Cementitious bond

In this chapter the nature of the cementitious bond and the acid-base reaction will be discussed. 

Wygant, J. F. (1958). Cementitious bonding in ceramic fabrication. In Kingery, W. D. (ed.) Ceramic Fabrication Processes, pp. 171-88. New York John Wiley Sons. 

Another feature of the metal ions that are typically involved in cementitious bonding in AB cements is that most of them fall into the category of hard in Pearson s Hard and Soft Acids and Bases scheme (Pearson, 1963). The underlying principle of this classification is that bases may be divided into two categories, namely those that are polarizable or soft, and those that are non-polarizable or hard. Lewis acids too may be essentially divided into hard and soft, depending on polarizability. From these classifications emerges the useful generalization that hard acids prefer to associate with hayd bases and soft acids prefer to associate with soft bases (see Section 2.3.7). 

Of the ions most often implicated in cementitious bonding in AB cements, Ca ", Mg " and AF are classified as clearly hard Zn " by contrast falls into the category that Pearson designated borderline , as... 

It must be stressed that these specific properties of carbonate sands do not imply that they are not suitable as a fill material (crushing, cementitious bonding). Ample experience, in particular in the Middle East, indicates that carbonate sands perform satisfactorily as a bulk fill provided that during design and construction their typical properties are taken into account. 

Concrete, a type of large-particle composite, consists of an aggregate of particles bonded together with cement. In the case of Portland cement concrete, the aggregate consists of sand and gravel the cementitious bond develops as a result of chemical reactions between the Portland cement and water. 

The essential property of a cementitious material is that it is cohesive. Cohesion is characteristic of a continuous structure, which in the case of a cement implies an isotropic three-dimensional network. Moreover, the network bonds must be attributed to attractions on the molecular level. Increasingly, recent research tends to show that cements are not bonded by interlocking crystallites and that the formation of crystallites is incidental (Steinke et al., 1988 Crisp et al., 1978). The reason is that it is difficult to form rapidly a mass which is both cohesive and highly ordered. 

The phosphate bonded cements described in this chapter are the products of the simple acid-base reaction between an aqueous solution of orthophosphoric acid and a basic oxide or silicate. Such reactions take place at room temperature. Excluded from this chapter are the cementitious substances that are formed by the heat treatment of aqueous solutions of acid metal phosphates. 

To improve the interfacial bonding between the PP fiber and a cementitious matrix, commercially available PP fibers with different geometries were fluori-nated and the effect evaluated with regard to crack control, impact resistance (AC1 Committee 544 Report), and water absorption (ASTM C-948).M,34a In all cases, as seen in Table 16.13, fluorination improved the induced properties. 

Acrylic-modified cementitious products excellent bond strength water resistance resistance to ultraviolet radiation low water/cement ratio superior handling and application properties various, depending upon type of product repair mortars floorings tile adhesives renders coatings, etc. 

The surface of the concrete substrate has to be prepared to provide sufficient bond of the repair material. Factors that may affect the bond are the strength and integrity of the substrate, the cleanliness of the surface, and the roughness. The surface should be rough and dust or incoherent residues should be removed (for instance by sandblasting or waterblasting). This operation is usually not necessary if hydro-demohtion has been used. If the cementitious repair material is appHed directly on the surface of the concrete, the surface should be saturated by water in order to avoid absorption of water from the substrate and subsequent plastic shrinkage and incomplete hydration of the repair material, which will result in loss of bond. 

Bonding agents may also be applied in some cases on the concrete surface in order to enhance adhesion of the repair mortar. These may be cementitious systems (cement paste or fine mortars), polymer latex or epoxy systems [4]. Epoxy systems should be used carefully since they create a moisture barrier between the substrate and the repair material, which can result in failure of the repair if moisture is trapped in the concrete. 

Polymer-modified mortars can be obtained by replacing part of the mixing water with a synthetic latex (e. g. styrene butadiene or acrylate) to the mix. Although the binder is still cementitious, and thus alkalinity is guaranteed, the latex may improve the workability, the waterproofness, the carbonation and chloride resistance, the tensile and flexural strength of the repair mortar [8]. It can also reduce the modulus of elasticity, increase the bond to the substrate, reduce the rate of drying out and thus the rate of shrinkage. 

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