دانلود رایگان مقاله منطقه دیواره سیمان مبتنی بر مواد - سد یا خط سرعت برای تخریب شیمیایی؟

Abstract

Sophisticated evaluation models for the long-term stability of cement-based systems demand a precise knowledge of the mechanisms of deterioration reactions, particularly respecting a permanent exposure to aqueous environments. Commonly, insights into these mechanisms are deduced from long-term investigations. However, these chemical reactions start immediately after exposure to aggressive environments causing rapid changes of composition and structure. Consequently, properties of its rim zone change, which affects transport processes in aqueous solutions. In laboratory experiments, the influence of these surface processes on the stability of cement-based materials exposed to different chloride solutions was studied as a function of time and temperature. Analysis of compositional and structural changes beneath the surface reveal the role of crystalline covering layers for chemical resistance. Such layers are often described as protective barriers. However, these processes in the rim zone can accelerate chemical degradation and subsequently reduce the resilience of the cement-based materials to aggressive aqueous environments.

5. Concluding remarks

These results highlight the complexity of physico-chemical processes affecting the properties of a several mm thick rim zone of Portland cement bound systems and, therefore, determining, if it acts as protective barrier or fast lane for reactive transport processes. Its structural properties, especially the capacity to inhibit transport processes, which protects the cement paste, are closely related to the boundary conditions during crystal growth. In the case of calcium carbonate on the surface of cement paste exposed to hard tap water, the structure of the covering layers is often dense and able to restrict transport processes. In contrast, the formation of the Mg(OH)2 capping due to the impact of the MgCl2 solution occurs in a way that harms the cement paste substrate and does not provide a protective effect. Hence, such a chemical attack instantly causes the alteration of a several tens of mm thin rim zone of a cement-based material. The development of the chemical and structural properties of the rim zone is closely related to the evolution of chemical degradation by means of reactive transport. Accordingly, processes that occur on the material surfaces within a very short time period may seriously affect the resistance against chemical attacks of a cementbased material and, thus, its durability. The relevance of such surface processes for the resistance against reactive transport processes emphasizes in particular the importance of the curing phase for the vulnerability of cementitious systems to chemical degradation.