Influence of fly ash content on macroscopic properties and microstructure of high-performance concrete

doi:10.21203/rs.3.rs-4821515/v1

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Influence of fly ash content on macroscopic properties and microstructure of high-performance concrete

https://doi.org/10.21203/rs.3.rs-4821515/v1

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To study the influence of fly ash content on the performance of high-performance concrete (HPC), Seven groups of tests were conducted to evaluate macroscopic properties (workability and compressive strength) and microscopic pore structure. Low field nuclear magnetic resonance (NMR) technology and SEM images were used to analyze the changes in the internal pore structure of the concrete. The results show that the workability of HPC initially increases and then decreases with the increase in fly ash content. When the content is 15%, the slump of HPC reaches a maximum of 264 mm, and the 28-day compressive strength increases by 23.2% compared to the 7-day compressive strength. The pore size distribution of the concrete varies with different fly ash contents, with over 95% of the pores in each group measuring less than 0.1 µm. At 15% fly ash content, secondary hydration of the fly ash is sufficient, refining the pores between 0 and 0.1 µm.

Physical sciences/Materials science/Structural materials

Physical sciences/Engineering/Civil engineering

high-performance concrete

fly ash

SEM

low-field nuclear magnetic resonance

T2 spectrum

pore structure

As a kind of industrial waste, fly ash is collected after burning coal ash in thermal power plants. It can be used as an active admixture to replace part of cement in concrete, which can reduce the amount of concrete cement, improve work performance and construction efficiency, and save energy [1, 2]. Due to the good pozzolanic activity of fly ash, it has been gradually applied to HPC as a concrete admixture [3]. Ismail [4] investigated the effect of fly ash on the water permeability and chloride ion permeability of alkali-activated slag mortar and concrete. Permeable pore volume and adsorptivity tests show that the water absorption rate of alkali-activated materials is higher than that of OPC-based materials. Increasing the amount of fly ash causes a decrease in mechanical strength and an increase in water absorption. Yazıcı [5] investigated the properties of self-compacting concrete which incorporate 30–60% fly ash, as a cement replacement, the experimental results showed that self-compacting concrete could be obtained with a high-volume fly ash. But fly ash replacement cement caused a reduction in compressive strength of concrete, including early and ultimate compressive strength. Jiang [6] reported carbonization and corrosion resistance of concrete mixed with a large amount of low-quality fly ash. The test results show that concrete with low-quality fly ash has good carbonization and corrosion resistance. Babu [7] studied the effect of natural admixture effect on performance of concrete containing high volume class F fly ash.

Low-field nuclear magnetic resonance (NMR) technology, as a new technology for concrete pore structure testing [8], has the advantages of fast and convenient operation, and repeatable tests without damaging the test block. It has been widely used in rock pore structure [9]. Díaz-Díaz [10] reported an improved low-cost imbedding miniature NMR sensor which can nondestructively measure the evaporation water loss and porosity refinement of cement-based materials with low and high water-cement ratios. Enjilela [11] studied the relationship between the pore structure and T₂ lifetime in concrete. The long T₂ lifetime and its associated amplitude, related to water in the pore space (micro and macropores), change with local moisture content. Porteneuvea [12] studied the pore size distribution of three reactive powder concrete formulations via the proton nuclear magnetic relaxation method. The discrete and fractal characteristics of this concrete distribution were confirmed, and the surface fractal dimension was evaluated for each formulation, revealing the hierarchical structure of pores. Zhang [13] used nuclear magnetic resonance (NMR) technology to test the porosity and pore size distribution of the test concrete and other microstructure parameters. The test results show that the optimal content to reduce the gas and water permeability of concrete is 30% and 40%, respectively. Using low-field NMR technology to study the influence of fly ash content on the internal pore size distribution of HPC, we can understand the essence of the influence of various factors on the performance of concrete more deeply [14]. In this study, the influence of fly ash content on workability and compressive strength of HPC is studied, and the pore size distribution of HPC with different fly ash content is studied by nuclear magnetic resonance technology.

