Hydrosodalite has been synthesized at low-temperature (105 °C) in the unstirred suspension at isothermal curing from NaOH, Al(OH)_3, amorphous SiO₂ × nH₂O and H₂O. By the process of modification of hydrosodalite ions exchange was carried out by reduction of Na⁺ and increasing Ca²⁺. Through the high puzzolanic activity of hydrosodalite and formation of hydroaluminate phases it has the positive effect on the hardened cement paste compressive strength, especially at the early state of cement hydration. This modified hydrosodalite and cement can undergo puzzolanic reaction which results in less void content and higher final compressive strength, depending on the amount of synthetic modified zeolite. The amount of the additive has various influences on the strength of the hardened cement paste. Addition of 5% of modified hydrosodalite slightly reduces the compressive strength of the hardened cement paste, however, with the increase of the additive amount from 5 to 20%, the compressive strength slightly improves, especially at the early state of cement hydration. The highest compressive strength was observed in samples with 15% of the modified hydrosodalite additive. The increase of the strength is related to the impact of SiO₂ and Al₂O₃ presented in modified hydrosodalite.

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