**1. Introduction**

Ferronickel slag is a byproduct of ferronickel alloy production. Approximately 0.64 million tons of nickel–iron alloys are produced globally each year [1]. China accounts for 48% of all ferronickel alloy production. According to different ferronickel alloy production methods, ferronickel slag can be classified as electric furnace ferronickel slag (EEFS) or blast furnace ferronickel slag (BFFS). The electric furnace method is the main method for producing ferronickel alloys and is used worldwide [2]. With the lack of nickel-rich minerals and the demand for ferronickel alloys in China, the blast furnace method is still the main production method [3]. The chemical and mineral composition of BFFS and EFFS are different with different production methods. EFFS is mainly composed of SiO2, Fe2O<sup>3</sup> and MgO; BFFS is mainly composed of CaO, Al2O<sup>3</sup> and SiO2. The main minerals in EFFS are crystalline substances such as forsterite and diopside; BFFS is mainly composed of an amorphous phase [4]. The resource utilization of EFFS has been extensively studied by many researchers. EFFS can be used as a supplementary cementitious material [5–8], as an aggregate [9–11] in concrete, and as a precursor in geopolymer synthesis [12,13]. Compared with EFFS, there are relatively few studies on BFFS.

Alkali-activated materials have recently become popular as green building materials in cement. Alkali-activated materials use solid waste as a precursor in the preparation of cementitious paste, effectively transforming solid waste into a construction resource. Researchers have studied different industrial solid wastes including blast furnace slag [14–16], red mud [17–19], ground coal bottom ash [20], silico-manganese (Si–Mn) slag [21], electrolytic manganese residue [22], Cu–Ni slag [23] and lead slag [24]. Alkali-activated

**Citation:** Huang, Z.; Zhou, Y.; Cui, Y. Effect of Different NaOH Solution Concentrations on Mechanical Properties and Microstructure of Alkali-Activated Blast Furnace Ferronickel Slag. *Crystals* **2021**, *11*, 1301. https://doi.org/10.3390/ cryst11111301

Academic Editors: George Z. Voyiadjis, Yifeng Ling, Chuanqing Fu, Peng Zhang and Peter Taylor

Received: 15 September 2021 Accepted: 23 October 2021 Published: 26 October 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

materials have greater strength and durability than Portland cement [25]. However, alkaliactivated materials also have workability problems such as large shrinkage and a fast setting time [26].

Using BFFS as a precursor to produce alkali-activated materials is a potential way to utilize BBFS. The amorphous aluminosilicate phase in solid waste is the active source of the precursor to produce alkali-activated material [27]. The calcium content in the amorphous phase has a great impact on the solid waste activity; the presence of calcium lowers the polymerization degree of the aluminosilicate framework in the amorphous phase and increase the activity for alkali-activated reaction. The most commonly used precursors are blast furnace slag (BFS), which has a high calcium content. BFFS is mainly composed of CaO, SiO2, Al2O<sup>3</sup> and MgO, which has a chemical composition similar to BFS. However, BFFS has a higher CaO content and a lower Al2O<sup>3</sup> content than BFS. Thus, BFFS can be classified as a medium-calcium slag for alkali-activated materials and has potential activity for alkali-activation. Few investigations were conducted on BFFS due to most BFFS is adopted only in some parts of China [28]. Wang [28] compared the reaction mechanism and mechanical properties of the alkali-activated BFFS and alkali-activated BFS. The early compressive strength of alkali-activated BFFS is lower than that of alkali-activated BFS, while the strength development of alkali-activated BFFS is better.

This study investigates the effect of NaOH concentration on the mechanical properties and microstructure of alkali-activated BFFS. The mechanical property is measured by the compressive strength. The pore structure is tested to understand the trend of strength on the microstructure scale. The phase composition and hydration heat are used to analyze the effect of reaction product on the pore structure. The polymerization degree and chemical bonding of C-A-S-H is tested by FT-IR.

### **2. Raw Materials and Test Methods**

### *2.1. Raw Materials*

BFFS was used as the precursor of the alkali-activated material. The specific surface area of the FFS was 439 m2/kg. The chemical composition of the FFS was determined by X-ray fluorescence (XRF), as shown in Table 1. FFS consists mainly of CaO, SiO2, Al2O<sup>3</sup> and MgO.


**Table 1.** Chemical composition of raw materials (wt.%).

The X-ray diffraction (XRD) patterns of the FFS are shown in Figure 1. The slag consists mainly of amorphous phases, indicated by the broad peaks from 17◦ to 37◦ in the XRD patterns. The main crystalline phases in BFFS are calcite (CaCO3), spinel (MgO·Al2O3) and gypsum (CaSO4·2H2O). It is widely accepted that the reactive CaO, SiO<sup>2</sup> and Al2O<sup>3</sup> contents in amorphous phases determine the precursor reactivity. The CaO content in BFFS is lower, and the Al2O<sup>3</sup> content is higher than that of BFS. The MgO content in BFFS is mainly in the form of spinel, which is stable and does not easily react.

The alkali hydroxide solutions of five NaOH concentrations (2 M, 5 M, 8 M, 10 M, 12 M) were prepared by dissolving sodium hydroxide pellets (99.9% purity) in water. The water-to-slag ratio was 0.5 for the mortar used in this study. Hydroxide was added to the water to obtain the required concentration. The slag-to-sand ratio for the mortar was 1:3. The paste and mortar specimens were cured in standard curing conditions of 20 ± 2 ◦C and 95% RH.

**Figure 1.** XRD pattern for BFFS.
