Supplementary cementitious materials (SCM) lower the environmental impact of cement and concrete but react more slowly than Portland cement, which therefore limits the levels of substitution possible as reasonable early strengths are necessary. One of the main factors affecting the reaction of the SCMs is the alkalinity of the pore solution. However, alkalis do not only affect SCMs, but also the clinker phases. It is generally accepted that alkalis accelerate the hydration of Portland cement but may have a detrimental impact on strength at later ages. The aim of this work is to clarify the impact of alkali salts (NaOH and Na2SO4) on the kinetics and microstructural development of cementitious systems to better understand their effect on the mechanical properties. The study of many different systems, from models such as alite to more realistic cement-slag systems, allows a general overview of the trends in each parameter. The novelty of this thesis is the study of the relationship between the development of the porosity over time, as well as the evolution of the phase assemblage, and the resulting mechanical strength in the presence of alkalis. Moreover, the use of 1H NMR brings new insights on the impact of alkalis on the amount of water and density of C-S-H in non-dried samples. For a same degree of hydration of cement, the addition of Na2SO4 leads to the same compressive strength as the alkali-free system while NaOH clearly lowers it. Ettringite is suggested as responsible for the differences. While Na2SO4 promotes the precipitation of ettringite, NaOH clearly inhibits it. Therefore, a higher volume of hydrates can better fill the space, the porosity decreases and consequently the strength is increased. Moreover, the addition of sodium gluconate to the cement-slag systems with Na2SO4 improves the strength development which seems to be related to the formation of a less dense C-S-H that better fills the space and lowers the porosity. The presence of Na2SO4 or NaOH does not affect the bulk density of C-S-H but major differences are observed in the morphology of C-S-H. Sulfate uptake leads to a divergent needle-like C-S-H while NaOH promotes a C-S-H that tends towards a foil-like morphology. Also, alkalis lead to a lower and more stable Ca/Si in C-S-H. However, the mechanical strength appears to be independent on the morphology and chemical composition of C-S-H. Alkali salts clearly accelerate the hydration rate at early ages but decrease the degree of hydration at later ages. From the studies on alite, white cement and white cement-slag systems, it has been observed that a slowdown in hydration kinetics could not be explained by changes in the relative humidity, i.e. the water activity. The increased concentration of aluminate ions could explain some of the slowdown but it appears there are other effects.