**1. Introduction**

Cementitious materials perform a necessary part in the construction sector and are therefore of paramount importance to improve their durability and mechanical characteristics [1–5]. Because of recent characterization technology progress, the characteristics of all these synthetic structures may be examined on a variety of length scales ranging from nano to macro [6–8]. Thus, cement materials' structure and behavior patterns at the sub-micrometer scale are better understood, which has improved their macro-properties [9]. For example, the durability can be improved by reducing the cement paste's total porosity by inserting additives in a range of pores primarily present at a micrometer length scale [10–12]. The studies performed on environmentally friendly and sustainable composites recently have gained importance. Tosee et al. [13] investigated the compressive strength of environmentally friendly concrete modified with eggshell powder using the hybrid ANN-SFL optimization algorithm. They found that the highest compressive strength was obtained for the samples containing 7–9% of eggshell powder and it was 55% higher than their control samples. Ziada et al. [14] produced environmentally friendly fly ash-based and basalt powder waste-filled sustainable geopolymer mortar with basalt fiber. They found that the produced sustainable mortar had high strength and durability properties and the use of 1.2% of basalt fiber increased samples' compressive strength by up to 18%

**Citation:** Ziada, M.; Tammam, Y.; Erdem, S.; Lezcano, R.A.G. Investigation of the Mechanical, Microstructure and 3D Fractal Analysis of Nanocalcite-Modified Environmentally Friendly and Sustainable Cementitious Composites. *Buildings* **2022**, *12*, 36. https://doi.org/10.3390/ buildings12010036

Academic Editor: Chiara Bedon

Received: 19 November 2021 Accepted: 22 December 2021 Published: 2 January 2022

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and flexural strength by up to 44%. ¸Sahmaran et al. [15] produced ECC mixes with various FA/PC ratios (1.2, 2.2, and 4.2). They found that the increase of FA content in ECC specimens exhibit more ductile behavior.

Ultra-high-molecular-weight polyethylene, carbon, and high-modulus polyvinyl alcohol (PVA) fibers are now used in cement-based products. PVA fibers are widely used because the first two fibers are too costly to be commonly used [16–18]. The modification of fibers in cement-based materials focuses on increasing toughness [19,20]. Overall, using fiber improves interfacial adhesion. However, this enhances the bridging result between such fiber and the interface, and it overlooks the influence of sliding friction on fracture energy. When fibers are detached, the increased bond strength at the interface causes a large amount of fracture energy to be generated quickly, raising the potential of brittle fracture [21]. Previous articles had improved the toughness of concrete by altering the content of cementitious composites, such as micro compounding, in which certain microscale particles are added to the concrete mix to increase the toughness of the concrete [22,23].

Unlike conventional concrete materials, Engineered Cementitious Composites (ECC) use a micromechanics-based design theory in the material design process. In a single tensile loading, PVA-ECC has tight and multiple cracking behaviors. The intrinsically cracking width of less than 100 mm is high ductility and improved durability [24]. The tensile strain capacity for PVA-ECC over five percent was demonstrated using commonly available materials and equipment in the concrete manufacturing sector [25]. Li et al. [26] found that the fiber volume should not be more than 2% to ensure good performance in ECC blends. Due to their composite performance and economic considerations, PVA fibers are among the types of fibers used by ECC and the other high performance cementitious composites [27–31].

Nanomaterials have been demonstrated to enhance the interfacial transition area of structures by speeding up the hydration reaction, considerably improving the porosity and durability of the hardened cement-based mixtures [32]. Furthermore, the addition of nanoparticles generally increase the matrix fracture toughness due to the shielding effect on the crack tip and improves the multiple cracking behavior of engineered cementitious composites by making the fiber distribution more homogenous [25]. Among the nanoparticles, nano-CaCO3 is one of the most used nanoparticles in cementitious composites [33]. The most stable shape of naturally abundant inorganic Calcium Carbonate (CaCO3) material in nature is nanocalcite. Calcium Carbonate (CaCO3) is immaculate, crystalline, and highly transparent. Nanocalcite offers advantages in addition to its excellent functions, such as enhancing resilience and rigidity, providing perfect stability and insulation in electricity.

