Modifications on Cement Matrix to Increase Durability

By mixing Portland cement with water, the hydration process begins. The hydration process is the combination of physical and chemical changes during which, the cement previously mixed with water becomes a porous sohd. During this process a number of exothermic reactions give rise to cement matrix. In the case of Portland cement, the main reactions are based on interaction between two mineral components present in higher proportion clinker and water. These reactions can be written in simplified form thus [30, 65, 66]  

Taking into account their stoichiometry, both reactions are similar and produce the same products in different proportions [30, 65, 66]. Hydrated calcium silicate (3Ca0.2Si0, AY[ O) is the principal hydration product and is the component that provides resistance to cement matrix and is formed on the surface of cement particles. The calcium hydroxide (Ca(OH)2) is responsible for the pH 12 of the cement matrix and is critical in protecting the steel reinforcement against corrosion however it deg-radates the hemicellulose and lignin of natural fibers decreasing the properties of the composite material. 

Pozzolanic additions are siliceous minerals added during mixing of cement matrix to react with hydration products, especially with CaCOH). This reaction is exothermic and occurs between pozzolan and Ca(OH)2 to form hydrated calcium silicate [30,65,66]. 

Pozzolanic reaction courses slower than reactions between clinker components and water however its speed can be increased by varying the composition and surface area of the pozzolanic additive. Pozzolans more commonly used to increase durability of cement composites with cellulosic fibers are silica fume, metakaolin fly ash and slag. 

Gutierrez et al. [50] investigated the effect of replacing part of Portland cement by pozzolans as silica fume, fly ash and metakaolin on the mechanical properties of cement mortar reinforced with synthetic and natural fibers. The mechanical behavior and durability of composite materials was improved by replacing 15 wt% of cement with metakaolin or silica fume however, because of its low pozzolanic power, fly ash had no effect on the durability of these materials. Khorami and Ganjian [62] increased flexural strength in 20% of cementitious materials reinforced with straw fibers and eucalyptus, by replacing 5 wt% of Portland cement with silica fume. 

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