The Influence of Mineral Powder Dosage on the Mechanical Properties and Microstructure of Self-Compacting Concrete

Abstract

The dosage of mineral powder has a complex influence on the compressive strength of self-compacting concrete, among which the pore structure is a key determining factor. This study investigated the effects of different dosages of mineral powder (0%, 5%, 10%, 20%, and 30%) on the workability, mechanical properties, and pore distribution in C80 self-compacting concrete. The research results show that an appropriate dosage of mineral powder (0–20%) can significantly improve the spreadability and fluidity of C80 self-compacting concrete. This phenomenon is mainly attributed to the shape effect and micro-aggregate effect of mineral powder, which improve the fluidity of concrete, reduce the viscosity of the paste, and thereby increase the spreadability and gap-passing rate. By testing the BSD-PS1/2 series fully automatic specific surface area and pore size analyzer, we found that there are columnar pores and ink bottle-shaped pores in C80 self-compacting concrete, as well as a small amount of plate-like slit structures. Our observations with an SEM scanning electron microscope revealed that the width of micro-cracks and micro-holes is between 1 and 5 μm and the diameter is between 3 and 10 μm. These microstructures may have an impact on the mechanical properties of the structure. By applying fractal theory and low-temperature liquid nitrogen adsorption tests, this study revealed the relationship between the fractal characteristics of internal pores in C80 self-compacting concrete and the dosage of mineral powder. The results show that with the increase in mineral powder dosage, the fractal dimension first decreases and then increases, reflecting the change rule of the complexity of pore structure first decreasing and then increasing. When the dosage of mineral powder is about 20%, the compressive strength of SCC reaches the maximum value, and this dosage range should be considered in engineering design.

1. Introduction

Self-compacting concrete is a mixture composed of coarse aggregate, fine aggregate, cement, and various mineral admixtures. Due to its high strength and good compactness, it is widely used. Under the “dual carbon” strategy, the demand for self-compacting concrete has surged in areas such as super high-rise buildings and marine engineering. As one of the main raw materials of self-compacting concrete, cement generates a large amount of dust and harmful gases during production, causing serious environmental pollution. Mechanically produced sand, such as the fine aggregate of self-compacting concrete, not only solves resource and environmental problems but also reduces the cost of concrete. Therefore, the use of mechanically produced sand to make self-compacting concrete has become the market mainstream. According to statistics, the consumption of mechanically produced sand in China will exceed 10 billion tons in 2024. However, the use of mechanically produced sand will lead to the deterioration of the flow performance of self-compacting concrete, resulting in a 10% to 15% increase in cement usage. Mineral powder is a common industrial waste product. In the production process of self-compacting concrete, the method of substituting cement with equal mass can be adopted to reduce cement usage. The large-scale application of mineral powder can reduce cement usage by 30% to 60% and reduce CO₂ emissions by 80 to 120 kg per cubic meter of concrete. C80 self-compacting concrete is widely used in modern construction engineering due to its high flowability, anti-segregation properties, and the ability to self-level and fill formwork without vibration. Mineral powder, as an excellent mineral admixture, possesses activity effects, morphology effects, and micro-aggregate effects. It can improve the durability, workability, and pumpability of fresh concrete, as well as reduce the water demand [1]. American scholars Smith et al. [2] found, through extensive experimental studies, that the appropriate addition of mineral powder can significantly improve the workability and fillability of self-compacting concrete, with a minimal impact on early strength. However, excessive addition can lead to slow early-strength development. The German scholar Schmidt [3] indicated that the incorporation of mineral powder can refine the internal pore structure of concrete, enhancing its density and thus improving its impermeability and durability. The Japanese scholar Tanaka [4] used scanning electron microscopy (SEM) and other microscopic testing techniques to conduct an in-depth analysis of the changes in micro-pores under different mineral powder contents. It was found that as the mineral powder content increases, the gel-pore content rises while the harmful large-pore content decreases.

