1-High chloride content in the sand induces the corrosion of the reinforced steel in hardened concrete. Thus, reduces the strength and durability of concrete.
2- High sulfate in the sand induces map cracks on concrete. Therefore will reduce the strength of concrete.
3- Nonetheless, carbonation increases the strength of concrete.
I totally agree with Prof @Ashraf Heniegal and Dr Sir Armand Augustin Fondjo
For more explanation, you can read important engineering standards and review articles related to this issue:
1. American Concrete Institute (ACI) Committee (2014) Building Code Requirements for Structural Concrete (ACI 318–14)
2. Neville AM (2011) Properties of concrete, Fifth Edition. Pearson Education Limited. 846 p. ISBN: 978–0–273–75580–7
3. USACE, 1994. Standard practice for concrete for civil works structures. Engineer manual 1110–2–2000, Washington DC
As well we have a new review paper with a case study that explains the consequences of salinity in the shallow coastal sandy zone to the urban infrastructures.Article Natural and Anthropogenic Coastal Environmental Hazards: An ...
The presence of Carbonates accelerates the strength of concrete. However, when Cl- concentration increases, the concrete surface get hardened and reduces its durability through the reduction of compressive strength. High content of Sulphates depletes the strength pf concrete due to resulting of concrete cracks
Sulfate attack leads to delayed ettringite formation (DEF) by the reaction with C3A that induce the volume expansion and crack localization. This is one of the reasons to decrease the concrete mechanical strength.