this group of specimens is dense. The shear modulus is increased by 20.31%, Young’s modulus is 1.3 times, and the volume modulus is 8.9 times that of the original high water material. The research findings suggest that high water materials can be modified by double mixing.


Introduction
Based on the two-carbon target, applying industrial waste to building materials effectively reduces carbon and sequesters carbon [1].High water material has no chemical pollution and good pumping ability.It can quickly fill and treat large areas of goaf and has been used in many fields such as backwall grouting [2].
The new high water material used in this paper is divided into A and B. A and B materials can be pumped separately for a long time without condensation and will condense a certain strength shortly after mixing [3].However, it is prone to condense rapidly at high reaction temperatures, uneven material slurry, and water layering and secretion phenomena [4].Sun et al. [5] found that the reaction temperature rise of fly ash high-water material with 60% content decreased by 22.9%, and the cost decreased by 54.6%.Feng et al. [6] believed that adding 15% fly ash into high-water materials was ideal, which reduced the peak strength by 25%, increased the residual strength by 50%, and reduced the elastic and deformation modulus.Zheng et al. [7] proposed that the optimal dosage of blast furnace slag (BFS) in cement-based materials is 30%.Shi et al. [8] found that the optimal content of fly ash was 20%, and the optimal content of carbide slag in fly ash was 5%.Some experimental results show that nano-silica (NS) can significantly improve the physical and mechanical properties of cementbased materials [9].
The research is mainly focused on the properties of high-water materials doped with a single material, while the research on high-water materials doped with two different materials is less [10].

Composition of raw materials
To break through the limitation of applying high water materials and improve the cement substitution effect of fly ash in high water materials, fly ash, slag, silica fume, and nano silica were used to replace part of cement in high water materials by single or double mixing.The chemical composition and basic properties of raw materials were tested to analyze the effect of mineral admixtures on high water content materials.It can be seen that the chemical composition of calcium sulfoaluminate cement (CSA), slag (S), fly ash (FA), and silica fume (SF) used in the test are shown in Table 1.
Table 1 To improve the testing effect, the experiment adopted ultrafint-slag powder with a density of 2.86 g/cm 3 , a specific surface area of 0.944 m 2 /g, and a median particle size D50 of 3.815 m  .The density of ultra-fine fly ash is 2.90 g/cm 3 , the specific surface area is 0.424 m 2 /g, and the median particle size D50 is 1.729 m  .The specific surface area of silica fume used is 21.709 m 2 /g, most particle size distribution is below 1.0 m  , and D50 is about 200 nm.The three materials are very different in chemical composition, and the activity is not completely the same.Nano-SiO 2 (NS) with a particle size of 10 nm and a specific surface area of 250±30 m 2 /g were used in the experiment.Because the particle size is very small and the surface adsorption capacity of particles is strong, it is difficult to achieve ideal results.Therefore, nano was first prepared as nano dispersion liquid with a nanometer content of 25% and a PH value 7 as a neutral liquid.