Research in this field by domestic academic circles has made certain progress, forming a series of original theoretical achievements and practical breakthroughs. Xu Hua-jing [5] et al. studied the impact of mineral powder content on the mechanical properties of C80 self-compacting concrete, indicating that increasing the mineral powder content within a certain range can enhance the later strength of the concrete. However, when the content exceeds 30%, the trend of strength increase slows down. Zhang Dong [6] compared the workability and mechanical properties of self-compacting concrete under different mineral powder contents through experiments, concluding that the optimal mineral powder content range is 20–25%. In terms of micro-porosity studies, Gao, X. B [7] et al. used a mercury porosimeter (MIP) to test the pore structure parameters of concrete with different mineral powder contents, finding that the addition of mineral powder reduces the minimum pore size and total porosity, thereby improving the microstructure of the concrete. Yu Yao-wei [8] used a C50 mix ratio and replaced cement with mineral powder at equal amounts of 0%, 10%, 20%, 30%, 40%, and 50% to conduct tests on slump, compressive strength, and carbonation resistance. It was found that when the mineral powder content is between 10% and 25%, the workability, mechanical properties, and carbonation resistance of the concrete are all in an ideal state. Mineral powder can be used as an admixture for high-performance concrete at C50 [9], and the appropriate amount of mineral powder can not only effectively reduce the amount of cement in the concrete but also significantly improve its overall performance. Currently, there is limited research on the impact of mineral powder admixtures on the relationship between the mechanical properties and pore structure of C80 self-compacting concrete. Existing standards [10] do not specify the compatibility requirements between mineral powder content and manufactured sand characteristics, leading to frequent cracking and leakage issues in engineering projects. Therefore, it is urgent to conduct research on dosage thresholds, long-term performance, and standardization. This paper focuses on mineral powder and C80 self-compacting concrete, studying the effect of mineral powder content on the compressive strength of high-performance C80 concrete. It aims to reveal the mechanism of mineral powder in concrete to support the revision of the “Green Building Evaluation Standard” [11]. This research provides valuable references for enriching and perfecting the theoretical framework of the relationship between concrete microstructure and performance. It also holds significant academic value for enhancing the prediction of concrete performance in the transition zone between cementitious materials and aggregates.

2. Sample Preparation

2.1. Test Materials

The materials used in the preparation of self-compacting concrete samples include cement, (carbonate rock) manufactured sand, gravel (carbonate rock and crushed stone), mineral powder, silica fume, microspheres, expansive agents, admixtures, and tap water. The cement used is P.O 42.5 ordinary Portland cement, produced by “Luzhou Sade Cement Co., Ltd.” Luzhou, China. For the test, two types of crushed stone were selected as coarse aggregate. The first type is “Guami Stone”, with a diameter ranging from 5 mm to 10 mm, and the second type of crushed stone has a diameter between 10 mm and 16 mm. The manufactured sand used in the test was prepared by “Panshi” Building Materials Company, Bijie, China. The mineral powder used in the experiment was S95 grade mineral powder provided by Guizhou “Guixin Building Materials Co., Ltd.” Guiyang, China. The silica fume, microspheres, and expansive agent used in the experiment were supplied by China Railway No.11 Engineering Group Co., Ltd., Wuhan, China. The cumulative particle size distribution and particle size distribution were measured using a laser particle size analyzer, as shown in Figure 1 and Figure 2. The chemical compositions of various powders are listed in

Table 1. Mineral composition and content of materials.

Material	Mass Fraction/%
	SO3	SiO2	Fe2O3	Al2O3	CaO	MgO	Cl−	K2O	Na2O	P2O5	Others
Cement	2.5	18.2	4.3	6.2	62.2	1.5	—	1.01	0.31	0.2	3.58
Mineral powder	2.7	24.2	3.2	16.7	37.8	7.5	—	1.2	—	0.2	6.5
Silica fume	0.7	92.1	0.2	0.8	0.2	0.5	—	0.3	—	0.04	5.16
Microspheres	0.4	64.4	5.4	10.2	10.0	1.1	—	2.6	0.7	0.7	4.5
Expansion agent	2.6	10.2	1.8	8.2	71.3	5	—	0.3	0.5	0.1	0

.

