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Ceramics are highly valued for their exceptional thermal resistance and ability to withstand high temperatures. This study investigated the production of ceramic-like pressed geopolymer, focusing on the effects of aging period and sintering temperature. The results showed that the 7-day pressed geopolymer achieved the highest compressive (134.7 MPa) and flexural (34.9 MPa) strengths after sintering at 1000°C, with a density retention of 93.7 %, a mass loss of 7.9 %, and a thermal shrinkage of 1.4 %. Microstructural analysis at 1000°C showed a dense ceramic-like structure with nepheline formation. Phase analysis of the 7-day pressed geopolymer revealed a decrease in the amorphous phase as temperature increased, with nepheline formed at 800°C (26.0 %) and maximized at 1000°C (61.0 %). This work offers an optimal aging period and sintering temperature to maximize the mechanical strength and nepheline crystal formation, making them perfect for fire-resistant panels and precast construction products. • Pressed geopolymer was formed at various aging times and sintering temperatures. • The highest mechanical strength was achieved by the 7d pressed geopolymer at 1000°C. • The residual compressive strength at 1000°C ascends in 28d < 7d < 1d. • Gel matrix transformed into a ceramic-like structure with rising temperature. • The 7d pressed geopolymer has the most nepheline phase formation at 1000°C. [ABSTRACT FROM AUTHOR]

This paper presents an innovative application of a one-part geopolymer for microwave absorption. The influences of frequency, ageing time and mixing formulations on the mechanical, dielectric, and microwave-absorbing performance of the one-part binary geopolymer using fly ash and ladle furnace slag were investigated. The mixing formulations included alkali activators-to-aluminosilicate sources, sodium metasilicate-to-sodium hydroxide, and water-to-binder ratios. The results demonstrated that the dielectric constant decreased while the dielectric loss and loss tangent increased and remained constant with increasing frequency. The geopolymers have the highest dielectric values at an early age and diminish with ageing time. These resulted in the geopolymers having excellent microwave absorption (50 – 80%) at high-frequency levels and later ages. A higher water content resulted in higher porosity, reducing mechanical strength but enhancing microwave absorption. An optimal water content must be attained to achieve dual mechanical strength and microwave absorption performance. • One-part binary geopolymer based on FA and LFS was synthesized. • Geopolymers show excellent microwave absorbing performance at higher frequency. • Microwave absorption of the geopolymers improved at later age. • Porosity creates path to attenuate the microwave energy. [ABSTRACT FROM AUTHOR]

[Display omitted] • The acid resistance of one-part geopolymers (OPGs) was studied. • Formation of hydrate gels during acid exposure led to a slight strength recovery. • High sorptivity made Na 2 SiO 3 and NaOH-based OPG more susceptible to acid attack. • The incorporation of NaAlO 2 and Na 2 CO 3 enhanced OPG's acid resistance. In this study, the durability of one-part geopolymers (OPGs) made from high calcium fly ash is assessed by investigating their resistance to acid attacks. The predominant use of less environmentally sustainable sodium metasilicate (Na 2 SiO 3) and sodium hydroxide (NaOH) in OPG, along with the limited understanding about the influence of solid alkali activators on OPG's acid resistance propelled the investigation and comparison of acid resistance of OPGs activated using both conventional activators and potential alternatives like sodium aluminate (NaAlO 2) and sodium carbonate (Na 2 CO 3). The OPGs developed were exposed to sulphuric acid (H 2 SO 4) solution for 28 days. The MH sample (activated with Na 2 SiO 3 and NaOH) was vulnerable to acid attack, with a 66% compressive strength drop after 5% H 2 SO 4 solution exposure, due to high sorptivity. In contrast, the MA sample (activated with Na 2 SiO 3 and NaAlO 2) demonstrated excellent acid resistance, with only a 32% strength reduction, attributed to its Al-rich hydrated gel. The MC sample (activated with Na 2 SiO 3 and Na 2 CO 3) exhibited a 41% strength reduction, where the formation of calcite reduced extensive ion exchange, gradually mitigated deterioration. The residual compressive strength of MA and MC samples increased by 7% and 9% from 7 to 28 days of immersion in 5% H 2 SO 4 solution, respectively. The NaAlO 2 and Na 2 CO 3 are potential activators for OPG, serving as alternative construction materials to OPC and traditional geopolymers in acidic environments. [ABSTRACT FROM AUTHOR]

[Display omitted] • A study of fly ash based pressed and cast geopolymer was proposed. • Compressive strength of pressed geopolymer nearly twice that of cast geopolymer. • Flexural strength of pressed geopolymer was 5 times higher than cast geopolymer. • The highly compacted pressed geopolymer reduced the tendency of efflorescence. • The low alkali activator of pressing method lowered EE, ECO 2 and ECI of geopolymer. The high liquid content of cast geopolymer not only limits its strength development and durability but also leads to high energy consumption and carbon dioxide (CO 2) emissions. Thus, a study of the cast and pressed geopolymer was performed. The geopolymers were cured for 1, 7 and 28 d before testing and characterizations. With the incorporation of pressure compaction, higher bulk density (2158–2227 kg/m3) was recorded for pressed geopolymer in comparison to cast geopolymer (1842–1854 kg/m3). The dense matrix in pressed geopolymer improved the inter-particle contact, increasing the 28 d degree of reaction to 39.7%, higher than that of cast geopolymer (33.0%). This feature was proved by SEM micrographs wherein the pressed geopolymer was well-compacted and denser in microstructure, with less unreacted/partially reacted fly ash and pores. The compressive and flexural strengths of pressed geopolymer reached 114.2 and 29.9 MPa after 28 d, higher than that of cast geopolymer (60.0 and 6.2 MPa, respectively). The strength reduction of pressed geopolymer (31.7%) after the accelerated efflorescence test was lower than that of cast geopolymer (60.2%). The ecological analysis inferred that pressed geopolymer was ecologically superior to the casting method in terms of embodied energy (EE) and embodied carbon dioxide emission (ECO 2), in which 50% and 59% of reductions are acquired. Besides, the embodied carbon index (ECI) of pressed geopolymer was about 21% of cast geopolymer. [ABSTRACT FROM AUTHOR]