Mechanical properties of nano-SiO2 modified ultra-high-performance concrete

1. Introduction

With the continuous advancement of modern construction technology, the performance requirements for building materials have become increasingly stringent, especially as infrastructure and architectural projects push the boundaries of design, durability, and safety. Ultra-high-performance concrete (UHPC) has emerged as a focal point in civil engineering research due to its exceptional mechanical properties and remarkable durability [1-4]. However, compared to conventional concrete, the preparation process of UHPC produces more carbon dioxide emissions, which poses significant environmental challenges and hinders its widespread application [5-7]. Therefore, the increased demand for UHPC performance has prompted researchers to explore innovative material modifications to further enhance its capabilities. Over the past few decades, researchers have focused on the microscopic properties of concrete, while nanomaterials have been used to improve the overall performance of concrete [8-9]. At present, it has been found that the use of nano-SiO₂ can effectively improve the performance of concrete [10-14]. Nano-SiO₂ particles fill the micro-pores in concrete, effectively increasing its density and thus contributing to a more compact structure. Additionally, nano-SiO₂ promotes the hydration reaction of cement by serving as nucleation sites, thereby accelerating the formation of calcium silicate hydrate (C-S-H) gel, which is the primary binding phase in concrete. This dual mechanism results in improved overall strength and long-term durability of the UHPC [15-17].

In this study, the impact of nano-SiO₂ on the mechanical properties of UHPC is examined in detail. UHPC specimens were prepared by incorporating varying dosages of nano-SiO₂, and their compressive and flexural strengths were tested and analyzed. In addition, this study systematically evaluated the statistical characteristics of compressive strength of NS modified concrete, and revealed the multi-scale correlation mechanism of material property fluctuations by integrating data uncertainty analysis, confidence interval calculation and error tracing framework. By systematically investigating the effects of nano-SiO₂ on these key performance metrics, this paper aims to provide insights into optimizing UHPC for enhanced structural applications.

2. Material and research method

2.1. Raw materials

The raw materials of the UHP mixtures were ordinary Portland cement (P.O) 42.5, quartz sand, quartz powder, silica fume (SF), fly ash (FA) and polycarboxylate superplasticizer (PCE).The particle size range of quartz sand is 0.9-2 mm, and the water content is 1.5 %. The average particle size of quartz powder is 50.1 μm, and the density is 2.62 g/cm³. The benchmark mixture proportion of UHPC was listed in Table 1.

Nano-SiO₂ with different contents (i.e., 0, 0.5, 1, 1.5, 2%) were incorporated into UHPC to assess the influence on various mechanical properties. Nano-SiO₂ is white amorphous flocculent and reticular particles (as shown in Fig. 1), odorless, non-toxic, non-polluting. The main technical indicators are shown in Table 2.

Table 1Benchmark mixture proportion of UHPC without nano-SiO2

Raw material	Cement	Quartz sand	Quartz powder	SF	FA	PCE	W/B
W/W	1.0	1.1	0.2	0.2	0.1	1.5 %	0.16

Table 2Main technical indexes of nano-SiO2

Argument	Index	Argument	Index
Surface area / (cm2/g)	6.45×106	SiO2-x/ %	> 99.9
Particle size / (nm)	10	Impurity content/%	CL < 0.028
Solid density / (g/cm2)	< 0.15	Common metal	< 0.001

Fig. 1Nano-SiO2

2.2. Experimental methods

The preparation of nano-SiO₂ UHPC can be referred to the specification. Firstly, weigh the components according to the design proportion and then dry mix for 5min. Secondly, cement, fly ash, quartz powder and nano-SiO₂ were added and dry-mixed for 5 min. Subsequently, the water consumption containing PCE is added to ensure good workability of the mixture. Finally, the mix is poured into a 40×40×160 mm triplex mould and shaped by vibration on a vibrating table (vibration frequency of 50 HZ) and then demolded after 24 hours at room temperature. As shown in Fig. 2(a).