Composition of raw materials
The experiment adopted a high water-cement ratio of 1.5:1.It should be noted that when adding nano admixtures because the nano dispersion was used, the water content in the dispersion was also considered when calculating the water consumption.The method of adding mineral admixtures was to replace calcium sulfoaluminate cement in material A with its equal mass, add additive A, and mix evenly with water three times to obtain A slurry.The B material was mixed with additive B and then mixed with water.Finally, the A slurry and B slurry were mixed.
High water material without admixture Jz-0 was used as the reference group.The amount of fly ash, slag, silica fume, and Nano-SiO 2 is determined according to the current research results of domestic and foreign scholars.The results proposed by Zheng et al. [11] show that the effect is that doped with Nano-SiO 2 is the first, doped with silica fume is the second, the strength of high-water materials doped with slag is slightly increased, and the strength of high-water materials doped with fly ash decreases with the increase of fly ash content.
When conducting a double mixing test, we replace the cement content with 5% fly ash in the high water content material and then use slag to replace 1% and 3% of cement in equal amounts, numbering the specimens as FS-1 and FS-2.If 1% or 3% silica fume is used instead of cement, the specimen numbers are FSF-1 and FSF-2.Specimens with 1% and 2% nano substitution are numbered FNS-1 and FNS-2, respectively.When the replacement amount of fly ash is 10%, then we use slag to replace 1% and 3% of cement in equal amounts, numbering the specimens as FS-3 and FS-4.If 1% or 3% silica fume is used instead of cement, the specimen numbers are FSF-3 and FSF-4.Specimens with 1% and 2% nano substitution are numbered FNS-3, FNS-4, and FNS-5.
A full-automatic pressure testing machine was used for the strength test of the specimen.The test acceleration was 2.4±0.2kN/s, as shown in Figure 1.The size of the specimen was 40 mm×40 mm×160 mm, as shown in Figure 2. The number of specimens in each group produced and maintained at the same age and under the same conditions is three.After curing to the age of 1 d, 3 d, and 7 d, the flexural test was carried out first, and the average flexural strength of the three specimens was taken.After the specimens were broken, the axial compression test was carried out on the 6 specimens, and the average compressive strength was taken.
The HKB-type intelligent ultrasonic automatic testing instrument was used for ultrasonic testing of test specimens, as shown in Figure 3.According to the testing results, the elastic modulus, shear modulus, and other material parameters of the specimens were calculated, and the material structure was analyzed.

Test results of fly ash replacement 5%
 According to the research in Section 3.1, adding fly ash could improve the working performance of high-water materials.Still, the early activity of fly ash was low, which reduced the early mechanical properties of materials.Adding slag, silica fume, and Nano-SiO 2 could improve the strength of high water materials to different degrees.To maintain the early physical and mechanical properties of high water materials, fly ash was used to replace cement in high water materials to obtain greater economic benefits.In this study, fly ash and silica fume, fly ash, slag, fly ash, and Nano-SiO 2 were mixed into high-water materials in different proportions, and the early mechanical properties of the specimens were tested. As for the double-doped high-water material with a 5% replacement amount of A material, it could be seen from Figure 4(a) that the double-doped effect of the FSF group was the best among the three groups of the FSF group (fly ash-silica fume), FS group (fly ash-slag) and FNS group (fly ashnano).Due to the low pozzolash activity of fly ash in the early stage, fly ash replaces part of cement in the same amount, reducing hydrated calcium silicate gel.Therefore, the lower the fly ash content in several groups of double-doped specimens is, the higher the compressive strength is.
 Figure 4(b) shows the 1 d~7 d compressive strength growth rate of each experimental group.It could be seen from Figure 2(b) that the experimental group effect of the two materials mixed with high-water materials was superior to that of single fly ash.Although the 7-d compressive strength of the double-admixture test group was close to that of the Jz-0 group, only the FSF-1 group had a higher 7-d compressive strength than the Jz-0 group.Although the increase rate of the strength of the FSF 1 group in 3 d~7 d was significantly reduced, the compressive strength of the FSF 1 group at 7 d age is 1.05% higher than that of Jz-0 and 3.38% higher than that of the F-1 group.
 The compressive strength of group FS-1 at 7 days is 0.92% higher than that of group F-1 but still lower than that of Jz-0. However, the mixed use of fly ash and nano admixtures significantly reduced the early compressive strength of the high water material, even lower than that of the high water material with fly ash alone.This indicates that Nano-SiO 2 and fly ash with small content do not synergize when the water-cement ratio is large.
 Therefore, the optimal ratio of mineral additives was 1% fly ash content and 4% silica fume content in these tests.

