Lately, the development of green chemistry methods with high efficiency for metal nanoparticle synthesis has become a primary focus among researchers. The main goal is to find an eco-friendly technique for the production of nanoparticles. Ferro- and ferrimagnetic materials such as magnetite (Fe₃O₄) exhibit superparamagnetic behavior at a nanometric scale. Magnetic nanoparticles have been gaining increasing interest in nanoscience and nanotechnology. This interest is attributed to their physicochemical properties, particle size, and low toxicity. The present work aims to synthesize magnetite nanoparticles in a single step using extracts of green lemon Citrus Aurantifolia residues. The results produced nanoparticles of smaller size using a method that is friendlier to health and the environment, is more profitable, and can be applied in anticorrosive coatings. The green synthesis was carried out by a co-precipitation method under variable temperature conditions. The X-ray Diffraction (XRD) and Transmission Electron Microscopy (TEM) characterization showed that magnetite nanoparticles were successfully obtained with a very narrow particle size distribution between 3 and 10 nm. A composite was produced with the nanoparticles and graphene to be used as a surface coating on steel. In addition, the coating’s anticorrosive behavior was evaluated through electrochemical techniques. The surface coating obtained showed good anticorrosive properties and resistance to abrasion.

By way of the sol–gel chemical synthesis method, it is possible to synthesize SiO₂ nanoparticles with a defined specific particle size, a surface area, and a defined crystal structure that can be effectively used as a nanoadsorbent to remove various organic dyes. SiO₂ nanoparticles were synthesized by the sol–gel method using sodium silicate (Na₂SiO₃) by a green method without using a tetraethyl orthosilicate (TEOS) precursor, which is very expensive and highly toxic. This sol–gel process involves the formation of a colloidal suspension (sol) and solid gelation to form a network in a continuous liquid phase (gel). In addition, it requires controlled atmospheres. XRD indicates the presence of an amorphous phase with a diffraction angle of 2θ = 23°, associated with SiO₂. UV-Vis spectroscopy reveals an absorbance value in the region of 200 nm to 300 nm, associated with SiO₂ nanoparticles. The application as a nanoadsorbent to remove dyes was measured, and it was found that the nanoparticles with the best performance were those that were synthesized with pH 7, showing a 97% removal with 20 mg of SiO₂ nanoparticles in 60 min. Therefore, SiO₂ nanoparticles can be used as a nanoadsorbent, using a low-cost and scalable method for application to remove methylene blue in an aqueous medium.

This study addresses a key unresolved question in alkali-activated materials: how externally supplied calcium from limestone influences calcium-induced hybrid gel formation under NaOH-only activation in precursor systems with different initial calcium availability. Unlike previous studies focusing mainly on strength development or using combined alkaline activators, this work establishes a composition-dependent mechanistic framework linking calcium availability, reaction kinetics, microstructural evolution, and engineering performance. Limestone additions (5–10 wt.%) were incorporated into fly ash (FA0) and slag–fly ash (FAS0) binders activated with 8 M NaOH and cured at 65 °C. In the low-calcium fly ash system, compressive strength increased from 29 to 38 MPa (≈30%), with nearly 90% of the ultimate strength achieved within the first 24 h. In contrast, slag-containing binders exhibited strength variations below 5%, indicating limited additional benefit once sufficient calcium was intrinsically available. Integrated characterization (SEM/EDS, XRD, FTIR, isothermal calorimetry, and TGA–DSC) consistently supports the progressive transformation from predominantly N–A–S–H gel toward hybrid calcium-containing binding environments. The combined evidence suggests that localized calcium availability associated with limestone accelerates hybrid gel formation and microstructural densification rather than acting solely as an inert carbonate filler. These findings provide practical guidelines for designing low-carbon hybrid alkali-activated binders with tailored reaction kinetics, optimized engineering performance, and improved material efficiency.