Soluble alkalis play a vital role during early age hydration and strength development in cement. This study investigates the influence of alkali addition on the hydration, phase assemblage, and strength development in ordinary portland cement (OPC) and limestone–calcined clay cement (
) produced with 50% clinker replacement. The alkalinity (%
) of OPC and
was increased using four different alkali salts: NaOH,
, KOH, and
. An acceleration of the early age hydration and an increase in the early age compressive strength development was observed with increasing alkalinity in OPC and
systems. The characteristic features of the calorimetry curves were seen to be significantly influenced by the presence of alkalis. The presence of additional sulfate ions was seen to modify the phase assemblage, with additional ettringite forming in these systems. However, increasing the alkalinity was also seen to reduce the later age clinker hydration and strength development.