This study explores the impact of Waste Brick Powder (WBP) on the microstructure and properties of High-Strength Concrete (HSC). WBP was utilized as a partial replacement for fine aggregates, which are derived from construction waste generated during the production of brick aggregate. The mortar was separated from the fresh concrete using a 4.75 mm sieve, and 50 mm cube specimens were fabricated to assess mechanical properties as well as the microstructure of the mortar. After a 56-day curing period, compressive strength, ultrasonic pulse velocity (UPV), absorption capacity, and apparent density of the concrete specimens were measured. Scanning Electron Microscopy (SEM) analysis was conducted to evaluate the microstructures of the interfacial transition zone (ITZ) at the interface between the cement paste and brick powder. Additionally, internal humidity within the concrete matrix over time was evaluated. Through these experiments and analyses, it was determined that the inclusion of WBP positively influences the microstructure of the mortar matrix of HSC.