The fracture surface morphology of cementitious materials resulting from crack propagation under loading would explain the differences in the mechanical behavior and the corresponding failure mode [34,35]. It is well established that several parameters control the roughness and texture of the fracture surface of the cementitious composite. The meandering of main crack (for example, tortuous or much less tortuous fracture surfaces) is considerably influenced by the use of micro and nano additives, properties of aggregate particles, and the concrete mix design [36]. Beginning with the pioneering work by Mandelbrot [37], the concept of fractal geometry and fractal dimension has become popular in construction technology and associated materials to better understand the relationship between the flexural response and the tortuosity of fracture surface in ECC [33] and for the design multiscale reinforcing fibers of composite materials [38].

The literature review above clearly indicates that research in this field has generally focused on evaluating strength properties, durability-related behavior, and thermomechanical performance at a macro level. However, research conducted thus far is still less to comprehensively evaluate the microstructure-associated mechanical and fracture characterization of engineered cementitious mixes modified with nanocalcite and 3D fractal characterization. This leads to the aim of this study, which is to analyze the mechanical performance and micro-structural damage characteristics of nano-modified ECC mixes and to improve the toughness, the multiple cracking behavior and the strength of the strain

hardening ECC composites for the development of super infrastructures, which are driven to attain higher strength and higher toughness. In this study, the effects of nanocalcite on ECC's mechanical and microstructural properties are investigated by modifying nanocalcite with 0%, 1%, and 1.5% mass of binder. Compressive strength, flexural tensile strength, and ultrasonic pulse velocity (UPV) tests were performed to investigate nano-modified ECC mixtures' mechanical and physical properties. In addition, scanning electron microscopy (SEM) and 3D fractal analysis were performed to examine the microstructural and crack analysis of the samples.

### **2. Materials and Methods**

#### *2.1. Materials and Mixing Procedures*

CEM I 42.5N Portland cement, fly ash (Class F), silica sand, water, high-range waterreducing admixture (HRWRA), and polyvinyl alcohol (PVA) fibers were used to prepare the ECC samples. The chemical and physical properties of the binder (Portland cement and fly ash) and filler (silica sand) are listed in Tables 1 and 2, respectively. In addition, the mixing ratios of the blends are listed in Table 3. Ding et al. [39] replaced the NCa material with Portland cement by 0%, 1%, 2%, 3% ratios to obtain nano-CaCO3 modified ultrahigh performance engineered cementitious composites mixes. In this study, the NCa was replaced with the binder by 0%, 0.5%, 1%, 1.5% ratios. Figure 1 shows the used nanocalcite materials and PVA fibers. In the mixing phase, the Portland cement (PC), fly ash (FA), and silica sand were dry blended for 3 min in a mixer. After that, HRWRA and dissolved water were added and mixed for another 5 min. Then, PVA fibers were added into the fresh mortar until it was homogeneous. Finally, nanocalcite was added with 0.5, 1, and 1.5 ratios and mixed homogeneously. The nanocalcite used in this study is white, with a purity of 99.9% and an average particle size of 900 nm. The freshly prepared mixture was poured into 15 × 50 × 350 mm molds and 50 × 50 × 50 mm cubic molds, and then these molds were covered with a plastic sheet. The specimens were cured at 23 ◦C. Figure 2 shows poured fresh nanocalcite-doped ECC samples.

**Table 1.** Chemical properties of binder and filler materials (% by weight).


**Table 2.** Physical properties of binder and filler materials.


**Table 3.** Mixture ratios of nanocalcite-doped ECC series.


<sup>1</sup> B: Binder materials (PC + FA).

**Figure 1.** (**a**) Nanocalcite materials (**b**) PVA fiber.

**Figure 2.** Poured fresh nanocalcite-doped ECC samples.