Test results of fly ash replacement 10% 
With the increase in the proportion of fly ash, the compressive strength of the specimen decreased continuously.It could be seen from the results of a single-doped test that when fly ash replaced 10% of A material in high-water material, the strength of material 7 d decreased seriously, which was much lower than that of group Jz-0.
 When the substitution amount of double admixture was 10%, it could be seen from Figure 5 that the compressive strength of specimens in the three groups of FSF (fly ash-silica), FS (fly ash-slag), and FNS (fly ash-nano) was significantly higher than that of specimens in the F-2 group (10% fly ash alone).The effect of double admixture in the FSF group was the best.
 When fly ash was mixed with silica fume, the addition of silica fume made up for the deficiency of the early strength of fly ash, and the compressive strength was even higher than that of the Jz-0 specimens.The compressive strength of specimens in group FSF-4 at 1 d, 3 d, and 7 d ages was increased by 50%, 67%, and 40% compared with group F-2 with the same cement replacement amount.The compressive strength of FSF-4 specimens at 1 d, 3 d, and 7 d age increased by 20%, 26.7%, and 15.8% compared with Jz-0 specimens, respectively.
 The composition of Nano-SiO 2 is the same as that of silica fume, and the specific surface area is larger, and the particle size is smaller.From the test results of additive alone, we know that the effect of single doping Nano-SiO 2 was very obvious, and the improvement of compressive strength of Compressive Strength(MPa) ICAMIM-2023 Journal of Physics: Conference Series 2720 (2024) 012018 high-water materials was better than that of silica fume.However, the double mixing test of nano and fly ash did not achieve the expected effect.Moreover, the more Nano-SiO 2 was mixed, the worse the test effect was.The compressive strength of the FNS-3 group mixed with Nano-SiO 2 and fly ash was 7.27% higher than that in the F-2 group but 16.69% lower than that in the Jz-0 group. The effect of fly ash and slag mixed samples in the FS group was between the FSF and FNS groups.The compressive strength of the FS-3 group was 14.55% higher than that of the F-2 group but 5.26% lower than that of the Jz-0 group.

Analysis of test results
The reasons for the test results are analyzed as follows:  The strength of high water material could be improved by adding fly ash and silica fume.Fly ash and silica fume have great differences in activity and physical properties.They can promote each other.The silica fume particles used in the experiment were small and reacted quickly with the initial hydration products, and the hydration product particles adhered to the silica fume particles.Silica fume plays a "nucleation role", which promotes the crystallization, growth, aggregation, and condensation of the hydration reactants.On the other hand, the content of SiO 2 in silica fume is 94.9%, as shown in Table 1.C-S-H and other gel materials produced by SiO 2 and Ca(OH) 2 filled the structural void.The high water material structure was denser.Therefore, the strength effect of high-water materials mixed with fly ash and silica fume was the best, especially the hydration speed was the fastest at 1-3 d age.
 The compressive strength of the high-water material with double fly ash and nano decreased with the increase of nano content.
The composition of Nano-SiO 2 is the same as that of silica fume; the surface area is larger, and the particle size is smaller.Therefore, the effect of nano-nanoparticles alone in high-water materials was very obvious, and the effect of improving the compressive strength of high-water materials was better than that of adding silica fume.However, the double mixing nano and fly ash did not achieve the expected effect.Nano silica can promote hydration reactions and can react quickly with calcium hydroxide.First, the cement in grade A was replaced by fly ash in equal amounts, which reduced the calcium silicate hydrate gel generated during hydration.Secondly, the absorption of Ca 2-by the rapid hydration reaction of Nano-SiO 2 led to the low concentration of Ca 2-in the solution.The formation and growth of hydration products in high water materials were delayed.The low concentration of Ca(OH) 2 could not stimulate the pozzolanic activity of fly ash.Therefore, the compressive strength of high water materials with fly ash and nano materials decreased seriously.The higher the amount of Nano-SiO 2 is, the lower the strength of the high-water material is.
 Although the strength of the high-water material with slag and fly ash was higher than that with single fly ash, it was still lower than the reference group.
Blast furnace slag (BFS) has a fast hydration reaction due to its amorphous properties and high volcanic ash activity.To obtain better activity, the ultrafine slag used in this experiment had a smaller particle size and could quickly consume a large amount of OH -.In addition, slag contains more CaO than fly ash.With high alkalinity, more Ca(OH) 2 was generated after hydration, and calcium silicate hydrate with low alkalinity was generated with SiO 2 .This gel product had high strength.Slag could replace cement for hydration reaction and provide strength, but the strength increase could not make up for the early strength decline caused by fly ash incorporation.

