Enhancing geopolymer stiffness via in-situ seed-mediated growth of LDHs on cellulose nanofibrils

Geopolymers are promising low-carbon cementitious materials, but their elastic modulus is constrained by the dominance of low-stiffness amorphous gels and the scarcity of high-modulus crystalline phases. To address this, a nanocomposite (CNF@LDHs) is developed by the in-situ growth of Mg-Al layered double hydroxides (LDHs) on cellulose nanofibrils (CNF), where the CNF skeleton anchors LDHs and facilitates seed-mediated growth of a rigid framework. In alkaline environments, CNF@LDHs undergoes selective Al3+ dissolution-recrystallization, achieving 89.2% LDHs growth and forming a continuous crystalline network. Quantitative phase analysis using a novel CNF internal standard method confirms that this seed-mediated growth leads to a 2.91-fold increase in total precipitate mass and a significant net accumulation of LDHs over 168 h, directly correlating with ion consumption from the pore solution. With only 0.3 wt% CNF@LDHs, the geopolymer exhibits accelerated reaction kinetics, a 308% increase in LDHs content to 5.60 wt%, and a reduction in macroporosity from 14.30% to 8.46%. After 28 days, compressive strength, flexural strength, and elastic modulus increase by 25.8%, 28.2%, and 30.5%, respectively. However, at a higher dosage of 0.5 wt%, a severe workability loss and nanoparticle agglomeration occur, introducing macro-defects and leading to diminished mechanical performance and stiffness. The enhancement mechanism is quantitatively interpreted using a homogenization-based effective modulus framework, demonstrating that the seed-mediated LDHs growth on CNF enables efficient multiscale stress transfer. This strategy improves the stiffness of geopolymers, advancing their suitability for structural applications.