Geopolymers are an emerging class of alumino-silicate compounds, amorphous, inorganic, with a polymeric-like structure. In detail, their structure consists of chains, sheet-like and three-dimensional networks constituted of various repetitive unit types of connected SiO₄ and AlO₄ tetrahedra. To maintain stability, the negative charge of AlO₄^- tetrahedra must be balanced by positive ions present in the system, mainly Na⁺ or K⁺ [1]. Geopolymers possess several attractive properties such as an excellent fire resistance, acid resistance, high compressive strength, and strong interfacial bonding ability [2]. Thanks to Cold Sintering Process (CSP) is possible to produce ultra-dense geopolymer artifacts. CSP is a recent (Randall,2016) low energy sintering technique where ceramics powders are densified with the aid of a liquid phase under an external pressure and limited heating conditions (< 350°C) [3].

In this study, a preliminary examination of the application of CSP technology on mixed alkali geopolymer powder is presented. Mixed alkali geopolymers have the simultaneous presence of both Na⁺ and K⁺ ions in their network [4]. Specifically, the adopted solution made it possible to improve the stability of the geopolymer artifacts at high temperatures (avoiding the typical swelling of pure Na-GP in these conditions) on the one hand, on the other to mitigate the excessive reactivity and viscosity (for high shear rate) distinctive of the potassium-based geopolymer slurry (during powder production). Furthermore, a high density (indicating low residual internal porosity) and a proven chemical stability of the product (as confirmed by passing the boiling test) were obtained.

Therefore, this study demonstrates the feasibility of production mixed alkali geopolymer dense bodies using CSP methodology. In particular, this approach will be beneficial for a variety of applications, such as matrices for composites, high temperature components and fire-resistant coatings. 

[1] J. Davidovits, Geopolymer-Chemistry and application, (5 TH ed.), Institut Géopolymère, (2020).        
[2] J. Davidovits , 30 Years of Success and Failures in Geopolymer Applications. Market Trends and Potential Breakthroughs, Geopolymer 2002 Conference, presented at Melbourne, Australia, on 28-29 October, (2020).
[3] C. A. Randall, J. Guo, H. Guo, A. Baker, M. T. Lanagan, Cold sintering ceramics and composites, U.S. Patent Application 15277553, filed 2015/09/29.      
[4] P.S.W.M. Duxson , S.W. Mallicoat S.W.,G.C. Lukey, W.M. Kriven, J.S. Van Deventer, The effect of alkali and Si/Al ratio on the development of mechanical properties of metakaolin-based geopolymers, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2007.