Effects of seawater and undesalted sea sand on the hydration products, mechanical properties and microstructures of cement mortar

Abstract

The effects of seawater and undesalted sea sand on the early-age hydration, mechanical performance, water absorption, leaching behavior and hydration products of Portland cement mortars with only sea sand, seawater, or both seawater and sea sand were fundamentally investigated in this study. The results show that the salts content in sea sand could accelerate the hydration reaction and increase the early-stage compressive strength, while the physical properties of sea sand have an adverse effect on the strength development, leading to the compressive strength of cement mortar with only sea sand lower than the ones with demineralized water and river sand. The addition of seawater could significantly accelerate the hydration due to its high salts content, which could compensate for the side effect of sea sand and make the cement mortar with seawater and sea sand with the highest compressive strength. The chloride, sulphate and magnesium ions in seawater and/or sea sand may lead to the phase changes of cement mortars. The chloride and sulphate ions in seawater and sea sand reacted with tricalcium aluminate (C3A) and the hydrated product of Portlandite to form much more Friedel’s/Kuzel’s salts and ettringite. Moreover, the magnesium ions primarily in seawater generated magnesium hydroxide (Mg(OH)2) and magnesium silicate hydrate (M−S−H) formation by replacing the Ca2+ and decalcification of calcium silicate hydrate (C-S-H). The substitute of seawater also can make the Si/Ca ratio of the C-S-H (Cl) be increased from the range of 0.2–0.4 to 0.6–1.2 respectively. Therefore, utilization of seawater and undesalted sea sand in concrete production provides a sustainable way to solve the shortage of natural freshwater and river sand.