Cement Paste-Aggregate Bond

Maso [10] has shown experimentally that at weak paste-aggregate bond its cracking does not imply the cracking of whole material, because in this case the strength of composite is only the function of the strength of cement matrix. 

Above some level of adhesion force, the cracking of paste-aggregate bonds results in cracking of material then the matrix cannot transfer all load. This can be demorrstrated based on an example (Fig. 6.5). The paste-aggregate bond is henee of substantial importance and the strength of concrete is thus determined [10], 

The following formrrla for concrete strength was derived by Alexander and Tap-lin [11] from the regression analysis  

Zaitsev and Wittmann [15] proposed a model of fracture, presented in Fig. 6.6. The authors assume in this model that the cracks start in the interfacial transition zone. 

The XRD studies of the interfacial transition zone (material produced by abrasion of paste layers) [16], as well as the SEM observations with EDS analysis [16] revealed the presence of transition zone surrounding the aggregate grains, determined by Maso as an aureole [ 10]. This relates to the former water film around the aggregate. This area shows higher w/c ratio and subsequently cement components can readily dissolve, as well as the hydration products crystallize from the solution. Calcium hydroxide crystallizes in this interfacial transition zone and the crystals are oriented in such a way that their (001) axis is perpendicular to the surface of aggregate, as it was reported by Barnes et al. [17]. The C-S-H is then formed and the two products occur together as a duplex film about 1 pm thick (Fig. 6.7). 

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Fig. 6.13 Cement paste-aggregate bond in concretes with different aggregates of natural origin, with rough and polished surfaees. (according to [18]) 1—polished limestone, 2—cracked limestone, 3—polished quartz, 4—quartz, natural gravel, 5—polished calcite, 6—polished feldspar, 7—polished granite...

H53.H62). The properties were attributed to a combination of effects. The particles of microsilica, being much finer than those of the cement, partially fill the spaces between the cement grains, and this, together with the superplasticizer, allows the latter to pack more uniformly. They also provide nucleation sites for hydration products, undergo pozzolanic reaction and probably improve the paste aggregate bond. 

Struble, L. (1988) Microstructure and fracture at the cement paste-aggregate interface , in Proc. Symp. Bonding in Cementitious Composites, Boston, 2—4 December 1987, Pittsburgh, Pennsylvania Materials Research Society, pp. 11-20. 

Hsu, T.T.C. and Slate, F.O. (1963) Tensile Bond Strength between Aggregate and Cement Paste or Mortar. ACI Journal, Proceedings 60,4, 465 -85. 

Reclaimed concrete material (RCM) is sometimes referred to as recycled concrete pavement (RCP), or crushed concrete [ 159-162]. It consists of high-quality, well-graded aggregates, bonded by a hardened cementitious paste. The aggregates comprise approximately 60-75% of the total volume of concrete. RCM is generated through the demolition of Portland cement concrete elements of roads, runways, and structures during road reconstruction, utility excavations, or demolition operations. 

First Step. When polymer latexes are mixed with fresh cement mortar or concrete, the polymer partides are uniformly dispersed in the cement paste phase. In this polymer-cement paste, flie cement gel is gradually formed by the cement hydration and the water phase is saturated with calcium hydroxide formed during the hydration, whereas the polymer partides dqrosit partially on the surfaces of the cement-gel-unhydrated-cement partide mixtures. It is likely that the calcium hydroxide in the water phase reacts witit a silica surface of the aggre tes to form a calcium silicate layer.I It is confirmed that tire formation of the calcium hydroxide and ettringite in the contad zone between tire cement hydrates and aggregates is attributed to the bond between them.I lPl... 

Concrete is a composite typically composed of aggregate and cement paste. It is the connection between these phases - the interfacial transition zone (ITZ) that is most important.. The ITZ is the weakest link in the composite system with lowest mechanical properties of the three [1-4], The ITZ can affect the overall elastic module and the stress distributions in a concrete material. The ITZ is comparatively more porous than that of bulk cement paste, and often less well bonded to the aggregate [3]. This region can have a low formation of calcium-silicate-hydrate (C-S-H gel), a product of Portland cement hydration responsible for the good mechanical properties and durability [5]. 

Cement paste can be considered as a concrete with micro—aggregate in the form of unhydrated cement grains. Its high strength is attributed to the occurrence of strong bonds between these unhydrated cement grains and C-S-H. 

An aggregate particirlarly suitable for use in combination with calcium aluminate cement is limestone. The calcium carboaluminate hydrate (C aCHjj) formed in the reaction of the CaCOj of the limestone with the calcimn aluminates of the cement contributes to an improved bond between the hardened cement paste and the aggregate sitrface. 

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