Designing a bi-component sprayed geopolymer paste with prolonged rheological stability and rapid setting

The adverse effect of accelerator on long-term strength and the substantial carbon footprint of Portland cement are the main challenges faced in cement-based accelerated shotcrete. High-calcium geopolymer, as a sustainable alternative, is well-known for fast setting and rapid early strength development, showing great potential to replace cement in current accelerated shotcrete. However, the fast-setting risk to construction is not yet addressed in conventional sprayed geopolymer material. Therefore, a new bi-component sprayed geopolymer paste is developed in this study where the activator is separately transported and mixed with other ingredients at the nozzle, characterized by prolonged rheological stability during transmission and quick-setting effectiveness after spraying. The determined component Ⅰ has a binder composition of 80% slag, 12% OPC and 8% silica fume by mass and a water to binder ratio of 0.20. 2% calcium hydroxide (CH) was used to further accelerate setting, achieving initial and final setting times of 4.5 min and 9 min respectively, and the 1-day compressive strength reaches 58.5 MPa. CH facilitates the formation of layered hydrotalcite (Ht). Particle bridging and network strengthening by Ht are proposed as possible contributions to the shorter setting time. The preliminary laboratory evidence of sprayability under the tested conditions of the presented bi-component sprayed geopolymer paste was provided by a spraying test. This study provides a proof-of-concept demonstration of a bi-component geopolymer spraying approach for accelerated shotcrete. The results indicate preliminary potential for addressing the rapid-setting issue and support further investigation into geopolymer as an alternative binder in shotcrete applications.