Mineral carbonation has been proposed as an option for carbon capture and utilisation in the construction sector, where it has been successfully demonstrated in the production of cementitious materials, concrete, and associated products. Synthetic magnesium carbonate trihydrate, nesquehonite, investigated in this article, develops cementitious properties via thermal processing, where the sample paste is heated and hardened inside a mould. Such material would incorporate around 30% of CO ₂ into its final composition, and near-maximum in terms of the current development of carbonate-bearing construction materials. Initially, the mechanism of strength development was thought to be linked to phase conversion (e.g., towards hydromagnesite). However, it has not yet been systematically investigated. Thus, this study provides statistically validated insights into some of the synthesis and processing factors that influence the resulting compressive strength of nesquehonite-based products. The results disclosed within suggest a more eco-efficient production technique and propose a working hypothesis on the relation between compressive strength and the nesquehonite—hydromagnesite conversion reaction.