**1. Introduction**

The past two decades have seen the rapid use of high-performance concrete (HPC) in constructing critical structural elements of super-tall buildings and other complex architecture, engineering, and construction (AEC) structures worldwide. HPC is an innovative high quality, and cost-efficient concrete compared to normal-strength concrete that meets the new generation's desire for complex engineering structures [1,2]. Such a fact is not surprising because HPC had provided a pleasant living environment and safety in high-rise buildings and other AEC facilities. Specifically, HPC is essentially a concrete with a lowwater-to-binder (W/B) ranging from 0.2–0.38 [3] that meets the performance challenges of structural elements regarding increasing heights, span length, and load. Moreover, many

**Citation:** Nduka, D.O.; Olawuyi, B.J.; Fagbenle, O.I.; Fonteboa, B.G. Effect of KyAl4(Si8-y) O20(OH)4 Calcined Based-Clay on the Microstructure and Mechanical Performances of High-Performance Concrete. *Crystals* **2021**, *11*, 1152. https://doi.org/ 10.3390/cryst11101152

Academic Editors: Adam Stolarski and Piotr Smarzewski

Received: 19 August 2021 Accepted: 28 August 2021 Published: 22 September 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/).

demonstrated AEC projects had been accomplished using HPC in many countries [4,5]. Therefore, the use of HPC in a developing country such as Nigeria would improve infrastructure projects' future performances.

Among the various constituents of HPC, supplementary cementitious materials (SCMs) of pozzolanic nature play significant roles in meeting HPC requirements. SCMs are mainly siliceous/aluminous finely divided solid minerals used as partial/whole substitutes for cement in concrete and mortar production. SCM in the cement matrix will react chemically to deplete calcium hydroxide to form a more cementitious product later [6]. Significance of SCMs includes the refinement and improvement of pore size distribution; capillary pores and interfacial transition zone of hardened concrete; reduced large pores; improved density of cementitious products; improved workability in the fresh state; and lowering of W/B in HPC mix [1,7]. In addition, SCMs have also been documented to have the potentials to reduce carbon emissions generated during cement production and promising resources in lowering clinker content [8].

In this respect, thermally transformed illite-based clay presents a viable option as SCM in HPC due to the global availability, economic and circular economy attributes [9] and little effects on water demand and 28-day compressive strength [10], among others. Illite is a 2:1 structured aluminosilicate clay mineral sandwich of silica tetrahedron (T)—alumina octahedral (O)—silica tetrahedron (T) layers in the mica family [11]. Song et al. [11] gave the chemical formula as KyAl4(Si8-y) O20(OH)4, here y is estimated to be 1.5. The dehydroxylation of illite commences at about 350 °C and terminates at 800 °C with loss in crystallinity [12]. In a pozzolanic reactivity test, Avet et al. [12] showed that low-grade kaolinitic calcined clays' compressive strength compared favourably with plain Portland cement. Zhuo et al. [6] demonstrated that abandoned London clay calcined up to 900 ◦C exhibited similar compressive strength properties with slag and pulverised fuel ash in concrete application. Trümer et al. [13] developed a binary cementitious material that consists of 30 wt.% of calcined bentonite clay and 70 wt.% of PC in concrete. They suggested that their calcined clay could apply to the majority of concrete works. Ferreiro et al. [14] indicated that 2:1 structured illite clay thermally activated at a temperature of 930 ◦C is appropriate for developing ternary calcined clay-limestone blended cement, which corresponds to improved workability and strength performance in concrete.

Also, Irassar et al. [15] experimentally studied the thermal transformation of raw illitechlorite-based clay to understand the material production's best calcination conditions for concrete use. Similarly, Laidani et al. [16] used bentonite-rich calcined clay as an SCM to improve self-compacting concrete's fresh and hardened (i.e., mechanical and durability) properties. The authors reported improved compressive strength at 15–20% content of the calcined bentonite clay. Finally, Marchetti et al. [10] demonstrated the applicability and performance of illite calcined clay thermally activated at a temperature of 950 ◦C on a low-energy mortar. The authors' findings revealed an enhanced packing density and compressive strength of mortar at a later age. Therefore, using illite calcined clay as a cement replacement could be regarded as a sustainable approach in HPC production due to the improved mechanical, durability and microstructural properties and minimisation of PC consumption.

With many studies conducted on HPC in many regions, information on HPC materials and structural properties produced with calcined clay is still scarce in Nigeria. The present study attempts to fill the gap in the literature by investigating the influence of meta-illite calcined clay (MCC) on Class 1 (50–75 MPa) HPC internally cured with superabsorbent polymers (SAP). The justification of this research is that for the first time, a Nigeria manufactured Pozzolan (NBRRI cement) named MCC in this study, which is readily available, is to be investigated as a binder component for use in HPC. This study will further provide direction to construction industry stakeholders on new materials that can revolutionise high-rise buildings and other heavy civil engineering infrastructural projects in developing countries such as Nigeria.

