**1. Introduction**

Geopolymer is an inorganic compound material used in construction as a sealant and heat-resistant material [1]. It is a three-dimensional (3D) aluminosilicate structure that is activated using suitable precursor raw materials. Kaolin is an inorganic material that has been identified as geopolymer-compatible with excellent performance. Wang et al. [2] reported that the kaolin structure is significantly influenced by the calcination temperature. A change in the aluminium species influences the structural changes of geopolymer after

**Citation:** Ramli, M.I.I.; Salleh, M.A.A.M.; Abdullah, M.M.A.B.; Aziz, I.H.; Ying, T.C.; Shahedan, N.F.; Kockelmann, W.; Fedrigo, A.; Sandu, A.V.; Vizureanu, P.; et al. The Influence of Sintering Temperature on the Pore Structure of an Alkali-Activated Kaolin-Based Geopolymer Ceramic. *Materials* **2022**, *15*, 2667. https://doi.org/10.3390/ ma15072667

Academic Editor: Claudio Ferone

Received: 14 March 2022 Accepted: 2 April 2022 Published: 5 April 2022

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being heated to 900 ◦C. The calcium aluminosilicate framework fills the pores between akermanite crystals after being heated up to 1200 ◦C.

Apart from the geopolymerization component, sintering plays a vital role in producing geopolymer ceramic. Sintering is defined as a thermally activated adhesion process, which increases the contact between particles and their respective coalescence. Sintering closes some of the open pores, decreasing the water absorption rate and increasing pore strength. The dense heated geopolymer has a glassy phase, making it a ceramic. Traditionally, ceramic vitrification begins at 900 ◦C, marked by the melting of several solid phases that bind present solid particles, enhancing bonding strength [3,4]. The solid reaction product usually consists of an open-pore volume fraction that was reported to be ~<1–40% [5]. After the sintering process, gas adsorption–desorption and mercury intrusion porosimetry are standard methods used to investigate pore structures [6]. Pores ranging from 1 μm to 0.5 mm are also commonly investigated using SEM and nitrogen adsorption. However, these measurements suffer from several drawbacks, rendering them unsuitable for observing cementitious materials. Both are destructive and can potentially alter pore structures. Therefore, advanced techniques such as tomography using neutron sources have been explored to understand the sintering process' effect on the pore structure of kaolin-based geopolymer. It has been demonstrated that neutron tomography imaging is a suitable characterization method for pore structures. An understanding of the pore structure after the sintering process can be applied for tailoring the resulting materials' properties. Also, it has been established that the nondestructive testing (NDT) of highresolution 3D tomography is beneficial as it elucidates qualitative and quantitative pore formations [7]. The utilization of this tomography technique to investigate porosity and pore size distribution is advantageous and effective. Moreover, extensive quantitative research has been conducted on the pores of ceramic materials such as alumina ceramic using X-ray computed tomography, per Lo. et al. [8]. Nickerson et al. also studied the porosities formed in ceramics and their permeability using X-ray computed tomography [9].

In this study, tomography imaging with a neutron source was used to elucidate the effect of sintering on the pore structure of kaolin-based geopolymer. Neutron attenuation coefficients resulted in different image contrasts relative to those generated by conventional X-ray tomography, producing high-resolution images suitable for determining correlations between pore size, density, and absorption performance. Correlations were linked to the elemental distribution obtained using micro-X-ray fluorescence at a synchrotron source. This work successfully characterized and investigated the pore structure of kaolin-based geopolymer.
