Spalling cement paste

The formation of ettringite is associated with a generation of expansive forces, which may cause expansion of the material and spalling of surface layers. At the same time the decomposition of the C-S-H phase causes a loss of strength and of cohesion. Ultimately, a complete disintegration of the hardened cement paste may occur. 

Chloride ions present in the cement paste promote the corrosion of the reinforcement in steel-reinforced concrete, by causing a breakdown of the protective oxide film that develops at the steel surface at high pH. Under these conditions iron oxide (mst) is formed at the steel/concrete interface, and the expansion associated with it causes cracking and spalling of the surronnding concrete. 

Freeze-thaw damage. Freeze-thaw damage is related to the porous nature of concrete. If the solution trapped in the pores freezes, a volume expansion occurs, which results in tensile stresses. When the hydraulic pressure exceeds the strength of the cement paste, cracking and spalling of the concrete results. Concrete with a high moisture content is most susceptible to this damage mechanism. 

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