2.1 Materials and mix proportion

Cement: P·O42.5 Portland cement whose fineness is 3400cm²/g. Silicone fume: Gray white fine powder with particle size less than 2µm, density of 2.214kg/m³, specific surface area of 143100cm²/g. Quartz sand: Choose coarse, medium and fine quartz sand. Coarse sand particle size is 0.63–1.25mm, medium sand particle size is 0.32–0.63mm, fine sand particle size is 0.16–0.32mm. Fly ash: Grade I fly ash is the fine ash collected from the flue gas after coal combustion, which is the main solid waste discharged from coal-fired power plants. Basalt fiber: Short-cut fiber with length of 12mm is used to improve the HPC compressive strength. Water reducing agent: 37% solid content of poly carboxylic acid system high performance water reducing agent.

Seven kinds of mix proportions with different fly ash contents were prepared in this experiment, respectively A0, A1, A2, A3, A4, A5 and A6. The amount of cement replaced by fly ash (mass fraction %) is respectively 0%, 10%, 15%, 20%, 25%, 30%, 35%. The mixture ratio of HPC is shown in Table 1.

Table 1

Base mix proportion of HPC
Cement (kg/m³)	Water (kg/m³)	Silica fume (kg/m³)	Sand (kg/m³)	Basalt fiber volume fraction (kg/m³)	Water reducing agent (kg/m³)
803.95	177.09	200.99	1306.42	5.5	6.03

2.2 Specimen preparation

According to the ratio and dosage of each group, after checking the mixer without error, first pour three kinds of quartz sand, start the machine and mix it thoroughly, about 2-3min can be done; Continue to pour several gelled materials, turn on the machine and mix it well. It can be observed to be uniform by naked eye for about 2-3min. Sprinkle appropriate amount of fiber into the blender and mix until evenly dispersed, for about 30s, then close the blender and continue to sprinkle fiber, and cycle until all the fibers are evenly mixed. Then pour the evenly mixed water-reducing agent and half of the water into the mixture, and stir for about 3min. It can be observed that the mixture changes from wet to thick. Finally, pour the remaining water into the blender and mix thoroughly for about 3min to finish the mixing process. After stirring, slump and expansion test should be carried out quickly. After that, the mold should be quickly installed to the full extent, put on the shaking table for 2min to vibrate for compaction and scrape the surface with a spatula. The high performance concrete specimen after molding by vibration is moved into the standard incubator with the mold, and stored in the environment of 95% humidity and about 20℃ for more than 6 hours. The complete specimen is removed from the mold and labeled. Move to high temperature curing box for 70°C high temperature steam curing for three days.

2.3 Test methods

All the compressive strength specimens in this test adopt the size of 100mm×100mm×100mm. The loading speed is an important factor that affects the results of the concrete strength test. In this test, the loading speed should be 0.5-0.8MPa/s for the concrete aged 7d and 28d. The microstructure was analyzed by scanning electron microscopy (SEM) using Quanta250FEG scanning electron microscope (from FEI Company, USA). The pore structure was determined by low-field nuclear magnetic resonance analyzer. In order to meet the requirements of NMR equipment, three R50mm×60mm cylindrical specimens were prepared in each group. Nuclear magnetic resonance (NMR) measurements were carried out on each group of specimens to measure the T2 relaxation value and porosity of the samples.

3.1 Concrete workability

The content of admixture has the greatest influence on the working performance of concrete mixture. In this test, the addition of fly ash also significantly affects the working performance of the mixture. The relationship between the content of fly ash and the degree of concrete slump and expansion is shown in Fig. 1.

(a) Slump

Figure 1 Workability of high-performance concrete

(b) Expansion

It can be seen from Fig. 1 that when fly ash content is less than 15%, the slump and expansion of HPC increase with the increase of the fly ash content, and when the replacement amount is 15%, the slump reaches the maximum of 264mm. When fly ash content is more than 15%, the slump and expansion of HPC decrease with the increase of fly ash content. When the fly ash replacement amount is increased by about 5%, the slump decreases by 6–40mm, and the decrease is obvious when the fly ash replacement amount is more than 25%.