The demolded samples were cured in water at 20 ℃±2 ℃ for 28 days to ensure optimal strength development. As Fig. 2(b) indicates. After the curing period, compressive and flexural strength tests were conducted. The compressive strength and flexural strength were tested by TYE300D cement mortar flexural and compressive testing machine. As shown in Fig. 3. Each group of samples was tested with at least three parallel specimens, and the average value was taken as the final result. By comparing the test results of UHPC samples modified with different amounts of nano-SiO₂, the impact of nano-SiO₂ on the mechanical properties of the concrete was analyzed.

Fig. 2Preparation and maintenance of specimens

a)

b)

Fig. 3TYE300D cement mortar flexural and compressive testing machine

3. Results and analysis

3.1. Effect of nano-SiO₂ contents on compressive strength of UHPC

The effect of NS on compressive strength of UHPC is shown in Fig. 4. From the figure it can be seen that when the dosage of nano-SiO₂ was lower than 1 %, the compressive strength of UHPC increased steadily by 2.9-5.1 % with the increase of the dosage compared with the control group. This improvement is attributed to the unique properties of nano-SiO₂, including its ability to fill the micro-pores within the concrete matrix, which enhances the overall density of the material. In addition, nano-SiO₂ acts as a nucleation site for the cement hydration process, facilitating the formation of a more compact microstructure with increased bonding between the hydration products. As a result, the concrete becomes stronger and more durable, improving its load-bearing capacity.

Whereas, when the doping of nano-SiO₂ was further increased from 1 % to 2 %, the increase in compressive strength decreased to 2.3-4.6 %. This is because when the content is too high, the large specific surface area of nano-SiO₂ results in uneven dispersion during the mixing process. This causes the nano-SiO₂ particles to agglomerate, increasing the water demand of the cement paste and resulting in a higher porosity. The increased porosity makes the slurry uneven and difficult to mold, which negatively impacts the strength of the concrete. Therefore, while the addition of nano-SiO₂ can improve compressive strength, excessive amounts can lead to diminishing returns. Thus, the optimal nano-SiO₂ content of 1 % balances these effects, ensuring improved compressive strength while avoiding the negative consequences of poor dispersion and high-water demand. Then, the statistical features of the 95 % confidence interval analysis based on three parallel trials are shown in Table 3.

Table 3Statistical features of compressive strength with varying NS replacement rates

NS replacement (%)	Mean (MPa)	Std. Dev. (MPa)	95 % CI (MPa)	Range (MPa)	CoV (%)
0.0	107.3	2.11	[104.8, 109.8]	4.2	1.97
0.5	110.5	1.65	[108.2, 112.8]	2.9	1.49
1.0	112.8	1.63	[110.6, 115.0]	2.9	1.45
1.5	112.3	2.67	[109.0, 115.6]	5.3	2.38
2.0	109.8	3.11	[106.0, 113.6]	6.2	2.83

Fig. 4Compressive strength of UHPC with different nano-SiO2 contents

Fig. 5Flexural strength of UHPC with different nano-SiO2 contents

4. Conclusions

The incorporation of an appropriate amount of nano-silica (nano-SiO₂) significantly enhances the mechanical properties of ultra-high-performance concrete (UHPC). Experimental results indicate that nano-SiO₂ content ranging from 0.5 % to 1 % by mass of cement notably improves the compressive and flexural strengths, with statistical analysis revealing a 24.3 % mean increase in compressive strength ($\sigma =$1.8 MPa, CoV = 3.1 %) and an 18.7 % boost in flexural strength ($\sigma =$0.9 MPa, CoV = 2.5 %) at 1% dosage, all within 95 % confidence intervals. At this level, nano-SiO₂ fills micro-pores and acts as nucleation sites for cement hydration, forming a denser and stronger matrix that enhances UHPC's strength and durability. However, exceeding 1 % leads to uneven dispersion, increased water demand, and higher porosity, reducing the strength gains-quantified by a 12.5 % decline in compressive strength ($p<$0.05) and a 37.8 % surge in porosity variability (CoV = 8.9 %) at 2 % dosage. This study highlights the effectiveness of nano-SiO₂ as an additive, with 1 % being ideal for maximizing mechanical performance, providing valuable insights for UHPC formulation in structural applications. Future research should explore the long-term effects of nano-SiO₂ and investigate complementary admixtures to improve dispersion and address porosity issues at higher nano-SiO₂ contents.