Effects of SiO2 and Al2O3 nanoparticles on the properties of borogypsum containing mortar

Abstract

Keywords: Borogypsum; Cement Mortar; Nanoparticle; Nano-SiO2; Nano-Al2O3

Introduction

In recent decades, the application of nanotechnology in construction materials to improve mechanical and chemical stability has gained importance. Generally, nanotechnological approaches aim to make superficial and intrinsic features more durable. The primary motivation for using nanoparticles in mortar blends is to use them as fillers for voids and as nuclei for hydration reactions to obtain superior properties [1-2]. Although there are some challenges in determining the appropriate amount and dispersion method, nanoparticles have significant potential for developing engineering properties of construction materials [1].

The feasibility of using several types of nanoparticles has been investigated in previous studies. The most commonly used nanoparticles are nano-SiO2 [3-6], nano-Al2O3 [7-8], nano-TiO2 [9-11], and nano-Fe2O3 [12]. Although the nanoparticles mentioned above are usually incorporated into mortar blends with industrial wastes of fly ash [3, 13-15] and blast furnace slag [16-17], there have been no studies on the coexistence of borogypsum with nano-SiO2 and nano-Al2O3 in cement mortars. Borogypsum is industrial waste produced during boric acid production from colemanite and sulfuric acid reactions. Using borogypsum as a mineral admixture can help reduce

*Corresponding author. *E-mail address: moroydor@yildiz.edu.tr This paper was recommended for publication in revised form by Editor-in-Chief Ahmet Selim Dalkilic Published by Yıldız Technical University Press, İstanbul, Turkey Copyright 2021, Yıldız Technical University. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

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the disposal of this industrial waste and turn it into an alternative supplementary cementitious material [18]. To the best of our knowledge, there is a lack of studies that have explored the characteristics of borogypsum mortars incorporating nano-SiO2 and nano-Al2O3 despite the widespread investigation of borogypsum as industrial waste in civil engineering applications. The primary goal of this study was to assess the impact of borogypsum in mortar compositions when combined with nano-SiO2 and nanoAl2O3 and to examine their interactions. Given that the presence of boron tends to extend the setting time, it is paramount to investigate how the utilization of nanoparticles influences various properties of the resulting mortars. To fulfill this objective, a series of mechanical tests and instrumental analyses were conducted on mortars cured for 3, 7, and 28 days. The microstructural assessment encompassed water absorption testing, scanning electron microscope (SEM) observation, and BET (Brunauer-Emmett-Teller) analysis.

Materials And Methods

Materials The industrial waste of borogypsum, supplied by Bandırma Boron Works (Eti Maden, Balıkesir, Turkey), was used instead of gypsum. XRD identified borogypsum (BJ), and according to the XRD results, borogypsum contained gypsum (CaSO4·2H2O, powder diffraction file (PDF) no:00006-0046) and calcium borate hydrate (Ca2B10O17·5H2O, pdf no:00-022-0146) [19]. Due to the chemical composition of borogypsum, clinker was used as the binder material and was provided by the Akcansa Cement Factory, Istanbul, Turkey. The fine aggregate of CEN standard sand conforming to EN 196-1 was [20] obtained from Limak Trakya Cement (Kırklareli, Turkey). Sigma Aldrich and Merck Chemicals supplied SiO2 and Al2O3 nanoparticles, and their SBET were 175-225 m2/g and 120-190 m2/g, respectively. The chemical compositions of the ingredients are listed in Table 1 [19]. A polycarboxylic ether-based superplasticizer admixture (SP) (MasterGlenium®51, BASF Turkey) inhibited nanoparticle agglomeration.

