# Strength Development of Metakaolin-Based Alkalial-Activated Cement
## 1 Introduction  
Portland cement has been used as construction material for nearly two centuries, where the cement is used as a binder material in concrete and mortar. However, the production of Portland cement represents about 6-8% of global anthropogenic CO\({}_{2}\) emissions. Therefore, there is a strong interest in finding alternative binder materials that have a significantly lower CO\({}_{2}\) footprint [1; 2; 3]. One potential alternative to Portland cement is alkali-activated materials (AAMs) or \"geopolymers\", which have already been used as construction material in several applications [2; 3; 4]. Alkali-activated materials are completely devoid of Portland cement, resulting in a considerably lower CO\({}_{2}\) footprint, but also having different reaction mechanisms and subsequent strength development. While Portland cement develops strength during a hydration reaction when the cement powder reacts with water and forms a C-S-H gel as the main strength-providing binder phase, the aluminosilicate precursors of AAMs react with an alkaline activator (usually NaOH, KOH, or their silicates) thereby forming other types of strength-providing reaction products. The calcium content of the precursors determines in large part which binder phases are formed, with a low Ca content leads to the formation of N-A-S-H or K-A-S-H phases and a high Ca content leads to the formation of C-A-S-H phases [5; 6]. Typical high Ca precursors are Blast Furnace Slag (BFS) and Fly Ash Type C, whereas typical low Ca precursors are metakaolin and Fly Ash Type F. Low-calcium alkali-activated materials are often referred to as \"geopolymers\". It should be noted that regardless of the Ca content, the precursors of AAMs dissolve into monomeric or small oligomeric Si and Al species in the activator solution before the aluminosilicatebinder phases precipitate. Therefore, the reactivity of the precursors (i.e., their dissolution potential) is of considerable importance for the subsequent strength development. In addition to applications as construction material, Portland cement is also commonly used as a barrier material during cementing operations of wells for oil and gas production, CO\({}_{2}\) storage, and geothermal energy production [7; 8; 9; 10]. During well construction, cement is pumped into the annulus outside the steel casing, both to provide support to the steel casing and to ensure zonal isolation in the well. Furthermore, cement is also pumped into wells during plug and abandonment (P&A) operations, to ensure that hydrocarbons and other downhole fluids do not leak into the environment after well abandonment [7; 11; 12]. There is currently a drive in the research community to find alternative well cementing materials as well, with geopolymers having been suggested as potential candidates [11; 12; 13]. Several different types of precursor materials have been used to prepare geopolymer well cements, such as fly ash [14; 15; 16; 17; 18; 19], rock waste [20; 21; 22], and volcanic ash [23]. Metakaolin as precursor material during alkali activation has so far not been widely studied for well cement applications, which could be due to the relatively large water consumption of the platelike metakaolin particles [24; 25], thereby causing potential slurry viscosity and pumpability issues. However, it has recently been shown that metakaolin-based geopolymers can have good potential as well cement during steam injection operations, due to their improved mechanical properties [26], and consequently, there is a need for further studies on the mechanical properties and strength development of these materials. With respect to sustainability, it should be noted that metakaolin is prepared by calcination of excavated kaolinite minerals, and therefore has a higher CO\({}_{2}\) footprint and environmental impact than other, typical AAM precursors such as BFS and fly ash which are waste materials.