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In most countries, limestone and pozzolanas are widely used in Portland cement because such additions increase the chemical resistance to sulfate and/or chloride attack, impermeability and lower the heat of hydration of concrete. This study aimed to investigate the effect of fly ash (FA) and limestone (LS) on the hydration and durability of ordinary Portland cement (OPC) subjected to Caron's Lake water. The results revealed that the increase of LS content increased the water of consistency (normal consistency) of cement pastes, whereas the initial and final setting times were reduced by up to 30%. The portlandite contents of mixtures containing FA decreased. At the same time, an increase of LS content from 10 to 20% decreased the portlandite content by up to 90 days. On the other hand, the portlandite content of 30% LS increased by up to 90 days. Cement pastes containing FA had the lower values of total sulfate and chloride contents than sulfate-resisting cement pastes. The compressive strength of mixture M.10 (20% FA and 10% LS) showed higher compressive strength values than the other cements when immersed in both curing media, namely tap and Caron's Lake water. It is recommended that cements that are to be used for underground concrete structures or marine structures that are exposed to sulfatecontaining seawater do not contain more than 10-20% of LS.

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