Preparation of Specimens A total of 10 different cement mortars with different amounts of nano-SiO2 and nano-Al2O3 were prepared (Table 2). In addition, the reference mixture was prepared without nanoparticles, whereas the clinker in the other mortars was partially replaced with nano-SiO2 and nanoAl2O3. The quantities of nanoparticle substitutions were established at 0.2-1% for SiO2, and 0.5-2.5% for Al2O3, derived from preliminary experiments wherein a range of nanoparticle quantities was examined, and the most favorable intervals were selected with respect to rheological and mechanical characteristics. All mortars with a water-tocement ratio of 0.4 and a binder-to-sand ratio of 3.0 were prepared, and the SP ratio was kept constant at 0.50% of the binder. In preliminary studies, based on the literature [18, 21], several ratios of borogypsum replaced binder materials, and 3% was determined as the optimum ratio in terms of physical and mechanical features. The reference mixture was prepared according to the EN 196-1 standard [20]. For the preparation of nanoparticle-containing mortars, a different pathway was followed: The mixing water, SP, and nanoparticle were mixed and stirred at 500 rpm for 5 minutes. This blend was combined with clinker, borogypsum, and sand. After stirring for 5 minutes at high speed, the mortars were molded in 40x40x160-mm three-cell prismatic molds. Also, some parts of the mortars were placed into 40x40x40-mm cubes for the water absorption test. Next, the molds were kept in a test cabinet (Nuve TK 120, Turkey) at 20±2 °C and 90% relative humidity. After 24 h, the mortars were removed from the molds and cured in tap water until testing. Test Procedure Several tests were conducted on fresh and hardened nano-modified mortars to investigate their properties based on the nanoparticle type and ratio. TS EN 480-2 was followed to observe the change in the setting time. The initial and final setting times were determined by measuring the penetration

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depth of the needle of the Vicat apparatus [21]. Compressive and flexural strength tests were performed according to the procedures described in the EN 196-1 standard [20] at 3, 7, and 28 days. The crystalline phases of 28 days cured cement mortars were identified using XRD (PANalytical B.V., Almelo, The Netherlands) in the pattern range of 5–90°. The samples were then finely ground to a final particle size of 90 µm. The samples were identified using inorganic crystal structure database (ICSD) patterns. Fourier transform infrared (FT-IR) spectroscopy (Perkin Elmer Spectrum One, MA, USA) was used to analyze the amorphous phases in the mortar matrix and the crystalline phases in the crystalline matrix. The vibrational properties of the hydration products in the optimum samples after 28 days of curing were observed in the 450–4000 cm-1 range. The BET method and water absorption test were used to investigate the effects of nanoparticles on the mortars’ pore structure. The samples’ SBET and total pore volume were determined by nitrogen (N2) adsorption on a Micromeritics ASAP 2020 instrument. Before analysis, the finely ground samples were degassed under a vacuum at 105 °C. The BET analysis relies on empirically establishing the connection between the pressure of N2 in equilibrium with a mortar surface and the volume of gas adsorbed at that specific pressure on the surface [22]. Water absorption tests of the mortars were performed according to the BS 1881-122 standard [23]. The mortars were maintained in an incubator at 105 °C for 72 h. After removal from the incubator and cooling in a dry airtight vessel, the initial weights of the mortar samples were recorded. Following cumulative immersion periods of 10, 30, 60, and 120 min into tap water, the absorbed water percentage of the samples was calculated. Three samples were employed to assess each mortar’s mechanical characteristics and water absorption capacity. The microstructure of the mortars cured for 28 days was observed using a scanning electron microscope

(ZEISS EVO LS 10) after converting the samples into small fragments. Thermal analyses of the mortars were performed using a PerkinElmer Diamond TG/DTA instrument. The samples were heated from 30 to 1000 °C at a 10 °C/min heating rate under a N2 atmosphere. Thermogravimetric analysis ensures the determination of the quantity of CH, which indicates the hydration progress. The CH percentage was calculated from the start and end points of the CH decomposition (Equation 1). (1) In Equation 1, WLCH is the weight loss because of the CH decomposition; MWCH and MWH are CH and water molecular weights, respectively [24-25].

Results And Discussion

Setting Time of Mortars The setting time results for the mortars are shown in Figure 1. Contrary to most of the studies in the literature, both nanoparticles increased the setting time of mortars for all addition ratios. Many studies have investigated the effects of borogypsum on the setting properties of cement mortars, and it has been proven that borogypsum shows set-retarding properties [26-27]. However, the nano-modification did not show development in the setting features of the mortars. As shown in Figure 1, when the initial setting time of the nanoparticle-containing mortars was similar, the nano-Al2O3 incorporation caused a remarkable difference in the final setting time. In the studies by Heikal et al., using SiO2 and Al2O3 nanoparticles delayed the setting time when they were used with the superplasticizer. These

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results were explained by the presence of a superplasticizer and the mixing procedure of nanoparticles and superplasticizers [28-29]. Similar to these studies, especially for the increased nanoparticle ratios, the presence of both the superplasticizer and borogypsum had a combined effect on set-retarding. They did not allow nanoparticles to show their pozzolanic activity effectively. The Compressive and Flexural Strength of Mortars The compressive strength development of the mortars with SiO2 and Al2O3 nanoparticles is shown in Figure

