*Materials, Sample Preparation, and Characterization*

A precursor of kaolin (supplied by Associated Kaolin Industries Sdn. Bhd., Petaling Jaya, Malasysia) was used for the synthesis of geopolymer. The NaOH was in pellet form with 97% purity, and the Na2SiO3 consisted of 9.4% Na2O, 30.1% SiO2, and 60.5% H2O, with SiO2/Na2O = 3.2. The other characteristics were: specific gravity at 20 ◦C = 1.4 kg/cm3 and viscosity = 0.4 Pa s. To form the geopolymer samples, the kaolin was activated with alkaline activator solution, namely, sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) solution, at ambient temperature. The NaOH clear solution was mixed with sodium silicate solution and cooled to ambient temperature one day before mixing [10]. The solid–liquid and Na2SiO3/NaOH were fixed at 1.0 (NaOH molarity 8 M) and 1.5, respectively, on the basis of previous research on the optimum design of kaolin geopolymer [11]. The kaolin materials were mixed with an alkaline activator solution for 5 min; then, the homogenized mixture was poured into a mold. Then, after curing for 14 days, the kaolin-based geopolymer was sintered at 900, 1000, and 1100 ◦C for 2 h at a heating rate of 10 ◦C/min in an electrically heated furnace. The details of sample preparation are illustrated in Figure 1.

**Figure 1.** The process of creating kaolin-based geopolymer ceramic.

The unsintered and sintered samples of pore microstructures were imaged using the JSM-6460LA Scanning Electron Microscope (JEOL, Peabody, MA, USA) equipped with secondary electron detectors. The voltage and working distance were fixed at 10 kV and 10 mm, respectively. The surface area and pore volume were measured using Brunauer– Emmet–Teller (BET) equipment (TriStar 3000, Micromeritics Instrument Corporation, GA, USA). The adsorbed quantity correlated with the particles' total surface areas and pore volume in the unsintered and sintered samples. The samples' thicknesses were 0.5–1 mm. For contrast variation measurements, the samples were placed horizontally in a sample holder and the solvent was added dropwise to the center of the disc.

Neutron images of the samples were acquired at the IMAT beamline, ISIS neutron source, Rutherford Appleton Laboratory, United Kingdom [12]. The IMAT tomography camera was equipped with a 2048 × 2048 pixel Andor Zyla sCMOS 4.2 PLUS. The camera pixel size was 29 μm. The samples were inserted into an aluminum tube that was fixed on the rotating platform and placed at a distance, L, of 10 m from the beam aperture and a distance, d, of 25 mm from the neutron screen. The diameter (D) of the beam aperture was 40 mm, resulting in an L/D ratio of 250. We collected 868 projections, with an exposure time for each projection of 30 s and a total scan time of approximately 6 h/tomogram. Several open-beam and dark images were collected for flat fielding before and after each tomography scan. The images were analyzed using ImageJ and the Octopus reconstruction package (XRE, Ghent, Belgium). The unsintered and sintered geopolymer samples' elemental distributions were determined using synchrotron μ-XRF at BL6b beamline at the Synchrotron Light Research Institute (SLRI) in Bangkok, Thailand. A polycapillary lens was used to initiate a micro-X-ray beam with a size of 30 × 30 μm on the samples, with continuous synchrotron radiation. The X-ray energy range used was 2–12 keV without the monochromator feature. The detection limit at the sub parts per million concentration level can be obtained at larger than 100 nm, with sensitivities approaching the attogram (10–18 g) level [13]. The experiments were conducted in a helium (He) gas atmosphere with 30 s of exposure at each point. The data were obtained and analyzed using PyMca [14].

The samples were fabricated in powder form for phase analyses. The XRD analysis was performed using an XRD-6000 Shimadzu X-ray diffractometer (Cu Kα radiation (λ = 1.5418 A)). The operating parameters were 40 kV, 35 mA, at 2θ of 10–80◦, at a 1◦/min scan rate. The XRD patterns were then analyzed using X'pert HighScore Plus. The density was calculated, and water absorption tests were conducted per ASTM C642-13 (ASTM C642-13, Standard Test Method for Density, Absorption, and Voids in Hardened Concrete, ASTM International, United States (2013)). The weight of the samples after and before the samples were immersed in water was recorded, and the percentages of water absorption for the samples after sintering at 900 and 1100 ◦C were determined.
