Civil Engineering Reference
In-Depth Information
Cheyrezy, 1995). The particle size and specific surface area of the silica fume are
secondary factors, and products with specific surface areas ranging between 10 m
2
/g and
20 m
2
/g may be considered acceptable. The most injurious impurities that may be present
in silica fume products are residual carbon and alkalis (de Larrard, 1989).
Typically, the silica fume/cement ratio in DSP materials is about 0.20-0.25. This ratio
corresponds to the best space-filling performance, but it is too high for a full conversion
of the silica fume to C-S-H in a pozzolanic reaction.
Besides microsilica, other ultrafine materials may be used as constituents of DSP
systems, as long as their particles are small enough to fill the existing spaces left between
the particles of cement. Aldridge
et al.
(1992) produced a DSP material using ultrafine
rutile (TiO
2
) instead of microsilica as filler. The resulting material had properties
comparable to those of a DSP product made with microsilica, in spite of the absence of a
pozzolanic reaction in this combination of starting materials.
To achieve good space-filling between the cement particles, the particles of microsilica
(or rutile) must be effectively dispersed in the liquid phase. This may be achieved by
introducing a superplasticizer to the system, in addition to cement and the microfiller.
Superplasticizers based on melamine or naphthalene may be used for this purpose. Even
better dispersion can be achieved with polyacrylate based dispersing agents (Richard and
Cheyrezy, 1995), but these tend to retard the hydration process. To be sufficiently
effective, the superplasticizer must be added in relatively large amounts, typically 1.5-
2.0% per weight of cement.
The strength of DSP material may be further improved by adding to the starting mix—
in addition to microsilica—medium-size particles, with average diameters between about
2.5 and 9
µ
m (Cheong
et al.,
1997). Such materials may include finely ground granulated
blast furnace slag, fine fractions of fly ash, paper sludge ash, or calcined bauxite. At a
dosage of cement : microsilica : medium-size particle=7:2:1 all these additives are able to
lower the water/solid ratio and increase the compressive strength of the DSP material;
granulated blast furnace slag is the most effective in this respect (see Fig. 14.1).
In addition to cement and microsilica, it is also possible to add larger, non-reactive
particles to the starting mix. The maximum particle size of this “aggregate” fraction
should not exceed 600
µ
m. Particles with diameters below 150
µ
m should be absent, to
prevent interference with the largest cement particles. If particles of this size class are
added to the system, the volume of the paste (that is, microsilica+cement+water) must be
at least 20% greater than the void index of the aggregate in the non-compacted state
(Richard and Cheyrezy, 1995). Under these conditions the aggregate particles will not
form a rigid skeleton within the hardened material, but instead will constitute a set of
inclusions trapped in a continuous matrix.
Fine steel particles with diameters not exceeding 800 µm may also be used as
aggregate in DSP products: this results in an even higher strength of the hardened
material (Richard and Cheyrezy, 1995). By limiting the size of the aggregate particles it
is possible to avoid the formation of microcracks in the material due to external
mechanical forces, autogenous shrinkage, and differential thermal expansion between
aggregate and the paste.