Based on the technical specifications of self-compacting concrete, the physical and chemical properties of raw materials and cement compatibility are comprehensively considered, and the self-developed polycarboxylate high-performance water reducer is selected as the main admixture. The main chemical components and mass ratio of the admixture are shown in

Table 2. Mix ratio of admixtures.

Admixture	Water Reducer	The Collapse Agent	Retarder	Defoaming Agents	Air entraining Agent	Water
Mix proportion/%.	34	18	0.25	0.002	0.001	47.747

, and the composition and content of admixture are shown in

Table 3. Composition and content of admixture.

Surveillance Project	Chlorine Ion Content/%	Total Alkali/%	Solid Volume/%	Density/%	pH Value/%	Sodium Sulfate Content/%
Detection result	—	0.35	24.9	1.04	5.6	0.1

.

2.2. Mix Ratio Design

The design strength grade of self-compacting concrete is C80, the design compactness is 2200 kg/m³, and the total water consumption is 130 kg/m³. The amount of mineral powder is 0%, 5%, 10%, 20%, and 30%. The detailed mix ratio parameters are shown in

Table 4. C80 Mix design of self-compacting concrete (unit: kg/m³ quantity).

NO.	Sand Ratio /%	Water-Binder Ratio/%	Mechanism Sand/kg	Total Amount of Cemented Material (640 kg/m3)					Melon and Rice Stone /kg	Gallet/kg	Water Reducer /kg
				Cement /kg	Microspheres/kg	Mineral Powder/kg	Silica Fume /kg	Expansion Agent /kg
SP-A	0.47	0.21	620	449	76	0	75	40	367	551	17.92
SP-B	0.47	0.21	620	417	76	32	75	40	367	551	17.92
SP-C	0.47	0.21	620	385	76	64	75	40	367	551	17.92
SP-D	0.47	0.21	620	321	76	128	75	40	367	551	17.92
SP-E	0.47	0.21	620	277	76	172	75	40	367	551	17.92

.

2.3. Test Sample Molding

The specimen size was 100 mm × 100 mm × 100 mm, and the physical properties were tested in Panstone Building Materials Laboratory, Bijie City, Guizhou Province. The physical properties included expansion, the gap-passing rate (J-ring test and V-funnel test), gas content, etc.Preparation process and physical property test of C80 SCC test samples are shown in Figure 3.

3. Test Methods

3.1. Mechanical Properties Test of Concrete

According to the relevant provisions of the “Standard for Test Methods of Physical and Mechanical Properties of Concrete” [12] and the “Standard for Test Methods of Long-term Performance and Durability of Ordinary Concrete” [13], this study tested the compressive strength of C80 high-performance concrete specimens with 28-day standard curing.

3.2. Scanning Electron Microscope (SEM)

The experiment selected specimens aged for 28 days (days) under standard curing conditions. After precise section preparation and surface polishing, the microstructure characteristics of C80 self-compacting concrete (SCC) were systematically analyzed using scanning electron microscopy provided by FEI company, Hillsboro, OR, USA (Type: Nova Nano SEM 450). The focus was on observing internal pore structure parameters, solidified particle geometric dimensions, and morphological features.

3.3. Low-Temperature Liquid-Nitrogen Adsorption Test

We prepared the sample into a 1 cm³ cube and conducted low-temperature liquid-nitrogen adsorption tests using the BSD-PS1/2 series of fully automatic specific surface area and pore size analyzers provided by Best Instrument Co., Beijing, China (Type: 3H-2000PS2). We then calculated the specific surface area parameters based on the Brunauer–Emmett–Teller (BET) multi-layer adsorption model and systematically analyzed key pore structure parameters such as pore volume, pore size distribution, pore volume distribution, and average pore size using the Barrett–Joyner–Halenda (BJH) theoretical model and gas isotherm adsorption curves.