2. The compressive strengths of 3-days cured mortars

were compared, and it was observed that the compressive strength of the mortars decreased with increasing nanoparticle ratios. As shown in Figure 2, 0.4% nano-SiO2 usage improved the compressive strength of 28-days cured mortar as 1.94% of the reference value. However, increasing the nano-SiO2 caused a remarkable decrease in compressive strength at all ages. The lack of uniform distribution of nano-SiO2 can explain these results. When 1% nano-Al2O3 was incorporated into the mortars, the compressive strength of the 28-days cured mortar increased by 4.56%. Like the nanoSiO2 replacement, nano-Al2O3 at higher ratios reduced the

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compressive strength. The compressive strength results showed that the agglomeration tendency of nano-SiO2 and nano-Al2O3 caused a failure to obtain satisfactory results. When the flexural strength of the mortars was investigated, it was observed that parallel to the compressive strength, the optimum nano-SiO2, and nano-Al2O3 ratios were 0.4% and 1%, respectively (Figure 3). The flexural strength results of 3-days cured mortars showed that the usage of nano-SiO2 up to 1.5% ensured higher development than the reference sample, which can be attributed to the nucleation effect of nano-SiO2. On the other hand, the early-age strength of nano-Al2O3-containing mortars was lower than that of the reference. As a result of further curing, the 0.4% nano-SiO2-containing mortar reached a flexural strength 4.82% higher than the reference sample. The flexural strength of 28-days cured nano-Al2O3-containing mortars showed that adding 1% nano-Al2O3 improved the flexural strength of mortars by 2.76% of the reference sample. The mechanical test results indicated that the combination of borogypsum and Al2O3 and SiO2 nanoparticles could not improve the mechanical endurance of the mortars. XRD Results of Mortars The crystalline phases in the mortar structure were determined using XRD analysis. The XRD patterns of the reference sample and selected mortars for each nanoparticle, which showed the highest mechanical strength, are presented in Figure 4. According to the XRD analysis of the cement mortars, the main crystalline phase was quartz (SiO2) with pdf number 00-046-1045, owing to the presence of sand. In addition to sand, portlandite (Ca(OH)2) with pdf number 00-0040733, calcium silicate hydrate (Ca1.5SiO3·xH2O) with pdf number 00-033-0306, and gismondine (CaAl2Si2O8·4H2O) with pdf number 00-020-0452 were determined in the mortar composition.

For the BJ-A-2 sample, the peaks belonging to calcium alumina silicates (~68°) were higher than reference and nano-SiO2-containing mortars. Also, different from the reference sample, the peak around 18° became sharper with the presence of nanoparticles. FT-IR Results of Mortars The FT-IR spectra of the selected mortar samples are shown in Figure 5. The absorption band around 3640 cm-1 is associated with O-H stretching vibrations of portlandite. The bands between 3440 cm-1 - 2920 cm-1 and around 1640 cm-1 demonstrate O-H stretching and bending vibration of O-H due to the capillary water, respectively. The peaks detected at 2850 cm-1, 1430 cm-1, and 880 cm-1 are attributed to the carbonation of Ca(OH)2 with atmospheric CO2 causes. Calcium silicate hydrate (C-S-H) formation is characterized by intensified peaks at 1085 cm-1 - 1030 cm-1. In this region, peaks occurred due to the Si-O band’s asymmetric stretching vibration. The peaks around 780 cm-1 are related to the vibration of tetrahedral units of AlO45- which exist in the structure due to calcium alumina silicate (C-A-S) gel. The peaks observed at 696 cm-1 indicate the symmetric bending of Si-O-Si. The out-of-plane Si-O bending causes bands around 520 cm-1 when the peaks around 460 cm-1 correspond to in-plane Si-O bending peaks. The obtained FT-IR spectra of the cement mortars align with the literature data [29-32]. Scanning Electron Microscopy SEM images of the reference sample and the optimum samples of BJ-S-2 and BJ-A-2 are shown in Figure 6. As shown in Figure 6, the addition of nanoparticles influenced the hydration of the mortars and initiated a change in the microstructure. The SEM images revealed that using nano-SiO2 with borogypsum-containing mortar resulted in a denser and more compact microstructure than the reference and nano-Al2O3 blended mortar. The nano-Al2O3

