Friedel's salt stability and sulfate durability of seawater mixed cement-based materials

As water scarcity continues to increase, using fresh water to produce cementitious material is not sustainable. This study aimed to explore the impact of using seawater as mixing water, varying seawater ion concentrations, and the addition of metakaolin on sulfate durability and the stability of Friedel's salt. For this purpose, the mortar specimens were subjected to sulfate attack for one year, after which the alterations in their length and compressive strength were determined. The effects of sulfate attack and other parameters (using seawater, different ionic concentrations, and substitution of metakaolin) on the microstructure and Friedel's salt stability of cementitious materials were explored quantitatively by X-ray diffraction and thermogravimetric analyses before and after the sulfate attack. Furthermore, the time-dependent Friedel's salt stability of the moisture-cured specimens was also examined by microstructural analyses. The results indicated that the stability of Friedel's salt was observed to be affected by carbonation and sulfate attack. Additionally, the use of seawater and increasing ion concentrations decreased sulfate resistance due to higher gypsum formation as a result of the higher portlandite content and instability of Friedel's salt. On the other hand, the addition of metakaolin reduced the portlandite content as a result of pozzolanic reactions and improved Friedel's salt stability against both sulfate attack and carbonation. Consequently, metakaolin mitigated the negative effects of using seawater on the durability of cementitious materials, thereby it is considered that metakaolin can facilitate the use of seawater as mixing water.