Ultrasonic testing of high water materials with double mineral admixtures
The Jz-0 basic group and FS-4, FSF-4, and FNS-3 test groups in 10% fly ash double-doped groups were selected for ultrasonic detection using HKB intelligent ultrasonic automatic testing instrument.The vertical and horizontal wave shapes of several groups of specimens were compared and analyzed. .An ultrasonic wave of the FNS-3 specimen.Generally speaking, with the increase of the porosity of the material, the wave velocity of the longitudinal and transverse waves will decrease, which also indicates the decrease of the elastic modulus of the material.However, the shape, aspect ratio, distribution, and other factors of the pore of the material will also affect the elastic modulus and Poisson's ratio of the material.

Ultrasonic waveform analysis
In the uniform cement mortar medium model, the ultrasonic wave is stable, and the ultrasonic law is attenuated.The ultrasonic wave propagates in materials, and the acoustic properties of materials and their defects will affect the propagation of the ultrasonic wave.Ultrasonic wave produces reflection, refraction, and wave pattern conversion at different interfaces, and the uniformity of material structure can be understood non-destructively through ultrasonic detection.
 Head-wave analysis.As shown in Figure 6~8, the ultrasonic head wave movement of each group was greater, which was caused by the porosity defects of high water materials.Compared with other groups, the head wave of the FSF 4 group was smooth and stable, the amplitude curve of the pwave was flat, the peak difference was small, and the periodic amplitude was uniform.
 Waveform and crack.However, at the time of 320 s, the longitudinal wave of the FSF-4 specimen showed the loss of a lower peak, and the shear wave was also attenuated (Figure 8).This was because the wave impedance between cement and cavity or crack was very different, and the sound wave would have a total reflection on the interface of the crack.The reflection wave formed by the P-wave and the first arrival wave of the S-wave would interfere with each other, and the S-wave could not propagate in the crack, resulting in the envelope surface of the S-wave being more chaotic.The energy of this part of the sound wave was reduced, and after the P-wave reflection, due to the attenuation of the wave, part of the waveform would also be missing.If a large crack was encountered in the propagation process, the wave had a wide range of total reflection at the interface, and the sound wave propagating outside the crack was less, forming a trapezoid area with weak energy. Waveform analysis.The ultrasonic waveform of FS-4 was similar to that of FSF-4, but the period and amplitude were relatively small, indicating that the two groups of high-water materials were of uniform material.The amplitude of the longitudinal wave of the FNS-3 specimen varied greatly.Still, no significant waveform loss indicated more holes in the high-water material after adding nano and fly ash.Still, the distribution was relatively uniform without large voids.

Basic physical property parameters of materials
After processing, the data of each group are shown in Table 2.It could be seen that the amplitude and wave velocity of group FSF-4 were larger than those of the other groups, and the elastic modulus was also the highest.The compressive strength test also showed that group FSF has the highest compressive strength.Compared with the JZ-0 group, Young's modulus, bulk modulus, and shear modulus of the doped high-water material experimental group were improved.The Young's modulus of the FSF-4 group was 2.3 times that of the original high water material, the bulk modulus was 8.9 times that of the raw material, and the shear modulus was increased by 20.31%.Young's modulus, volume modulus, and shear modulus increased with the decrease of porosity, indicating that the highwater material mixed with fly ash and silica fume (FSF-4) was more compact in its internal structure.

