This article presents the influence of both Portland cement replacement and sand to binder ratio (S/B) on slant shear strength between concrete substrate and geopolymer mortar. Fly ash (FA) was replaced with Portland cement (PC) at the dosages of 0, 5, 10 and 15% with various sand to binder ratios of 1.00, 1.25 and 1.50, respectively. The alkali solution for used as liquid alkali were sodium silicate (Na2SiO3) and 10 molar sodium hydroxide (NaOH) solutions. The Na2SiO3/NaOH ratio of 2.0 and the liquid/binder (L/B) ratio of 0.60 and cure at ambient temperature were used in all mixture. Test results indicated that the slant shear strength between concrete substrate and geopolymer mortar tends to increase with increase in PC replacement. While the S/B ratio increased, the slant shear strength between concrete substrate and geopolymer mortar tends to increase. However at S/B ratio of 1.50, the slant shear strength tends to decrease. The 15% PC replacement with S/B ratio of 1.00 gave the highest slant shear strength between concrete substrate and geopolymer.