Alkali-activated concrete (AAC) has emerged as a viable low-carbon alternative to Portland cement; however, the scarcity of conventional precursors such as fly ash (FA) and granulated blast furnace slag (GBFS) poses challenges to its scalability. This study explores the utilization of waste powders from the stone processing industry, including marble waste powder (MWP), granite waste powder (GWP), and pumice waste powder (PWP), as sustainable precursors for alkali-activated lightweight aggregate concrete (AAC-LWAC). Locally available lightweight pumice stones were used to replace the normal coarse aggregates, and the FA precursor was replaced with waste powders in proportions ranging from 25% to 100%. X-ray diffraction analysis was conducted to investigate the characterization and reactivity of the powders. Furthermore, the mechanical properties of the resulting materials were evaluated to determine the compressive strength, modulus of elasticity, and stress-strain relationships. The results showed that at lower replacement levels (25–50%), lightweight pumice waste powder (LPWP) concrete exhibited superior performance among the studied waste powders, achieving strength close to the control sample and demonstrating better ductility and strain capacity, particularly at 50% replacement. In contrast, at higher replacement levels (75–100%), all powders showed significant reductions in strength and stiffness. Granite & Marble waste powder concretes were acceptable at lower replacement levels but are more suited for non-structural or insulating uses where high mechanical performance is not critical.