I know if you add too much sand or lime to a mortar it can become weak. Is there any problem with adding too much portland cement to a mortar?
Portland cement gives a mortar added durability, high early strength, a consistent hardening rate, and high compressive and bond strengths. However, mortar with excessively high cement contents can affect water permeability and durability of the masonry. Water permeability is increased for three reasons. First, mortars with high cement content have greater shrinkage and more frequent shrinkage cracks. Shrinkage cracking often takes the form of evenly spaced vertical cracks in bed joints and evenly spaced horizontal cracks in head joints. The mortar may also separate or pull away from the body of the brick at the bond interface. Although high cement content mortar itself is less permeable, water can penetrate the shrinkage cracks. The second reason that high cement content mortars may increase water permeance is that these mortars are stiff and not easily workable. Because of this, good bond may not be achieved at the time of construction. Finally, the high strength (high cement content) mortars are hard (brittle) and don't easily accommodate movement of the masonry (due to environmental changes or foundation settlement) and therefore may result in cracking of the masonry wall. Cracks make the walls more susceptible to freeze-thaw deterioration even though high cement content mortar itself is more resistant to frost damage. Mortar should always be weaker than the masonry unit to accommodate small movement without damage to the masonry unit.