EL PLAN DE INMANENCIA Y LA MULTITUD

The hardened cement paste produced by the interaction of portland or blended cement with water is composed of anhydrous phases (unreacted clinker minerals, mineral additions), hydrate phases modified by foreign ions incorporation and adsorption (C–S–H, ettringite, portlandite, AFm), minor phases such as hydrotalkite, pore solution, and pores. The hydrate phase most abundant in hardened cement paste is calcium silicate hydrate.

Figure 2.17 presented modelled changes during hydration of a) OPC and b) blended cement with fly ash replacement. The majors changes in the presence of fly ash are related to the C-S-H composition which tends to have lower Ca/Si and higher Al/Si ratios than in OPC paste.

2.6.1 Phase assemblage

Accelerated hydration of cement paste and faster formation of calcium hydroxide at initial period was observed in the nanoparticles added cement paste [47, 48]. Nanoparticles were reported to favor the formation of small-sized crystals (such as Ca(OH)2 and AFm) and small sized uniform clusters of C-S-H [49, 50].

Besides the physical effects, nanoparticles may react chemically in the cement hydration process. Limestone is now well acknowledged to interact with AFm and AFt phases. In an ordinary portland cement without limestone powder, the C3A and at a slower rate also the C4AF will react with the calcium sulfate to form ettringite (C3(A,F)·3CaSO4·32H2O). Upon depletion of the sulfates, the remaining C3A and C4AF will react with the ettringite to form monosulphate (C3(A,F)·CaSO4·12H2O) or hydroxy-AFm solid solution [51, 52]. In the presence of limestone, the AFm-carbonate equivalents such as monocarbonate (C3(A,F)·CaCO3·11H2O) are formed

rather than the sulfate containing AFm phases. The AFt-carbonate equivalent has been observed by some researchers [53], but it is unlikely to form in a significant amount at ambient temperatures in a hydrating cement as it is less stable than the AFm phases. The decomposition of ettringite to monosulphate when reacting with the remaining C3A and C4AF upon sulfate depletion is prevented as monosulfate is less stable than monocarbonate in the presence of limestone. The stabilization of the voluminous, water rich ettringite instead of the less voluminous monosulphate, gives rise to an increase of the total volume of hydration products [51, 53]. However, there are many disagreements about the time when the hemicarbonate and monocarbonate phases are formed and in which order.

Nano-silica participates in the pozzolanic reactions, resulting in the consumption of Ca(OH)2 and formation of C-S-H. Dense and compact microstructure with lesser amount of calcium hydroxide crystals was observed (Figure 2.18) [21, 54]. Gaitero and his co-authors reported that nano-silica particles increase the average length of the silicate chains of C-S-H gel [55]. SEM analysis results showed that the cement paste containing 2% nano-montmorillonite addition was denser and more uniform [56].

2.6.2 Pore structure

When cement powder and water are mixed, reactions begin to consume the cement particles and to produce solid products. This is accompanied by a net decrease in volume, called chemical shrinkage, because the volume of the solid products of hydration is less than the initial volume of solids and water from which they form. However, the solid hydration products have greater volume than the initial solids alone, so the water filled space is gradually replaced by solids during the reaction. The space not filled by solid products of hydration is traditionally called the capillary pore space. The principle hydration product, calcium silicate hydrate (C-S-H) gel, also contains a

significant volume of very small pores called gel pores. In this sense, the pore system naturally divides into two distinct populations of pores, one population becoming less numerous, and the other becoming more numerous, as the reactions proceed (Figure 2.19). The distinction between capillary pores and gel pores is somewhat arbitrary, with a cut-off of 10 nm being typically used (Table 2.3). It should be kept in mind that the sizes of capillary and gel pores overlap, and the spectrum of pore sizes in a cement paste is continuous. The capillary pore network remains fully connected until its volume fraction is reduced to about 18% [2].

The properties of porous materials are strongly affected by the characteristics of their pore system, such as porosity, pore size distribution, connectivity, etc. Thus, materials with the same total pore volume (porosity) may exhibit quite different mechanical and transport properties. In cement-based materials, compressive strength and elasticity primarily depend on the porosity, while transport properties, such as permeability and diffusivity, are influenced by the total volume, size distribution, shape and connectivity of the pores [57].

In general fly ash blended cements have a higher total porosity at early age compared to plain portland cement caused by the lower clinker content and the slow reaction of fly ash. At later ages fly ash could refine the pore structure of cement paste by the secondary pozzolanic reaction. The hydration products produced by the reaction of fly ash with calcium hydroxide (CH) fill in the pores, which does not only reduce the pore volume but also the pore size [58, 59]. Nevertheless, earlier studies indicate that there is an optimal replacement of SCM above which the refinement and the reduction of the pore volume are not occurring which, as a consequence, decreases the mechanical properties of the paste [60].

Acting as nano-reinforcement and as filler, nanoparticles can densify the microstructure and the interfacial transition zone (ITZ), and thereby lead to a reduced porosity [61]. Oil well cements

designed with nano-silica showed a decrease in porosity and permeability [62]. The addition of nano-silica refines the pore structure of designed concrete pavement and enhances the resistance to chloride penetration. However, for the concrete pavement containing polypropylene fibers and nano-silica, the pore structure is coarsened and the resistance to chloride penetration is reduced [63].

Through addition of ultrafine layer silicates, it was found that the C-S-H growth on the clay particles resulted in structures determined by the size and shape of the clay particles and the negative charge of some of the clay minerals [64]. Portland cements containing palygorskite and bentonite have a more open structure consisting of interconnected fine pores. In mortars with clays added the interfacial transition zone (ITZ) is connected to the system of interconnected fine pores.