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

#### *2.1. Materials*

Portland-limestone cement (CEM II B-L, 42.5 N)—"3X" produced by Dangote cement PLC., Ibese Plant Ogun, State, Nigeria conforming to BS EN 197-1 [17], and NIS 444-1 [18] was used as the main binder. A commercially available Pozzolan (MCC) manufactured by the Pozzolan Cement Plant of the Nigeria Building and Road Research Institute (NBRRI), Ota, Ogun State Nigeria, served as the SCM. The SCM was incorporated in powdered form for the various HPC mixtures required by the mix design. Masterglenium Sky 504—a polycarboxylic ether (PCE) polymer-based superplasticiser supplied by BASF Limited (West Africa) was used to improve the workability of the HPC mixtures and administered within the manufacturer's optimum specification of ≤2% by weight of binder (bwob). The specific gravity of the superplaticiser was 1.115, and it is chlorine-free. Superabsorbent polymers (SAP) tagged "FLOSET 27CC" ≥ 300 μm as described in an earlier publication of Olawuyi and Boshoff [19] at a constant content of 0.3% by weight of binder (bwob) was used as an internal curing agent. As specified by BS EN 1008 [20], potable water available within the concrete laboratory of the Department of Building Technology, Covenant University, Ota, was used for mixing.

The river sand used as fine aggregate was at the air-dry condition with a minimum particle size of 300 μm (i.e., all the particles smaller than 300 μm were removed using the sieving method) in compliance with the requirement of fine aggregate specification for HPC production [21–23]. Crushed granite stone passing through 13.50 mm sieve size and retained on 9.50 mm sieve size was used as coarse aggregate in compliance with typical HPC mixes found in the literature [4,22–24]. The crushed granite was used in saturated surface dry conditions after it has been washed to eliminate fine content that will likely increase water demand. Results of the physical characteristics of the materials are presented in Section 3.1. For a more scientific explanation of the binders, laser diffraction PSD, Brunauer–Emmett–Teller (BET) specific surface area (SSA), specific gravity, initial and final setting times, and soundness CEM II and MCC were determined. The binders' PSD was performed using a Malvern Mastersizer 3000. The specific surface area of each binder was measured by nitrogen adsorption for the BET model using Nova Station B Quantachrome Instrument.

The powdery binder samples' chemical compositions were investigated using a wavelength dispersive X-ray fluorescence (XRF) [Bruker AXS. S4, explorer]. The scanning electron microscopy/energy-dispersive X-ray(SEM/EDX) was performed to examine the binder's samples' morphology and microstructure using SEM (Model: Phenom ProX, PhenomWorld Eindhoven, Netherlands). The mineralogical phases of the binders were measured using a Rigaku Miniflex 600, Japan X-ray diffraction (XRD) technique adopting the reflection-transmission spinner stage of Theta-Theta settings. The two-Theta starting position was 2◦ and ended at 75◦ with a two-theta step of 0.026 at 8.67 s per step.

The MCC in powdery forms was analysed for functional groups using Cary 630 Fourier-transform infrared spectroscopy/Attenuated total reflectance (FTIR/ATR) spectrometer made by Agilent Technologies, Malaysia. The powdery sample was placed on the sample stage while the optimal spectra in the range of 500–4000 cm−<sup>1</sup> were obtained at a high speed greater than 110 spectra/s. The calcination efficiency of MCC was analysed using the PerkinElmer thermogravimetry (TGA), the Netherlands, operating at a maximum temperature of 1200 ◦C and a maximum heat rate of 20 ◦C.

#### *2.2. HPC Production*

#### 2.2.1. Mix Proportions

Table 1 shows the mix compositions of the seven HPC mixtures designed for 28-day Class 1, HPC having characteristic cube strength of 67 MPa (i.e., C55/67) following the margin for mix design. The equation can be written as: *fm* = *fc* + *ks*; where *fm* = target mean strength; *fc* = the specified characteristic strength; *ks* = the margin, which is a product of *s* = the standard deviation and *k* = a constant. The binder Type 1 (i.e., CEM II only) was adopted as an HPC control mixture and denoted as control. Binder Type 2 (i.e., CEM II + MCC) was adopted for MCCC-5 to MCCC-30 HPC mixtures. The MCCC-5 to MCCC-30 refers to the blend of CEM II with MCC from 5% to 30% at 5% intervals of SCM content adopted for the HPC mixtures. All the mixtures were prepared at a fixed W/B of 0.3%, fixed SAP content of 0.3% (bwob), and superplasticiser content (1.5% bwob). Additional water of 12.5 g/g of SAP was provided on the ground of the SAP absorbency determined by the work of Olawuyi [25]. Every constituent of HPC was measured by weight (kg/m3), and hence, the MCC addition was taken to be by weight of the binder. This same measurement was also adopted for SAP contents, as reflected in Table 1 following the British method of HPC design. Several studies have adopted the weight (%) method of cement replacement to arrive at their desired mix [6,26–30]. The HPC groups were designated with the name MCCC with MCC content reflected. For instance, the HPC containing MCC with 5% MCC content was coded as MCCC-5.



\* W/B = ((water + liquid content of superplasticiser)/(cement + MCC).
