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This study investigates the effects of carbon nanomaterials (CN), specifically nanohorns (CNHs) and nanotubes (MWCNTs), on geopolymer carbon nanocomposites (GCN). The research investigates 0.2%, 0.5% and 1.0% of CN and two curing conditions to understand their impact on the microstructure, morphology, and performance of GCN. Analytical techniques such as MIP, TGA-DTA, FTIR, Raman, SEM, and EDX are utilized to analyze the nanoadditives in alkali-silicate suspensions, fresh and hardened GCN. Ultrasonication is important for achieving better dispersion of MWCNTs, while CNH disperses more easily. Both MWCNTs and CNHs enhance flexural strength by up to around 58% and 44%, respectively, and reduce crack formation compared to the pristine geopolymer. Hydrothermal curing, despite leading to higher porosity, improves flexural strength due to increased geopolymerisation. The formation of a cross-polymerized network with oxo bonds and the release of water during the process contribute to increased strength and porosity, respectively.

In the pursue of environmentally-friendly binders for the construction industry, metakaolin (MK) has emerged as promising material, with its hardening performance primarily driven by pozzolanic reactions. However, in systems containing MK and calcium hydroxide (CH), transformation reactions from metastable to stable phases, particularly in excessive CH conditions, can adversely affect material properties. To anticipate these processes, this study introduces a stoichiometry-based reaction modeling approach for pozzolanic MK reactions, encompassing both short-term kinetics and long-term transformation processes. The primary pozzolanic reactions of MK are briefly outlined, highlighting two sequential, partially overlapping reactions forming C4AH13 and C2ASH8. Short-term reaction kinetics are modeled using isothermal calorimetry measurements and deconvoluting the two reaction peaks. The model is in good agreement with experimental quantitative X–ray diffraction (qXRD), thermogravimetric analysis (TGA) and helium pycnometer (density, i.e. solid volume) results. Model limitations are discussed based on qualitative scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDX) analysis. Leveraging the proposed model, the temperature dependency of pozzolanic MK reactions is analyzed, revealing an activation energy for the primary reaction of 84 kJ/mol. The model is intended to lay a foundation for designing innovative binder systems based on metakaolin, while paving the way for sustainable construction in the future.

Metakaolin (MK) has emerged as a highly promising supplementary material in low carbon binders for the construction industry. In wide range of applications, from lime to cement–based materials, its hardening performance relies on the pozzolanic reactivity between MK and calcium hydroxide (CH), resulting in the formation of diverse calcium-aluminate/silicate hydrates. The reaction sequence is affected by specific conditions dictated by the binder system employed in various applications. To advance the design of binders that reduce the carbon emissions, a systematical review on MK-based reactions is crucial. This review encompasses a broad range of MK/CH ratios and examines the effect of alkali hydroxides, carbonates and sulfates. The focus is on the formation and stability of pozzolanic hydrate phases over time and under different curing temperatures. Additionally, the review addresses the characteristics that directly affect MK reactivity, such as the (calcined) clay structure and the dissolution of the reactants. The systematic findings shed light on the hydrate phase assemblage, enabling a better understanding of the reaction mechanisms in complex systems, like MK cementitious blends. The results of this review serve as a valuable foundation for the development of novel “low-carbon” binder designs and compositions for both cementitious and lime-based binders.