After finishing slump test, the viscosity and water retention of concrete mixtures were observe. It was found that there was no excessive moisture on the concrete surface under the ratio of each group, indicating that the water retention was good. When the slump bucket is knocked gently from the side, the mixtures will not collapse loosely, indicating that the concrete mixtures had good cohesion. In addition, the greater the slump, the greater the fluidity of concrete. When fly ash content lower than 15%, the liquidity of high performance concrete increase with the increase of the dosage of fly ash. The reason is that fly ash has a ball effect - fly ash contains a large number of compact and smooth glass beads. The incorporation of fly ash into HPC can reduce the friction between aggregate and mortar, so that the newly mixed fly ash concrete has better plasticity and cohesion than ordinary concrete.

When the content of fly ash is higher than 15%, the fluidity of HPC begins to decrease; the fluidity of HPC with a content of 20% and 25% drops slightly and remains the same as the control group. However, when the content of fly ash in concrete is higher than 25%, because the density of fly ash is lower than that of cement, the volume of fly ash is about 1.3 times the volume of cement under the same quality. Therefore, when the fly ash content is too large, the volume of slurry increases significantly. A large amount of slurry fills in the gap of aggregate and is wrapped on the surface of quartz sand, thickening the lubrication film and reducing the viscosity of the mixture, thus reducing the fluidity of HPC.

3.2 Compressive strength

The test results of 7d and 28d cube compressive strength of HPC are shown in Table 2. It can be seen that the early strength of concrete without fly ash is higher than that of HPC with fly ash, and its strength decreases significantly with the increase of fly ash content. The compressive strengths of the A1-A6 of the 7-day curing age of HPC can only reach 92.5%, 88.1%, 84.0%, 82.9%, 78.4%, and 74.8% of that of the control Group A0, respectively. This is because the early strength concrete mainly depends on the C-S-H produced by hydration reaction of cement. For HPC with different amounts of fly ash substitution, the amount of cement decreases with the increase of fly ash content, and the amount of C-S-H produced by cement hydration is reduced, thereby the early strength of concrete decrease.

Table 2

Compressive strength of HPC
Mix ID	Fly ash substitution amount%	Compressive strength(MPa)		f₇/ f₂₈
Mix ID	Fly ash substitution amount%	f₇	f₂₈	f₇/ f₂₈
A0	0	86.14	96.62	0.892
A1	10	79.64	91.78	0.868
A2	15	75.62	93.14	0.812
A3	20	72.39	85.27	0.849
A4	25	71.47	80.64	0.886
A5	30	67.55	72.56	0.931
A6	35	64.41	70.29	0.916

f ₇: cubic compressive strength of concrete curing for 7 days, f₂₈: cubic compressive strength of concrete curing for 28 days.

The 28d strength of HPC with fly ash content of 10%, 15%, 20%, 25% can reach more than 80% of that of the control group. The activity of fly ash in the concrete increases significantly, the pozzolanic reaction is significantly accelerated, and the activity and gelation are more significant. Volcanic ash effect is the main reason for the growth of the late strength of concrete mixed with fly ash, which makes the late strength of HPC greatly improved under a certain amount of mixture. However, when the fly ash content is 30% and 35%, the strength of the concrete is still very low. It can be seen that if the fly ash content is too large, the generated hydration products are reduced, which has a significant impact on the strength of the concrete.

From the above data, it can be concluded that the late strength of HPC containing fly ash has a faster growth rate than that of control group, and the growth rate is related to the amount of fly ash. The relationship between the compressive strength of the concrete cubes from 7d to 28d is fitted, and the fitting formula is shown in Eq. (1).

$${f_{28}}=\left[ {1+\left( {10.22+\frac{{366.64}}{\pi } \cdot \frac{{9.02}}{{4{{(100x - 15)}^2}+{{9.02}^2}}}} \right) \cdot 0.01} \right] \cdot {f_7}$$

In which, x is the fly ash content.

