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Modified High Performance Concrete

3. Mechanism of MB action on cement systems

The mechanism of MB action on cement systems consists of the following. At mixing with the water and other components of a concrete mixture, a de–aggregation of granules takes place, which is accompanied by dissolution of the solidified layer of superplasticizers and by the gradual inflow of SP into a liquid phase. As a result of a complete de–aggregation during mixing, the particles of microfiller are distributed evenly in the cement system (Fig. 21).

Figure 21  De–aggregation of Modifier’s Granules in Water (SF–microsilica; SP–Superplasticizer; MB–modifier)

The gradual inflow of superplasticizers and setting regulator into a liquid phase is a condition of stable consistency of concrete mixtures in time. This is very important for long distance transportation of mixtures and for in–situ concreting.   Cement paste and concrete structure forming occurs in conformity with characteristic tendencies for a system containing microsilica, fly ash and superplasticizer [28, 43].

Complex influence of organic (superplasticizer and setting regulator) and mineral (microsilica or its mix with fly ash) parts of MB on cement system results in the following: at initial stage of structure forming the system being plastic becomes increasingly viscous and adhesive and is characterized by strongly pronounced thixotropy. At the late stage the cement paste is characterized by a special quality composition and a special structure geometry [27].   The first shows in increased cement hydration and in balance shift between hydrate phases towards volume increase of more strong and stable calcium hydrosilicates CSH(I) type proportion instead of primary crystalline hydrates of portlandith type and calcium hydro silicates CSH(II) type [28, 33, 34, 35] (Fig. 22).

Figure 22	Сement Hydration Degree and the Quantity of Ca(OH)2, CSH(II) and CSH(I) in Portland Cement Paste VS Modifiers Dosage

The second shows in increased content of gel pores and decreased volume of capillary pores correspondingly [27, 28, 33, 34, 43, 47] (Fig. 23).

Figure 23	Differential Porosity of Portland Cement Paste with MB-01 (Total porosity =33.5%; W/(C+МB)=0.3)

Proportion of microsilica and fly ash in modifiers (and, therefore, MB type) influences on the phase composition and structure of cement paste. With the increase in the level of fly ash the quantity of highly dispersed and strong hydrosilicates such as CSH(I) is reduced and that of lower strength CSH(II) is increased (Fig.24).

Figure 24	Relative CSH(I) and CSH(II) Content VS Fly Ash Quantity in Modifiers Mineral Part 1 – W/(C+МB)=0.18 2 – W/(C+МB)=0.14

The increase in the level of fly ash results in changes in the balance between pores of different degrees of dispersion in the concrete structure – a decrease of volume of gel pores and an increase of volume of micro- and macrocapillary pores (Fig.25) [8].

Figure 25	Differential Porosity of Mortars VS Fly Ash Quantity in Modifiers Mineral Part 1 – W/(C+МB)=0.18 2 – W/(C+МB)=0.14

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