Reaction mechanisms of alkali-activated materials

Abstract

The Alkali-Activated Materials (AAM) are defined as materials obtained through the reaction between precursors and activators, and are separated into two classes depending on the products formed in the reaction, those rich in calcium, as the blast furnace slag, whose Ca/(Si+Al) ratio is higher than 1; and poor in calcium, which is the geopolymers subclass. In this review article, some bibliographical aspects were discussed regarding the discovery of these materials, through research conducted by Victor Glukhovsky and through the characterization of historical monuments by Davidovits, which began in the 50s and 60s and persist to the present day. The main products obtained in the alkaline activation reaction were also addressed, using the definition of polysialates and zeolites, in the case of geopolymers, and the tobermorite structure, in the case of materials rich in calcium. The main steps of the alkali-activated reaction, such as dissolution, condensation, polycondensation, crystallization, and hardening, were discussed. Some techniques for characterizing the AA reaction products were also examined, such as X-ray diffraction (XRD), nuclear magnetic resonance spectrometry (NMR), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). Finally, the main factors that interfere in the kinetics of AA reactions were explored, in which the type of cure and the activating solution used in the alkali-activated materials production stands out.

Materials for Production of High and Ultra-High Performance Concrete: Review and Perspective of Possible Novel Materials

“…Another type of HPC and UHPC that has emerged as a great potential for application in civil construction is based on alkali-activated cement [ 231 ]. This type of cement, unlike OPC, is composed of two materials: a powdery and amorphous component named as a precursor, and another liquid composed of an aqueous solution of a dilute alkali metal, named as an activator [ 232 , 233 ]. The types of precursors and activators will be presented in detail in the following sections; however, in summary, they must result in composites with similar characteristics to OPC-based HPC and UHPC: adequate flowability, high mechanical strength, and good durability.…”

“…This is also related to the fact that the OPC industry is highly polluting. Alkali-activated precursors and cement are considered alternative and eco-friendly binders [ 232 , 236 , 237 ], usually resulting in concretes with lower CO 2 emissions than OPC-based concrete (around 10%, according to [ 238 ]). The accurate determination of the environmental impact associated with these binders—and consequently compare it with those associated with OPC—would require a detailed analysis which will not be addressed in this paper; nonetheless, further details can be found in [ 239 , 240 , 241 ].…”

“…It is observed that the OPC hydration reaction produces large calcium chains linked to Si tetrahedrons and with the presence of chemically linked interstitial water. This structure is called the Dreiketten structure [ 232 , 233 , 275 ]. In the case of alkaline activation of blast furnace slag, very similar calcium chains are formed.…”

“…Some authors highlight the importance of using activated agents obtained by dissolving silicates since, in the alkaline activation of precursors rich in aluminosilicates, alumina dissolves before silica [ 232 ]. Thus, the use of silicates increases the reaction kinetics because it already provides, in a faster way, the necessary silica species for the activation reaction [ 233 , 275 ].…”

Natural Fibers as an Alternative to Synthetic Fibers in Reinforcement of Geopolymer Matrices: A Comparative Review

“…Geopolymers are inorganic polymers formed by the polymerization of aluminosilicate precursors (such as fly ash, metakaolin, and blast furnace slag) in an alkaline solution [ 1 ]. Geopolymer composites are today seen as excellent alternatives to Portland cement-based materials, not only for having superior mechanical properties, such as high initial strength gain, high fire resistance, and acid and sulfate resistance attacks, but also for be considered eco-friendly materials, considering that, unlike what occurs with Portland cement, the synthesis of geopolymer does not release harmful gases to the environment nor does it depend on an extremely high energy expenditure [ 2 , 3 , 4 ]. An alternative found to improve environmental issues related to the use of traditional Portland cement is the use of geopolymeric binders with high mechanical performance, such as fiber reinforced ones, which can be processed in different ways, up to the most modern such as 3D printing [ 5 ].…”