The construction industry extensively uses cement-based composites for their strength and durability, but large-scale cement production significantly increases carbon emissions. This calls for sustainable construction methods to reduce environmental impact. Cementitious composites face deterioration from chloride ion penetration, carbonation, and sulfate attack, leading to reinforcing steel corrosion and reduced structure lifespan. Nano-admixtures offer a promising solution to enhance cement composites. This chapter examines the impact of nano-ferric oxide (NFO) and nano-silica (NS) on cement and concrete performance. We explore their effects on microstructure, mechanical strength, and durability. NFO and NS improve hydration kinetics; boosting the production of calcium silicate hydrate (C–S–H) gel and creating a more compact microstructure, greatly improving the mechanical performance of concrete. These nanoparticles reduce permeability and improve resistance to harmful agents, enhancing the composite’s integrity. They also act as crack-bridging agents and micro-fillers, boosting toughness and resistance to crack propagation. Moreover, NFO and NS increase durability by reducing chloride ion penetration and improving sulfate and carbonation resistance. Challenges like nanoparticle dispersion, dosage optimization, and agglomeration are also discussed, along with strategies to address them. This chapter provides an in-depth look at utilizing NFO and NS to create a longer-lasting, eco-friendly infrastructure.