Fault zones usually present a granular gouge, coming from the wear material of previous slips. This layer contributes to friction stability and plays a key role in the way elastic energy is released during sliding. Considering a mature fault gouge with a change in the percentage of mineral cementation between particles, we aim to understand the influence of interparticle bonds on slip mechanisms by employing the Discrete Element Method. We consider a direct shear model without fluid in 2D, based on a granular sample with realistic grain sizes and shapes. Focusing on the physics of contacts inside the granular gouge, we explore contact interactions and effective friction coefficient within the fault. Brittleness is enhanced with cementation and even more with dense materials. For the investigated data range, three types of cemented material are highlighted: a mildly cemented material (Couette flow, no cohesion), a cemented material with agglomerates of cemented particles changing the granular flow and acting on slip weakening mechanisms (Riedel shear bands R1), and an ultra-cemented material behaving as a brittle material (with several Riedel bands followed by shear-localization). Effective friction curves present double weakening shapes for dense samples with enough cementation. We find that effective friction of a cemented fault cannot be predicted from Mohr-Coulomb criteria because of the specific stress state and kinematic constraints of the fault zone.