In order to verify the applicability of this formula, the compressive strength values of HPC with fly ash content of 0%, 10%, 20%, and 40% in literature [15] were examined. The results show that the average error between the theoretical value and the actual value of 28d compressive strength obtained by substituting 7d compressive strength into Eq. (1) is only 5.15%.

4.1 SEM test

The microstructure of HPC at 28d age was experimentally studied. From the results of scanning electron microscopy, it can be seen that the main hydration products that can be observed in the 28-day-old HPC are C-S-H gel, hexagonal sheet layered Ca(OH)₂ crystals, needle-like Aft crystals and powders that are not involved in hydration fly ash particles.

The results indicated that different amounts of fly ash substitution have a greater impact on the high-performance microstructure. It can be seen from Fig. 2(a) that the hydration products of HPC without fly ash are basically type III C-S-H gels, and the hydration products are not well connected as a whole. The pores in the concrete are still more obvious, which will limit the improvement of the performance of HPC. In HPC without fly ash admixture, although the mineral components have reacted sufficiently with water, the internal density of the concrete is not high;

It can be seen from Fig. 2(b) that there are fewer fly ash particles observed in the HPC with a content of 10%, but more Ca(OH)₂ crystals are produced. It shows that when the amount of fly ash is small, it does not completely react with Ca(OH)₂ crystals and the secondary hydration reaction results in a large amount of cement hydration products in the concrete. It can be seen from Fig. 2(c) that the HPC with 15% fly ash content shows that the morphology of various mineral components is not very obvious and the resulting gel particles of hydrated product are intertwined and connected together. Concrete mixed with fly ash, due to the presence of a large amount of SiO₂ and A1₂O₃ in the fly ash, the secondary hydration reaction occurs with the hydrated product Ca(OH)₂ crystals, and a large amount of C-S-H gel is formed. The products are tightly combined together, increasing the density of the cement stone, the pores around the interface become smaller, and the structure is denser.

From the SEM images of HPC with different fly ash content, it can be concluded that the amount of fly ash affects the amount of hydration products inside the concrete. The addition of fly ash affects the degree of hydration inside the concrete. When no fly ash is added, the cubic compressive strength of HPC is higher due to the sufficient hydration of cement. Increase in fly ash content will reduce cement content, more hydration products, sparse distribution→more hydration products, dense distribution, high degree of hydration→more and dense hydration products, but wrapped A small amount of unhydrated fly ash particles→less hydration products, sparse distribution, and a large amount of unhydrated fly ash. This process explains the change process of the cubic compressive strength of HPC at 28d age.

4.2 Influence of fly ash content on pore structure

Nuclear magnetic resonance uses CPMG pulse sequence test on fully saturated HPC to obtain the attenuation signal of water signal in the pore. There are a variety of exponential decay processes in the pores inside the specimen. The decay amplitude of each spin echo train can be accurately fitted with a set of exponential inversion to obtain the attenuation constant, thereby obtaining all the attenuation constants, and then forming the nuclear magnetic resonance T₂ Spectral distribution. And from the multi-exponential decay curve in the porous medium, it has been proved that the relaxation time will be affected by the pore structure: the smaller the pore, the shorter the relaxation time T₂; the larger the pore, the longer the relaxation time T₂.

4.2.1 T₂ spectrum distribution of HPC

The T₂ spectrum distribution of the transverse relaxation time is related to the pore size. The T₂ spectrum distribution diagram actually reflects the distribution of the pore size; small pores T₂ small; large pores T₂ large. Therefore, the T₂ spectrum distribution directly reflects the size of the pore volume. Figure 3 shows the NMR T₂ spectrum distribution of HPC with different fly ash content. It can be seen that the NMR T₂ spectrum distribution of HPC mainly shows three peaks and the first peak of is the largest; it indicates that most of the pores of HPC are tiny pores.

