DSP Cement

In order to achieve better mechanical properties and a higher corrosion resistance, the maximum particle density must be combined with a rriinimum total porosity and, in particular, a virtual absence of large pores. There are several options towards achieving this goal, including  

Mechanical treatment by oscillatory or vibrational compaction, high-pressure densification, or shear mixing. 

The addition of dispersing agents with a simultaneous reduction in the amount of tempering water. 

The filling of large pores with sulfur ( suUcrete ) or resin. 

The mixing of cements with variable grain size distributions. 

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Fig. 9.35 DSP cement paste. (Densified system containing homogeneously arranged ultra-fine particles)...

Srrrt, G.K., and Young, J.F. (1993) Hydration reactions in autoclaved DSP cements. 

Special cements with silica fume, giving condensed, compact pastes (DSP) with high content of ultra-fine particles, have very good freeze-thaw resistance [80], It is caused by extremely low capillary porosity and hence very low content of freezable water in these pastes. The same rerrrark can be related to the reactive powder concrete (RPC), for example the Ductal type composite [335], This question will be discussed in Chap. 10. 

In eighties of twentieth century the two new technologies of very high strength cement pastes were developed the DSP— Densifted Systems containing homogeneously arranged ultra-fine Particles (in Denmark), and the MDF— Macro Defects Free Cements. 

The DSP technology is based on the high silica addition, very quick reacting with calcium hydroxide [176, 177]. This silica, in the form of silica fiune, plays a dual role as the pores filling material (micro-filler), as well as the calcium hydroxide bonding reagent calcium hydroxide is released as a product of cement hydration. 

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 constiments 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 mtile (TiOj) 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. 

Figure 14.1 Effect of medium-size particles on compressive strength of DSP material. Mixing ratio cement microsihca medium-size particles=7 1 2. FA, fly ash PSA, paper sludge ash BX=calcined bauxite GBFS= granulated blast furnace slag.

As for the cement, most of the silica fume remains non-reacted, and does not undergo a pozzolanic reactiom In one particular case only 20% of the cement and 9% of the silica fume had been consumed within 28 days of hydration at 20°C (Zanni et al., 1996). These low degrees of reaction must be attributed to the very low water/solid ratios employed in producing DSP materials. 

D. Lange-Kornbak and B.L. Karihaloo, Design of fiber reinforced DSP mixes for minimum brittleness . Advanced Cement Based Materials. 7,1998, 89 101. 

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