4. Analysis of the Test Results

4.1. Compressive Strength and Slump Expansion

The results of the compressive strength tests at 7 d and 28 d are shown in Figure 4.

It can be seen from Figure 4 that in the early stage (7 days), with the increase in mineral powder content, the strength increases slightly and then decreases, and with the extension of age (28 days age), as the production of mineral powder increases, the strength increases first and then decreases and the growth rate is significantly higher than that of 7-day age, with the maximum value reached at a 20% mixing ratio. The reason may be that in the early stage of the hydration reaction, the activity of mineral slag powder has not been fully exerted, so its contribution to the compressive strength of concrete is relatively small. However, as the age increases (28 days), the activity of mineral powder gradually enhances, leading to secondary hydration reactions with cement hydration products, generating more calcium silicate hydrate (C-S-H) gel, which significantly increases the density and strength of the concrete. When the mineral powder content exceeds 20%, excessive mineral powder may cause micro-segregation within the concrete, resulting in an uneven pore distribution, disrupting the internal structure of the concrete, and thereby reducing its compressive strength.

As shown in Figure 5, with the proportion of mineral powder increasing from 0 to 5%, 10%, 20%, and 30%, the spread of self-compacting concrete first increases from 620 mm to 730 mm and then decreases to 710 mm, reaching the maximum value at a dosage of 20%. The height difference between the inside and outside of the J-ring first decreases and then increases with the increase in mineral powder dosage, reaching the minimum value at a dosage of 20%. This indicates that the shape effect and micro-aggregate effect of S95 mineral powder can effectively improve the fluidity of concrete.

4.2. SEM Micromorphology Analysis

SEM observations of the SP-D samples were conducted for 28 days, and the results are shown in Figure 6.

Figure 6 shows the microstructure inside the concrete, where many spherical particles and some larger pores can be seen. These spherical particles are cement hydration products (such as C-S-H gel) or other added fine particles (such as silica fume). The larger pores indicate that there may be some underfilled areas within the concrete, and it can be seen in Figure 6 that the cementitious matrix contains numerous micrometer and nanometer-sized internal pores, cavities, and interconnected cracks. The compatibility between mineral powder and cement is also a critical factor affecting concrete performance. An appropriate amount of mineral powder can react chemically with the hydration products of cement, forming stable hydration products that enhance the strength and durability of concrete. However, excessive mineral powder may lead to incomplete chemical reactions, resulting in unstable hydration products or even harmful chemical reactions such as alkali–silica reactions, further reducing the strength and durability of concrete [14]. Through the analysis of SEM images, it can be seen that the microstructure of high-performance self-compacting concrete is relatively complex, featuring spherical particles, pores, cracks, and fibers. These characteristics significantly impact the performance of the concrete. By adjusting the mineral powder content and water-to-binder ratio and adding fibers, the microstructure of the concrete can be further optimized, enhancing its density, strength, and crack resistance.

4.3. Based on Low-Temperature Liquid Nitrogen Adsorption–Desorption Isotherm Characteristics and Pore Fractal Features

4.3.1. Low-Temperature Liquid-Nitrogen Adsorption–Desorption Isotherm Line

The samples numbered SP-A, SP-B, SP-C, SP-D, and SP-E were cut to about 1 cm³, and 5–10 g samples were degassed in a 200 °C vacuum for 120 min. After degassing, the sample was moved to the adsorption station for the gas adsorption test. The ambient temperature of the low-temperature liquid-nitrogen adsorption–desorption test is 25 °C, the test adsorbent is N2, and the adsorption/desorption temperature is 77.3 K. The adsorption/desorption isotherm of the five matched samples is shown in Figure 7.

As can be seen from Figure 7, the comparison between SP-A and other groups shows that when the mineral powder is added to self-compacting concrete, the adsorption–desorption isotherm line shows an obvious convex phenomenon within the pressure range $0.4\le P/P_0\le 0.5$, which indicates that part of the pores in the self-compacting concrete after adding mineral powder are ink bottle type and non-permeable type.

