Addition of fine limestone provides an excellent means to reduce the amount of clinker in cement. It is now well accepted that limestone partially reacts in cementitious systems with C3A to produce hemi- and monocarboaluminate phases and as a consequence more sulfate is available to form ettringite and the total volume of hydrates increases. The mechanism by which limestone affects the hydration is crucial in understanding its influence on the properties of cementitious materials. Laboratory and Commercial Cements with two different types of clinker, low and high C3A with different gypsum and limestone addition, were investigated. Hydrated pastes and mortars were investigated in terms of kinetics, phases assemblage, microstructure development, porosity, mechanical properties and durability (sulfate attack and sorptivity). An improvement in sample preparation for XRD measurement was made, which allows preferential orientation to be avoid and improves Rietveld Analysis quantification. The elastic modulus was found to correlate well with compressive strength and could be used as a nondestructive method to measure compressive strength. Monocarboaluminate formation was found to increase with increasing C3A. For high C3A cement it is visible at 2 days of hydration and at 720 days 4.5% of monocarboaluminate is measured in the system. For Low C3A cement it is visible at 7 days of hydration and at 720 days 1.6% of monocarboaluminate is measured in the system. Mc is formed only after all gypsum, which is more reactive than limestone is consumed to produce ettringite. No monosulfate is observed in the limestone systems. The optimum gypsum, was found to have as much effect at early ages on the hydration as 10% of limestone addition. Consequently variations in the gypsum level were investigated but it was difficult to quantify differences between samples with increasing gypsum addition. Studies of behavior in sulfate solution indicated that C3A is the dominant factor. Limestone addition produces slight changes in the form of degradation but do not fundamentally change whether deterioration takes place or not.