Effects of nano-SiO2 and nano-Al2O3 on mechanical and durability properties of cement-based materials: A comparative study
Introduction
Cement-based materials refer to engineering materials using cement as cementitious materials. Concrete and mortar are the most common cement-based materials. Nanomaterials which have unique properties and broad application fields are new materials developed in the early 1980s. Nanomaterials are known as new materials across the century and have attracted great attention from the scientific and business circles. Many properties of cement-based materials, such as workability, strength, and durability, can be improved by introducing nanomaterials. A lot of studies have been carried out on the incorporation of nanomaterials into cement, including nano-SiO2 [1,2], nano-Al2O3 [3,4], nano-Fe2O3 [5], nano-CaCO3 [[6], [7], [8]], nano-TiO2 [9,10], nano-Cr2O3 [11], and nano-CuO [12], etc.
The particle size of nanomaterials ranges from 1 nm to 100 nm, which is conducive to participate in cement hydration as nucleating agent. Even when the content of nanomaterials was low, it would produce compact microstructure [13,14]. It was reported that nano-SiO2 (NS) was able to react with free lime to form calcium silicate hydrate (C–S–H) gel in hydration reaction, namely pozzolanic activity of NS [15]. From the physical point of view, the particle size of NS was 100 times smaller than that of cement particles, which could fill the remaining pore in cement paste and increase the final density of concrete at the initial age [16]. In addition, the microstructure of concrete containing NS was more uniform and denser than that of concrete without NS. The durability of cement-based materials could be improved by increasing the density of the microstructure [[17], [18], [19], [20], [21]]. Previous studies have shown that the addition of 2% NS reduced capillary water absorption by 58% [22]. Another paper noted that the water absorption of the sample decreased by introducing 5% and 10% NS [23]. In addition, the chloride ion penetration depth and diffusion coefficient of concrete decreased greatly when the content of NS was 0.3% and 0.9% of the binder by weight [24]. Furthermore, the charge passing through the sample in rapid chloride permeability test and chloride ion transport coefficients by adding 5% NS were decreased [25,26]. This was mainly due to the refinement of pore structure, the increase of pore tortuosity and the blocking of more transmission channels, leading to the decrease of permeability. Moreover, the precipitation of small size C–S–H gel was also responsible for the formation of compact microstructure [27,28].
Apart from NS, studies on the effect of nano-Al2O3 (NA) on cement-based materials were launched as well. The microstructure, water absorption and resistivity of cement-based materials were improved by introducing NA [29,30]. It was found that the compressive strength of sample increased by 30.67% by adding 1% NA into cement [31]. Nazari and Riahi also found that the compressive strength of mortar could be significantly improved by introducing NA [3]. However, Salim found that adding 2% and 4% NA could not increase the compressive strength of the samples at 7 days. Microscopic analysis showed that no new phase produced, but the micro-morphology was more compact [30]. In terms of durability, the amount of Friedel's salt increased with the incorporation of NA under chloride attack environment [32]. Furthermore, the frost resistance of concrete with 8 nm NA was obviously higher than that of concrete with 20 nm NS. After 300 freeze-thaw cycles, the length loss, weight loss and water absorption of the samples with nanomaterials were less than 10%, 20% and 50% respectively [33].
Much research has been carried out on the introduction of NS and NA into cement-based materials single. However, there is a lack of research on composite nanomaterials. Meral Oltulu [34] investigated the effects of NS, NA and NF (nano-Fe2O3) powders on cement mortar properties. The results showed that the compressive strength of compound-doped NS + NA mortar (S0.5NSA) was always higher than that of single-doped NS (S0·5NS) and NA (S0.5NA) from 3 days to 180 days, and the capillary permeability coefficient decreased obviously for the composite nanomaterials (S0.5NSA). But the dosage studied (0.5%, 1.25%, and 2.5%) was less and the test method was simpler in this paper. Ehsan Mohseni did a similar study [25] and found that the strength and durability of binary combinations and ternary combination nanomaterials systems were better than that of single-doped nanomaterials. But there was a lack of research on small dosage (<1%) of nanomaterials, and the research process did not involve microanalysis, which could not explain the macro-phenomena very well. Therefore, this paper attempts to study the single and combined effects of NS and NA on mechanical and durability properties of cement-based materials. The contents of nanomaterials were 0.5%, 1.0%, 2.0% and 3.0% of the binder by weight. Setting time, compressive strength, flexural strength and durability, including rapid chloride permeability test (RCPT), electrical resistivity, drying and water absorption processes were measured. Finally, X-ray diffraction (XRD) and Differential thermal analysis/thermogravimetric analysis (DTA/TGA) were used to analyze the composition and content changes of the hydration products.
Section snippets
Materials and mix proportions
The chemical composition and physical properties of 42.5 Portland cement (PC) used in this study are respectively displayed on Table 1 and Table 2. Nanomaterials, including nano-SiO2 and nano-Al2O3 (γ phase), were supplied by Beijing Shenghe Haoyuan Technology Co. Ltd. The average particle sizes of NS and NA were 15 nm and 10 nm, respectively. The properties of nanoparticles are presented in Table 3. The XRD patterns of NS and NA are shown in Fig. 1. ISO standard sand with a fineness modulus of
Setting time
The hydration process of cement could be divided into five stages: Pre-induction stage, induction stage, acceleration stage, deceleration stage and stabilization stage. The initial setting time was basically equivalent to the end of induction period. The completion of the acceleration period indicated that the final setting time has passed. The nucleation and growth of Ca(OH)2 and C–S–H (O-type and I-type) mainly occurred during acceleration and deceleration periods. Nanomaterials could be used
Conclusions
The following conclusions could be drawn from the obtained experimental data:
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The introduction of NS and NA could shorten the setting time of cement paste. The setting times of the systems single-doped with 2% and 3% NA and the system compound-doped with NA + NS at 3% were longer than that of the control group. Among all groups, the setting time of the system compound-doped with 0.5% nanomaterials (0.25% NS + 0.25% NA) was the shortest.
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The introduction of NS and NA reduced the content of calcium
CRediT authorship contribution statement
Ai Zhang: Conceptualization, Methodology, Data curation, Formal analysis, Writing - original draft. Wencui Yang: Supervision, Writing - review & editing. Yong Ge: Conceptualization, Funding acquisition. Yuanbo Du: Visualization, Investigation. Penghuan Liu: Visualization, Investigation.
Declaration of competing 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.
Acknowledgement
The authors would like to acknowledge the National Natural Science Foundation of China (No. 51278157) from China and editors and reviewers for their hard work.
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