In the region ($P/P_0<0.8$) with relatively low pressure, the curve rises slowly, and the adsorption–desorption curve separates, forming a larger hysteresis loop, indicating that the adsorbate inside the concrete has not completely detached when the relative pressure decreases. At the relative pressure $0.8\le P/P_0<1$, the adsorption–desorption curves tend to overlap, suggesting that the sample contains a structure of intergranular gaps or flat plate-like particle interstices.

4.3.2. Based on the Fractional Feature of Low-Temperature Liquid Nitrogen Adsorption

As a kind of engineering material, self-compacting concrete shows a series of fractal characteristics both in its formation and in the process of work. This paper uses fractal theory to study the process of surface cracks of concrete structures quantitatively. The liquid nitrogen adsorption data were treated using the Frenkel–Halsey–Hill model [15,16,17] proposed by Pfeifer P. The FHH model of the gas adsorbed on the fractal surface is indicated by Formula (1):

$$\ln V=K\ln \left(\ln \left(p_0/p\right)\right)+C$$

(1)

In the formula, P is the equilibrium pressure; V is the adsorption volume corresponding to the equilibrium pressure; K is a constant, referring to the linear relationship coefficient, whose value depends on the adsorption mechanism; and C is a constant. When the adsorption mechanism is capillary condensation, K = D − 3, where D is the fractal dimension. When the adsorption mechanism is van der Waals forces and capillary effects are neglected, K = (D − 3)/3 [18]. The relationship between the fractal dimension and pore structure characteristics shows that the closer the fractal dimension is to 2, the more regular the pore surface, the simpler the pore structure, and the weaker the non-homogeneity. The closer the fractal dimension is to 3, the more irregular the pore surface, the more complex the pore structure, and the stronger the non-homogeneity of the pores [19].

According to the adsorption–desorption data measured in Figure 8, the x-axis is $\ln \left(\ln \left(p_0/p\right)\right)$, the y-axis is $\ln V$, and using the least-squares principle, the fractal dimension value and fractal characteristics are obtained from the slope value of the straight line K. The analysis results are shown in Figure 8, Figure 9, Figure 10, Figure 11, Figure 12 and Figure 13.

It can be seen from Figure 8, Figure 9, Figure 10, Figure 11, Figure 12 and Figure 13 that the fractal dimension is between 2.3 and 2.5, and the curves of $\ln \left(\ln \left(p_0/p\right)\right)$ and $\ln V$ have a good correlation, both close to 1, indicating that the internal pores of C80 SCC gel have good fractal features. As the water-to-gel ratio gradually increases, the fractal dimension shows a decreasing trend. This suggests that within the experimental testing range, as the water-to-gel ratio increases, the complexity of the pore structure within the gel structure decreases, and the homogeneity of pore distribution and pore structure improves.

4.3.3. Effect of Ore Powder Content on the Structural Characteristics of C80-SCC Pore

From the specific surface area, pore diameter, cumulative hole volume, and cumulative hole area measured by the low-temperature liquid-nitrogen adsorption test, the fractal dimension D was obtained according to the slope K of the fitting equation in Figure 8, Figure 9, Figure 10, Figure 11 and Figure 12, as shown in

Table 5. Summary of cryogenic liquid nitrogen adsorption test results and fractal dimension.

Number	Calcium Powder	BJH Cumulative Pore Volume/mL/g	BJH Average Pore Diameter/nm	Cumulative Hole Area/m2/g	K	D	R
SP-A	0	0.0142	13.368	4.279	−0.534	2.46	0.97
SP-B	5%	0.0058	5.901	3.689	−0.521	2.47	0.97
SP-C	10%	0.0052	5.814	3.577	−0.453	2.54	0.97
SP-D	20%	0.0039	5.9053	2.642	−0.493	2.51	0.95
SP-E	30%	0.0058	6.0932	3.807	−0.443	2.45	0.99

.

