Improving the Mechanical Properties of Mortars by Adding Metal-Doped Zinc Oxide Nanoparticles †

: The evolution of construction engineering depends on the development of cementitious materials with optimized properties and lower environmental impacts, such as the preparation of mortars with higher mechanical resistance and durability. Nanotechnology is a promising tool for industrial innovation, enhancing material properties like durability and mechanical performance. Thus, herein, we prepared mortars incorporating ZnO nanoparticles and evaluated their properties. The results showed that smaller percentages of ZnO presented better performance in consistency tests, and all samples containing ZnO showed higher mechanical resistance than the reference, thus suggesting the great potential of nanoparticles in optimizing the mechanical properties of mortars.


Introduction
Civil construction is considered one of the keys to world urban development, which makes the materials used in this sector some of the most consumed in the world.The concept of cementitious composites is related to a range of materials made with cement.Among them, mortar stands out due to its properties, performance, and versatility [1].Mortar is basically composed of water, cement, and fine aggregates, and may contain an additive to improve a specific property.These components are fractionated in the proper proportion and mixed in order to obtain a homogeneous mass with specific characteristics, both in fresh and hardened states, such as density, consistency, hardness, and resistance to compression [2].
However, the production of these materials has been the target of criticism due to the damage caused to the environment, mainly owing to the release of polluting phases during cement production [3].
In recent decades, researchers around the world have shown interest in the use of nanomaterials as a strategy for optimizing various materials, including civil construction materials, such as concrete and mortar, in order to improve existing knowledge about the cementitious matrix of these materials and to expand our understanding of nanometric incorporation in these materials [4].
Among the nanomaterials incorporated in cementitious composites, nanoparticles (NPs) based on metal oxides, such as SiO 2 , Al 2 O 3 , Fe 2 O 3 , ZnO, and TiO 2 , stand out [5,6].Therefore, the objective of this work was to prepare mortars incorporating different percentages of zinc oxide nanoparticles doped with cobalt (ZnO:Co) to evaluate the properties of the mortar in fresh and hardened states.

Synthesis and Characterization of ZnO:Co Nanoparticles
ZnO:Co nanoparticles were prepared by dissolving ZnCl 2 (2 mmol), CoCl 2 •6H 2 O (0.10 mmol), and mercaptosuccinic acid (MSA) (8 mmol), acting as a surface stabilizing agent, in 50 mL of deionized water, and the pH of the solution was adjusted, under constant stirring, to approximately 11 with a solution of NaOH (4 M).The system was maintained under constant stirring at a temperature of 80 • C for 90 min [7].After the end of the synthesis, the colloidal suspension was frozen to promote colloidal destabilization.After thawing at room temperature, the precipitate was purified and washed with distilled water and ethanol using a porous plate funnel for filtration.Subsequently, the material was dried in an oven for 1 h at 120 • C, followed by the calcination of the material in a muffle furnace for 2 h at 300 • C [8,9].For the structural and morphological characterization of ZnO:Co NPs, X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis were performed.
The X-ray diffraction (XRD) profile of colloidally prepared ZnO:Co nanoparticles, in a range of 20 • < 2θ < 100 • , and with a step size of 0.02 • and a scan speed of 0.235364 • /min, was obtained using a Rigaku SmartLab diffractometer (Matsubara-cho, 3-9-12 Akishima-shi, Tokyo, Japan), using CuKα1 radiation with a wavelength of 1.54059 Å and an accelerating voltage of 40 kV.The particle size and the microstrain were calculated using the average model of the Scherrer equation [10], Equation (1), and the microstrain of network [8], according to Equation (2).
In the above equations, D is the size of the particle, λ is the wavelength of CuKα radiation, β is the full width at half-maximum (FWHM) intensity, and θ is the peak position.

