This study investigates the influence of organic and inorganic hardeners on the microstructure, hydration, and physical properties of foamed gypsum–cement composites for lightweight construction applications. Raw materials, including gypsum plaster and white OPC, were first characterized using XRF, XRD, FTIR, TGA, and particle size distribution (PSD) analysis. Three hardeners were studied: two organic (styrene–butadiene rubber [SBR] and polyvinyl acetate [PVA]) and one inorganic (sodium metasilicate [SMS]). The impact on hydration of foamed gypsum pastes was assessed via XRD, FTIR, TGA, and SEM analyses. XRD and FTIR confirmed partial hydration of hemihydrate to dihydrate. Residual hemihydrate was most pronounced in PVA- and SBR-modified samples, indicating inhibited hydration. TGA revealed the presence of multiple CaCO₃ phases, indicating carbonation of OPC and residual gypsum carbonate. SEM micrographs highlighted differences in pore structure and crystal morphology among mixes, with denser matrices and finer crystallinity observed in sodium silicate-modified pastes. PVA significantly lowered the bulk density of foamed gypsum pastes by stabilizing foam, resulting in highly porous structures with reduced compressive strength and increased water absorption. SBR achieved moderate porosity with enhanced strength due to its film-forming properties. SMS significantly improved early setting, compressive strength, and shrinkage control, indicating accelerated hydration and matrix densification. While PVA delayed setting and reduced shrinkage moderately, SBR offered balanced performance in strength and dimensional stability.