According to Table 5, the relationship between the amount of mineral powder and the fractal dimension is shown in Figure 13. The amount of mineral powder BJH average pore size, cumulative pore volume, cumulative pore area, fractal dimension, and so on have a certain correlation. The amount of mineral powder has a certain influence on the internal structure of SCC. Due to the small particle size of mineral powder, it can fill the void between aggregates, improve the pore structure, reduce the porosity, and form a microscopic self-tight packing system, so as to improve the durability of concrete. The activity of the mineral powder is high, and a certain amount (less than 20%) can react with the cement matrix to form more hydration products, thus improving the density and strength of concrete. The micro-aggregate effect of the mineral powder can reduce the early hydration heat release of concrete, thus reducing the risk of cracks caused by temperature changes.

5. Conclusions

This paper focuses on C80 high-strength self-compacting concrete, incorporating 0%, 5%, 10%, 20%, and 30% mineral powder (as a percentage of total binder materials). By combining pore size analysis using a specific surface area analyzer with SEM scanning electron microscopy, fractal theory was employed to experimentally study the internal pore structure and distribution characteristics of self-compacting concrete. The following conclusions were drawn:

(1) Adding S95 mineral powder (≤20%) in appropriate amounts has a minimal impact on early strength, but excessive addition (e.g., >20%) can delay the development of early strength. The micro-aggregate effect of mineral powder can enhance density and optimize pore structure at a lower water-to-binder ratio (0.21). This is consistent with the findings of Smith’s experimental studies in the United States: when the mineral powder content exceeds a certain threshold, early strength growth slows down. For the C80 high-performance self-compacting concrete studied in this paper, it is advisable to control the total amount of cementitious materials to 20%.

(2) In C80 self-compacting concrete, the appropriate amount (0–20%) of mineral powder can significantly enhance workability and fluidity. This is mainly due to the morphological effect and micro-aggregate effect of S95 mineral powder, which improve the fluidity of the concrete and reduce the viscosity of the paste. Therefore, it can significantly enhance the expansion and void passage rate of self-compacting concrete.

(3) Using the BSD-PS1/2 series of fully automatic specific surface area and pore size analyzers, based on the principle of low-temperature nitrogen gas adsorption, experiments were conducted on five ratio samples to measure specific surface area, pore volume, pore size, isotherm adsorption, and desorption. The analysis results from the adsorption/desorption isotherm lines show that C80 SCC contains a large number of cylindrical pores and non-porous ink bottle-shaped pores, with a small number of flake particles forming flat intergranular structures. The observation results from SEM scanning electron microscopy show that the internal surface of the gel structure is uneven, with numerous primary pores and void structures. The width of the cracks is approximately 1 to 5 μm, and the diameter of micro-holes is about 3 to 10 μm. Under external forces, these micro-cracks may first form fracture surfaces, affecting the mechanical properties of the structure.

(4) By applying fractal theory in conjunction with low-temperature liquid nitrogen adsorption test results, this study aims to explore the relationship between the fractal characteristics of internal pores in C80 self-compacting concrete and the amount of mineral powder added. The research examines the pore characteristics of different C80 SCC cementitious structures. The results show that the internal pores of C80 SCC cementitious specimens exhibit significant fractal features, which can be quantitatively described using fractal dimensions. As the amount of mineral powder increases, the fractal dimension first decreases and then increases. This indicates that as the amount of mineral powder increases, the complexity of the internal pore structure in C80 SCC decreases, and the homogeneity of pore distribution and pore structure improves. However, when the addition exceeds a certain level (20%), the complexity of the internal pore structure increases again.

(5) Mineral powder plays a significant role in self-compacting concrete, enhancing its workability and strength. In practical engineering applications, controlling the proportion of mineral powder is crucial for improving the performance of self-compacting concrete. The research findings indicate that when the mineral powder content is around 20%, the compressive strength of SCCC reaches its maximum. Therefore, in engineering design, the mineral powder content can be controlled within this range to achieve optimal concrete performance. Additionally, by adding appropriate amounts of water reducers, air-entraining agents, and other admixtures, the mix proportion of self-compacting concrete can be further optimized, enhancing its overall performance.