During the hydration of CA cement exists a dormant period, which is responsible for the setting kinetics of CA cement bonded refractory castables. The dormant period is believed to be caused by an Al–O–H passivation layer on the CA cement particles. Recent publications indicate that the dissolution (aging) of the passivation layer is closely connected to the prevailing pH value of the pore solution during setting. The higher the pH value, the faster the dissolution of the passivation layer.As phosphate additives, frequently used as dispersing agent, differ in their pH value when dissolved in water it is reasonable that they change the setting kinetics of CA cement bonded refractory castables. Commonly used sodium polyphosphate (S-PP, special case S-TPP for sodium tripolyphosphate) dispersing agents mainly differ in chain length, and therefore in Na+ and phosphate concentration, with different setting kinetics as a result.To investigate the influence of S-(T)PP on setting kinetics, sonic velocity and pH measurements were conducted on S-(T)PP/citric acid dispersed and CA cement bonded refractory castables during their setting. Different phosphates, differing in portion of Na+ to phosphate and medium chain length of the phosphate molecules, were used in varied concentrations within refractory castables to quantify the shift in setting kinetics and verify the role of the pH value during the hydration of CA cement.It was found that the chain length of the phosphate molecule of S-(T)PP influences the setting kinetics, and especially the length of the dormant period of the hydration of CA cement, in an indistinct way. Minor differences within the Na+ concentration at differing chain length S-(T)PP showed a comparable tendency to influence the hydration kinetics of CA cement as the chain length of the phosphate molecule was varied. However, an increase of the concentration of the applied S-(T)PP, which goes along with a distinctly increased Na+ concentration, resulted in a significant shorter dormant period and correspondingly an accelerated main hydration of CA cement. The corresponding pH value during the dormant period of the hydration of CA cement increased concurrently to the increase of the S-(T)PP and Na+ concentration.It is discussed that this increase of the pH value of the pore solution of the refractory castable results from an increase of the Na+ concentration from the S-(T)PP, while phosphate molecules are precipitated as Ca-phosphate. This indicates that the dissolution of the Al–O–H passivation layer accelerates with an increasing Na+ concentration (equivalent to increasing pH value) in the pore solution. Thereby, the dormant period is shorter and the hydration of CA cement starts earlier.The general tendency that additives of alkaline nature are accelerating the setting kinetics could be linked to the pH value during the dormant period. Therefore, the theory of the pH dependent aging of the Al–O–H passivation layer on CA cement particles was confirmed. As a directly applicable result, S-(T)PP may be used as accelerator (accelerating the CA cement hydration) beside their dispersing effect.