Mortar Preparation and Characterization
The materials were used without further purification, using early-strength Portland cement (CP V), natural sand, and water as provided by the suppliers.
To prepare the cementitious composite, 600 g of cement and 2400 g of natural sand were used, and a water/cement ratio (w/c) of 0.78 was used (considering the volume of the ZnO:Co NP suspension).
To prepare the mortar, first, the water was mixed with the solution of the nanoparticles (0.5 and 1% of ZnO:Co NPs in relation to the cement mass).Next, all the anhydrous mortar (3.0 kg) was placed in a mixer tank, overlapping layers of cement and sand.The mixer was turned on at low speed, and 75% of the mixing water was added in the initial 10 s, mixing until 30 s had elapsed.The speed was changed to high and the material was mixed for a further 60 s.After this phase, the mixer was stopped for 90 s to scrape the entire internal surface and the blade; then, we turned the mixer on at low speed and added the rest of the water (25%) over 10 s, continuing the mixing until 60 s had elapsed.
The mortar was characterized in triplicate through consistency tests (Flow Table ) and tests of compressive strength at 7 and 28 days.

Synthesis and Characterization of ZnO:Co NPs
The synthesis of Co-doped ZnO NPs using chloride salts as precursors was successfully performed.Information regarding the structural properties of ZnO:Co NPs was obtained from the analysis of X-ray diffractograms to identify the crystalline phase, estimate the NPs' average size, and determine the microstrain of the network.
The X-ray diffraction (XRD) profile of colloidally prepared ZnO:Co nanoparticles is shown in Figure 1a.The diffraction pattern showed peaks at 31.63 • , 34.28 • , 36.09 • ,  The average size was at about 16 nm and the microstrain for each reflection was about 0.0023.The widening of the diffraction peaks may be due to the size and microstrain of the nanoparticles, possibly caused by uniform compression deformation effects due to the presence of Co 2+ with an ionic radius smaller than Zn 2+ , 58 Å, and 60 Å, respectively, which decreases the equilibrium distances between Co 2+ and O 2− in the crystal lattice of ZnO [11].Furthermore, the presence of the stabilizing agent (MSA) influences the nucleation and growth processes of nanoparticles, favoring greater size control, resulting in smaller crystals than those obtained using normal synthesis methods such as coprecipitation.
Morphological analyses were carried out using the TEM technique, in which a quasispherical morphology was observed for the nanoparticles.Using the ImageJ Open-Source software (version 1.53c 26 June 2020), with a reference sample of 350 particles, we obtained an average diameter of around 9 nm, as shown in Figure 1b.

Mortar Preparation and Characterization
The mortar preparation was successfully carried out, as observed in the consistency test (Flow Table ).However, better performance was observed in mortar spreading with the addition of 0.5% ZnO:Co NPs when compared with the control (without NPs), as shown in Figure 2a.By increasing the percentage of nanoparticle addition to 1% of ZnO:Co NPs, it is possible to notice a decrease in the scattering.This fact may be related to the relationship between the surface area of the NPs and the water present in the mixture, since a greater amount of nanomaterial may have caused greater absorption of water from the mixture, decreasing its consistency [12].The compressive strength tests were performed at 7 and 28 days.The mortars containing nanoparticles performed better than the control in terms of mechanical strength, and the samples containing 0.5% of ZnO:Co NPs performed better in terms of compressive strength at 7 days, while the samples containing 1% ZnO:Co NPs showed better values in terms of strength at 28 days, as shown in Figure 2b.This feat is related to the hydration process and microstructure of the cementitious matrix, as the presence of nanomaterials increases the initial hydration rate and the amount of hydrated calcium silicate gel CSH, in addition to reducing the porosity of the material [13].

Figure 1 .
Figure 1.(a) Structural analysis of Co-doped ZnO NPs, obtained via colloidal synthesis, using chloride salts as precursors: X-ray powder diffraction patterns of ZnO and ZnO:Co.(b) TEM image of ZnO:Co NPs.The symbol * in (a) represents impurity phases, probably CoO or Co clusters.

Figure 2 .
Figure 2. (a) Consistency (flow table) of the mortar with the addition of 0.5 and 1% of ZnO:Co NPs, and (b) compressive Strength at 7 and 28 days with the addition of 0.5 and 1% of ZnO:Co NPs.

Table 1 .
Values of ZnO:Co crystal size extracted using the average method based on Scherrer Equation.