It can be seen that the first peak value of Group A0 is small, and the peak value of the third peak is the largest. The reason is that the internal pore size of the control group without fly ash is mostly large pores, and there is more saturated water in the large pores, and the relaxation time is longer. The first and second peaks of Groups A3, A4, and A5 are similar, and the pore water content in the concrete is less, so the signal is also less. It shows that adding fly ash can make the concrete denser and reduce the distribution of tiny pores. The amount of fly ash in Group A2 is 15%, and its signal amplitude is the smallest, indicating that 15% fly ash will significantly reduce the pores in the HPC. With the increase of fly ash content, the first peak of the curve increases, and the internal pores increase. From the third peak of each curve, it can be seen that when the fly ash content is 35%, the three peaks of the Group A6 are much larger than those of the other groups. The amount of signal is the largest at each relaxation time, and the internal water content is significantly greater than that of other groups.

4.2.2 Pore size distribution of HPC

The pore size distribution of HPC with different fly ash content is shown in Fig. 4(a) it can be seen that the pores of HPC specimens are mainly nano-pores. The most distributed signal between 0.001 and 0.1um, the peak value is at 0.01um. There is a small peak between 0.1 ~ 1um and 1 ~ 100um. In the 7 groups of concrete, the pore size of 0ཞ0.1um accounts for more than 95%. The Group A2 with 15% fly ash has a smaller pore size distribution than other groups, and a smaller pore throat distribution. The Group A6 has the most pore size distribution between 0 ~ 0.1 and 0.1 ~ 1um;The content of fly ash has a great influence on the pore size distribution of HPC between 0 ~ 0.1 um, and the pore size decreases first and then increases with the increase of fly ash content. It can be seen from Fig. 4(b) that the pore size of the Group A2 is 0 ~ 0.1um, and the distribution within 0.1 ~ 1um is less. The Group A3 has a smaller pore size distribution in the range of 1-100um, so the influence range of fly ash content on the pore size is also different.

The particle size of fly ash is much smaller than that of other aggregates in concrete. In addition, the addition of too much fly ash leads to the reduction of cement content, which is not good for the early hydration of HPC, making the hydration products decrease with the increase of fly ash content. The combined effect of these two factors causes the pores of HPC to decrease first and then increase with the increase of fly ash content.

4.2.3 Porosity and T₂ spectrum area of HPC

When the HPC specimen is saturated with vacuum pressure, the internal pores are occupied by water. Nuclear magnetic resonance technology can calculate the volume of the pores in the porous medium by measuring the quality of water and the density of the known water, thereby obtaining the pores degree size. The area of NMR T₂ spectrum is directly proportional to the amount of fluid contained in concrete. The total area of T₂ spectrum is generally equal to or less than the effective porosity of concrete, which can intuitively reflect the change of pores and the damage process of concrete can be obtained. Table 3 shows the porosity and nuclear magnetic resonance T₂ spectrum area of HPC with different fly ash content.

It can be seen from Table 3 that the porosity and T₂ spectrum area of HPC under different fly ash content show a trend of first decreasing and then increasing. It shows that the pore volume first decreases and then increases with the increase of fly ash content. The porosity of the concrete without fly ash is 5.67%. After adding fly ash, the porosity of the Groups A0 ~ A6 is reduced by 14.11%, 35.45%, 8.82%, 10.05%, 3.70%, -20.11%, respectively. The change rate of T₂ spectrum area relative to the baseline group was 5.21%, 36.32%, 2.93%, 5.28%, 4.45%, -12.07%; From the percentage of the area of the first peak, it can be seen that when the fly ash added to the HPC does not exceed 25%, the small pores corresponding to the first peak in the concrete increase with the addition of fly ash; When the fly ash content is greater than 25%, the proportion of small pores decreases, while the larger pores corresponding to the second and third peaks increase. It shows that adding appropriate amount of fly ash can significantly improve the internal pores of HPC. The particle size of fly ash is smaller than that of cement. When the amount of fly ash is less than 25%, the hydration product will be densely distributed and have a high degree of hydration. The inside of concrete is mostly nano-sized pores. When the content is greater than 25%, part of the fly ash only plays a role in filling the concrete, and at the same time affects the early hydration of the concrete, resulting in the increase of large pores.

