Presented by: Asst. Prof. Dr. Mayada Waheed Fallah Head of the Civil Engineering Department – Al-Mustaqbal University Replacing a portion of Portland cement with Recycled Glass Powder (RGP) is considered one of the promising sustainable solutions in materials and construction engineering. The physical and mechanical behavior of cement mortar incorporating glass powder depends primarily on particle fineness and the replacement percentage. 1. Chemical Mechanism (Pozzolanic Activity) Glass contains a high proportion of amorphous silica. When glass is ground into very fine particles, generally smaller than 75 micrometers, the silica reacts with calcium hydroxide produced during cement hydration through a pozzolanic reaction. This reaction produces additional calcium silicate hydrate (C-S-H) gel, which fills fine pores and densifies the microstructure of the mortar. 2. Effects on Mechanical Properties Compressive Strength At early ages (3–7 days): Compressive strength may decrease slightly as the replacement percentage increases, due to the slower pozzolanic reaction compared with the rapid hydration of cement. At later ages (28–90 days): Strength can increase significantly at optimal replacement levels, typically between 10% and 20%, and may sometimes exceed that of the control sample. At high replacement levels (above 20–25%): Strength may decrease because the reduced cement content limits the availability of calcium hydroxide required for the pozzolanic reaction. Flexural and Tensile Strength Flexural and tensile strength generally follow a similar trend to compressive strength. Bonding at the interfacial transition zone (ITZ) between the cement paste and aggregate may improve due to the formation of additional calcium silicate hydrate (C-S-H) gel. 3. Effects on Durability Water Absorption and Porosity Fine glass powder reduces overall porosity and the connectivity of capillary pores, thereby decreasing water absorption and permeability. Resistance to Chloride Ion Penetration The incorporation of finely ground glass powder can significantly improve resistance to chloride ion penetration, helping protect reinforcing steel in concrete from corrosion. Alkali–Silica Reaction (ASR) Unlike coarse glass aggregates, which may cause harmful alkali–silica reaction and concrete expansion, very fine glass powder can mitigate ASR by consuming alkalis and calcium hydroxide, thereby reducing the formation of expansive reaction products. Resistance to Chemical Attack The resistance of cement mortar to sulfate attack and certain forms of mild acid exposure may improve due to reduced free calcium hydroxide content and a denser internal microstructure. However, the degree of improvement depends on the exposure conditions and the characteristics of the glass powder. 4. Workability and Fresh Properties Workability Glass powder may slightly improve mortar flowability in some cases because of its smooth, relatively non-absorbent surface. However, very fine particles may increase the mixture's viscosity if the water-to-cementitious-materials ratio and mixture proportions are not properly controlled. Conclusion The use of recycled glass powder as a partial replacement for Portland cement offers a promising approach to sustainable construction. When particle fineness and replacement percentages are carefully optimized, RGP can improve the mechanical performance and durability of cement mortar while reducing cement consumption and promoting the recycling of glass waste. This approach supports environmentally responsible construction practices and contributes to the development of more sustainable building materials.