Strength gain mechanisms of blended-cements containing marble powder and brick powder were investigated in the presence of chemical composition changings of cement pastes. Blended Cements (BC) were prepared blending of 5% gypsum and 6%, 20%, 21% and 35% Marble Powder (MP) or 6%, 20%, 21% and 35% Brick Powder (BP) for CEMI42.5N cement clinker and grinding these portions in ball mill at 30 (min). Pastes and mortars were also drawn up with these cements. Chemical compositions of pastes, Compressive Strength (CS) and Flexural Strength (FS) of mortars were determined at 7th-day, 28th-day and 90th-day according as standard methods. Experimental results indicated that fluctuation of SiO2, Na2O and alkali at Marble Powder-Blended Cement Paste (MP-BCP) and continuously increasing of SiO2 at Brick Powder-Blended Cement Paste (BP-BCP) positively induced strength gain mechanism of BCM since MP up to 6% or BP up to 35% were blended for cement.