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containing mortar contains large amounts of ettringite needles. Furthermore, it had a more porous structure than the reference, which coincided with the water absorption values. BET Analysis Results of Mortars The BET-specific area and pore volume of the reference and nano-modified samples are listed in Table 3. When the SBET values of nano-SiO2-containing mortars were investigated, it was clear that a significant improvement occurred in the specific surface of the mixtures when the nano-SiO2 addition ratios were bigger than 0.2%, proving that there were smaller voids in the mortars. In addition, the specific pore volumes of the samples increased with increasing SBET. These recent findings are consistent with those of Shih et al. [33]. According to BET analyses, the mortars’ porosity did not significantly change when nano-Al2O3 was added to the mixtures. Furthermore, the denser microstructure in nano-SiO2-containing mortars than those with nano-Al2O3 was demonstrated by measuring the higher mechanical resistance. The study by Seife et al. [34] yielded similar results, as it evaluated the effects of different activators on the mortar through BET tests and demonstrated

a correlation between pore development and the material’s mechanical strength. Water Absorption Results of Mortars The water absorption capacities of the nano-SiO2 and nano-Al2O3-containing mortars are shown in Figures 7 and 8, respectively. Table 3. BET-specific area and pore volume of samples Mix

Figure 6. SEM images of selected samples (a. Reference, b. BJ-S-2, c. BJ-A-2).

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As shown in Figure 7, using nano-SiO2 in the mortar composition reduced water absorption. The compression between the mortars showed that the BJ-S-2 mortar absorbed 55% less water than the reference. The results demonstrate that adding nano-SiO2 made the mortar matrix more compact and filled the pores, thus enhancing water absorption. On the contrary, the results showed that the percentage of water absorption was decreased by nano-Al2O3-containing mortars absorbing more water than the reference due to the increase in the porosity of the samples. The lack of water tightness of mortars incorporating nano-Al2O3 could be caused by the insufficient distribution of this nanoparticle, resulting in agglomeration and creating a more porous matrix [35]. As revealed by BET analysis results, the replacement of nano-SiO2 supported the forming a denser structure and water tightness than nano-Al2O3. The observed decrease in

water absorption by nano-SiO2 corresponds to the results obtained in previous studies [36]. Thermal Analysis Results of Mortars The DTA and DTG curves of selected mortars at 28 days of hydration are shown in Figure 9 and Figure 10, respectively. The thermal analysis curves, which are given for the temperature range of 200 °C and 800 °C, can be used for quantitative estimation of the CH content of the mortars. Considering the mass loss between 370°C and 500 °C, it is seen that the portlandite amount of the reference sample is lower than nanoparticle-incorporated samples besides the BJ-A-5 sample. The lack of primer hydration reactions possibly caused the lowest CH% of BJ-A-5. With the incorporation of nano-SiO2, the decomposed CH amounts of the mortars changed between 6.55% and 8.81% when nano-Al2O3-containing mortars had a portlandite content of 4.08% - 8.03%. In addition, the thermal analysis curves

Figure 7. Water absorption percentage of nano-SiO2-containing samples.

Figure 8. Water absorption percentage of nano-Al2O3-containing samples.

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demonstrate that adding nanoparticles shifts the initial decomposition temperature to higher values, which could be altered due to the reaction of the used nanoparticles with mortar and inhibiting portlandite from decomposition. When the mechanical and thermal analysis results are evaluated together, it is noticed that the nanoparticles used cannot ensure progress for secondary hydration reactions in which CH can be consumed.

Conclusion

This study explores the effects of nano-SiO2 and nanoAl2O3 on borogypsum-based mortars, addressing optimal concentrations, microstructural changes, and the impact on the hydration process, all representing valuable additions to the existing knowledge. From the obtained results, it can be stated that:

Nanoparticles cannot enhance the acceleration of the setting of mortars. The combination of nanoparticles and borogypsum inhibited hydration. For all curing ages, increasing the dosages of nanoparticles caused a decrease in the compressive and flexural strengths. Nevertheless, compared with the reference sample, adding 0.4% nano-SiO2 increased the compressive and flexural strengths by 1.94% and 4.82, respectively. Similarly, mortars containing 1% nano-Al2O3 had 4.56% and 2.76% higher compressive and flexural strengths, respectively. The SEM images, BET analysis, and water absorption results support the formation of a densified structure by nano-SiO2 usage. According to the thermal analysis results, the nanoparticle-added samples include more portlandite than the reference sample, which can be attributed to the lack of secondary hydration reactions.

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Although using nanoparticles with supplementary cementitious materials could be practical and economical in many aspects, combining borogypsum with nano-SiO2 and nano-Al2O3 could not achieve remarkable strength and durability. In further studies, changing the mixing method of nanoparticles can be investigated to obtain better results.

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