Conclusion
Based on the new high-water material, this paper studies the influence of fly ash, slag, silica fume, and nano mixture on high-water material through compressive test and ultrasonic detection, and the following conclusions can be drawn:  Fly ash, silica fume, and slag are common industrial wastes.If they can be used to replace part of the cement in high-water materials, the cost can be reduced, the reaction temperature can be controlled, and the high-water materials can be prevented from setting too fast.
 When the fly ash content was 5%, the 7-d compressive strength of flyash-silica fume (FSF) was 1.05% higher than that of JZ-0 and 3.38% higher than that of the F-1 group.The 7-d compressive strength of the high-water material with fly ash-slag was 0.92% higher than that of F-1 but still lower than that of JZ-0.However, the strength of the fly ash-nano silica high water material decreased obviously with the increase of the nano content.
 When the fly ash content was 10%, the compressive strength of the FSF-4 specimens of high water material mixed with coal ash and silica fume at the 1 d, 3 d and 7 d ages was 50%, 67% and 40% higher than that in the F-2 group of single fly ash with the same replacement amount.The compressive strength of FSF-4 specimens at the 1 d, 3 d, and 7 d ages increased by 20%, 26.7%, and 15.8% compared with that of JZ-0 specimens, respectively.The compressive strength of the FS-3 group of high water material mixed with coal ash and slag at the 7 d age was 14.55% higher than that of the F-2 group but 5.26% lower than that of the JZ-0 group.The compressive strength of the NFS-3 group doped with Nano-SiO 2 and fly ash was 7.27% higher than that of the F-2 group doped with 10% fly ash but 16.69% lower than that of the JZ-0 group. Through ultrasonic detection, it was obtained that Young's modulus of the FSF-4 group was 2.3 times that of the original high-water material, the bulk modulus was 8.9 times that of the raw material, and the shear modulus was increased by 20.31%.The high-water material (FSF-4) mixed with fly ash and silica fume was known to be dense in the internal structure.The fly ash and slag material mixed with high water was more uniform but less dense than the FSF-4 specimen.There were more holes in nano and fly ash double-doped high water materials, but the distribution of holes was more uniform, and there was no large cavity.
At present, the high water material has high water content, a small Poisson ratio, low hardness, and low strength, which limits the popularization and application range of the material.Adding mineral waste to high-water materials not only saves environmental protection and costs but also improves the strength of high-water materials, which provides a reliable idea for further research of the material and can broaden the application of materials in the future.

Figure 1 .
Figure 1. Figure with pressure testing machine.

Figure 2 .
Figure 2. Figure with the specimen.

Figure 4 .
Figure 4. Figure with the influence of replacing 5% cement with double mixed mineral admixture on compressive strength of high water material. The early strength of specimens in group FSF 1 increased faster from 1 d to 3 d, especially the 1 d compressive strength increased by 10% compared with Jz-0 and 17% compared with group F-1.Although the increase rate of the strength of the FSF 1 group in 3 d~7 d was significantly reduced, the compressive strength of the FSF 1 group at 7 d age is 1.05% higher than that of Jz-0 and 3.38% higher than that of the F-1 group.The compressive strength of group FS-1 at 7 days is 0.92% higher than that of group F-1 but still lower than that of Jz-0. However, the mixed use of fly ash and nano admixtures significantly reduced the early compressive strength of the high water material, even lower than that of the high water material with fly ash alone.This indicates that Nano-SiO 2 and fly ash with small content do not synergize when the water-cement ratio is large.Therefore, the optimal ratio of mineral additives was 1% fly ash content and 4% silica fume content in these tests.

Figure 5 .
Figure 5. Figure with the influence of replacing 10% cement with double mixed mineral admixture on compressive strength of high water material.
. Chemical composition of raw materials wt%.

Table 2 .
Test mix ratio of the two materials added