Hot-pressed geopolymers have been introduced as ultra-high early strength ceramic-like materials with an almost pore-less structure. This study investigates the combined effects of pressure and heat on geopolymerization kinetics, microstructure, physical properties, and mechanical performance of geopolymers. The results demonstrate that about 65% of the trapped air is easily removed from the fresh geopolymer matrix using an initial impact pressure. When the pre-compacted matrix is subjected to the hot-pressing, it is further densified in the range of 1–10% as a result of continuous free-water evaporation and compaction. This process not only reduces the size and volume of porosity but also changes the continuous pore network to a closed one. Besides, the increase in hot-pressing promotes higher geopolymer gel formation and accelerates the polycondensation which possesses relatively high mechanical strength up to 160 MPa immediately after the processing. Collectively, these results demonstrate that the hot-pressing technique is a promising processing route for designing geopolymer with a diverse range of mechanical properties in a short and fast manner.

This research explores the use of simultaneous heating and pressing techniques in order to enhance the mechanical properties of fly ash (FA) based geopolymer under relatively low temperature conditions to ensure minimum-porosity. Four effective parameters of pressing force, alkali activator/FA, duration of hot-pressing and sodium concentration are studied. Together with detailed experimental studies, our results reveal that the most dominant factor is the induced pressure. The main results indicated that the highest compressive strength of the geopolymer (134 MPa) could be obtained by employing the hot pressing, temperature and duration of 41.4 MPa, 350 °C and 20 min, respectively. The microstructure of the hot-pressed specimens showed more developed geopolymer matrix compared with conventional ones leading to higher compressive strength in much shortest time. The improved mechanical properties are generally attributed to the dense structure of the material and higher geopolymer gel production during the hot pressing process. However, further reaction of partially reacted particles improves the mechanical properties over time.