Table 3

Porosity and NMR spectrum area
Group	Porosity (%)	T2 spectrum area	Percentage of the first peak (%)	Percentage of the second peak (%)	Percentage of the third peak (%)
A0	5.64	5054.49	89.84	7.31	2.70
A1	4.86	4791.37	93.01	4.90	2.01
A2	3.66	3218.63	92.34	3.99	1.79
A3	5.12	4906.44	93.82	4.19	1.93
A4	5.13	4787.41	92.09	5.17	0.73
A5	5.46	4829.62	88.43	9.30	3.27
A6	6.81	5664.86	84.16	10.87	4.96

(1) The appropriate addition of fly ash can fill the pores between aggregates and ensure an enough slurry on the aggregate surfaces, thus improving the workability of high-performance concrete. When the fly ash content is 15%, the slump of the concrete reaches a maximum of 264 mm.

(2) At fly ash contents of 15% and 20%, the internal hydration of HPC is sufficient, resulting in densely distributed hydration products and small internal pores. With the increase of age, the compressive strength increases rapidly. However, when the fly ash content reaches 25%, the insufficient secondary hydration of fly ash only plays a filling role in the concrete, leading to deterioration in concrete performance and a decrease in the strength growth rate.

(3) The pores in the range of 0-0.1um account for more than 95% of all pore diameters. The fly ash content affects changes in internal pore diameters. the pore diameter larger than 10um in the HPC without fly ash is more prevalent than in other groups. When the fly ash content is 15%, the pore size of concrete between 0-0.1um is significantly refined. When the fly ash content exceeds 25%, the decrease in the degree of hydration leads to an increase of the pore size between 0.1-1um.

Conflicts of Interest:

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Author Contribution

Mengxin Kang: Conceptualization, methodology and writing- review & editing. Yabo Jia: Formal analysis, Writing—original draft preparation. Peng Guo: Data curation, Methodology, Writing - original draft. Yanzhong Ju: investigation and resources, supervision, writing draft. Hongji Zhang: Investigation.

Acknowledgement

The authors are grateful for the financial support provided by the National Natural Science Foundation of China (Project No. 51878128) and the Science and technology project of Education Department of Jilin Province (Project No. JJKH2023015KJ).

Data Availability

Data associated with the present study will be available upon request from the corresponding authors.

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No competing interests reported.

Download PDF

Version 1

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You are reading this latest preprint version

Influence of fly ash content on macroscopic properties and microstructure of high-performance concrete

Status:

Version 1

Abstract

Figures

1 Introduction

2 Experimental program

2.1 Materials and mix proportion

2.2 Specimen preparation

2.3 Test methods

3 Macroscopic test results and analysis

3.1 Concrete workability

3.2 Compressive strength

4. Microscopic characteristics of HPC containing fly ash

4.1 SEM test

4.2 Influence of fly ash content on pore structure

4.2.1 T₂ spectrum distribution of HPC

4.2.2 Pore size distribution of HPC

4.2.3 Porosity and T₂ spectrum area of HPC

5 Conclusion

Declarations

Conflicts of Interest:

Author Contribution

Acknowledgement

Data Availability

References

Additional Declarations

Status:

Version 1

Influence of fly ash content on macroscopic properties and microstructure of high-performance concrete

Status:

Version 1

Abstract

Figures

1 Introduction

2 Experimental program

2.1 Materials and mix proportion

2.2 Specimen preparation

2.3 Test methods

3 Macroscopic test results and analysis

3.1 Concrete workability

3.2 Compressive strength

4. Microscopic characteristics of HPC containing fly ash

4.1 SEM test

4.2 Influence of fly ash content on pore structure

4.2.1 T2 spectrum distribution of HPC

4.2.2 Pore size distribution of HPC

4.2.3 Porosity and T2 spectrum area of HPC

5 Conclusion

Declarations

Conflicts of Interest:

Author Contribution

Acknowledgement

Data Availability

References

Additional Declarations

Status:

Version 1

4.2.1 T₂ spectrum distribution of HPC

4.2.3 Porosity and T₂ spectrum area of HPC