Microsilica cements

The amount of free calcium hydroxide in Portland cement-microsilica mixes increases initially, as its formation in the hydration of tricalcium silicate is faster than its consumption in the pozzolanic reaction with microsilica. Later on, however, the amount of free calcium hydroxide may start to decline, when the amount of it consumed in the pozzolanic reaction exceeds the rate by which it is formed in the hydration of tiicalcium and dicalcium silicate (Papadakis, 1999). This crossover point— that is, the time at which the rate of Ca(OH)2 consumption exceeds the rate of its formation— will depend on the amount and reactivity of the microsilica present, as well as on the reactivity of the clinker, and can occur after several hours or a few days of hydration, or not at all (especially at low microsilica additions). The amount of residual free calcium hydroxide in mature paste will generally decline with increasing amounts of nucrosilica in the original mix. It will also decline with decreasing watei/solid ratio, as under these eonditions the C/S ratio of the formed C-S-H phase tends to increase. 

Compositions of high-alumina cement containing quartz or glass, calcium carbonate, microsilica, carbon black, iron oxide red mud or screened fly ash, and styrene-butadiene latex have been described [141,1803,1804]. 

Traetieberg (T47) showed that microsilica used as an addition with cement has considerable pozzolanic activity, mainly in the period 7-14 days after mixing, and that the reaction product formed with CH probably had a Ca/Si ratio of about 1.1. Several subsequent studies have shown that the pozzolanic reaction is detectable within hours and also that the early reaction of the alite is accelerated (H37,H54,H55). Huang and Feldman (H54,H55) studied the hydration reactions in some detail. In pastes with 10% or 30% replacement and w/s ratios of 0.25 or 0.45, the CH content passed through maxima usually within the first day before beginning to decrease in those with 30% replacement, it had reached zero by 14 days. Table 9.9 gives some of the results obtained for CH content and non-evaporable water in these pastes. As with pfa cements, and for the same reason, the non-evaporable water contents of mature pastes are considerably lower than those of comparable pastes of pure Portland cements. 

Table 9.9 Contents of calcium hydroxide and non-evaporable water in some pastes of Portland cement with and without microsilica (percentages on the ignited weight) (H53)... 

Calculations based on reaction stoichiometry and densities of phases support the conclusions from experimental observations that mature pastes of composite cements are more porous than comparable pastes of Portland cements. This is indicated by the results in Table 7.3, 9.4 and 9.6. Similar calculations for 180-day-old pastes of w/s 0.45 indicate free water porosities of about 24% for a typical Portland cement, 35% for a cement with 40% slag, 35% for one with 40% pfa and 32% for one with 30% microsilica. The calculated values are in all cases somewhat higher than observed mercury porosities (F34,F41). 

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. 

Many factors influence the ability of reinforced concrete to resist carbonation induced corrosion. As the carbonation rate is a function of thickness, good cover is essential to resist carbonation. As the process is one of neutralizing the alkalinity of the concrete, good reserves of alkali are needed, that is, a high cement content. The diffusion process is made easier if the concrete has an open pore structure. On the macroscopic scale this means that there should be good compaction. On a microscopic scale well cured concrete has small pores and lower connectivity of pores to the CO2 has a harder job moving through the concrete. Microsilica and other additives can block pores or reduce pores sizes. 

After milling, and deeper removal where cracking and spalling has occurred, the deck is then patched in the delaminated areas and a dense cementitious overlay of microsilica, polymer modified or low slump, low water/cement ratio concrete is put back on. This will slow the corrosion rate and the appearance of further delaminations. 

Pozzolanic materials or pozzolanas do not exhibit cementing properties if mixed with plain water. However, they possess the capacity to react at ambient temperatures with calcium hydroxide, in the presence of water, to yield strength-developing calcium silicate/aluminate hydrates. They include a variety of materials of natural and artificial origin, such as fly ash, microsilica, burnt clays, and diatomaceous earths. 

The specific gravity of microsilica is around 2.20. Because of its very small particle size and high specific surface area, the bulk unit weight of the loose material is very low, and ranges typically between 240 and 300 kg/m. To lower the transportation costs, it is common to ship the material in a predensified form, by which the unit weight may be increased to 540-600 kg/m. It is even more convenient to transport and handle the material in the form of a slurry with a high solid content. Blended cements, in which microsihca is combined with Portland clinker and calcium sulfate, are also commercially available. A comparison of different forms of microsilica indicates that densification of this material decreases its chemical reactivity (Sanches de Rojas et al, 1999). 

Because of the high reactivity of the amorphous form of Si02 and its extreme fineness, microsilica reacts readily with calcium hydroxide in the presence of water, yielding an amorphous C-S-H phase similar to that formed in the lydration of Portland cement (Justines et al, 1990 Justines, 1992 Guindy, 1993 Papadakis, 1999). This pozzolanic... 

It is common to add to the fresh mix appropriate amoimts of a superplasticizer, by which measiue the negative effect of microsihca on rheology may be largely or completely eliminated As well as reducing the amount of water boimd to the surface of the microsilica particles, the superplasticizer also causes an effective dispersion of these partieles within the concrete mix. Under such conditions the individual particles of microsilica can occupy spaces between the coarser cement grains that would otherwise be filled with water. This may result in a reduction of the amoimt of water needed to fill the empty spaces between cement particles, and in the formation of a denser matrix with a... 

In concrete mixes made with Portland cement, the addition of microsilica increases the initial hydration rate of the cement and especially that of the alite and belite phases (Wu and Young, 1984 Huang and Feldman, 1985a, 1985b Durekovic, 1986 Lilkov et al.,... 

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