Strength microsilica cements

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. 

Figure 9.3 Effect on concrete strength of replacing Portland cement with microsilica.

An exposme to elevated temperature (up to 600°C) tends to increase the strength of microsilica-modified Portland cement based concrete. This may be attributed to the formation of additional C-S-H in a reaction between residttal calcium hydroxide and microsilica (Saad et al, 1996). 

The use of microsilica-modified Portland cement concrete may be indicated especially in applications where reduced permeability and high chemical resistance of the material are required. The use of this material is also essential in making concrete of extremely high strength. Compressive strengths exceeding 100 MPa may be obtained under these conditions. 

A material closely related to silica firme is colloidal silica, which differs from the former by having a significantly higher specific surface area (up to 80 m /g). If added to Portland cement mixes this material reacts with the liberated calcium hydroxide at a faster rate than microsilica, and leads to the development of higher early strengths (Chandra and... 

Figure 18.2 Effect of microsilica (MS) addition on compressive strength of Portland cement based concrete at equal slump values [W/(C+MS) =0.30].

The main advantages of using foam concrete is that the density of the material can be varied between 300 and 1600 kg/m, and the foam provides good insulation properties and it is easy to handle by pumping. However, several properties still need to be modified in order to make foam concrete an even more competitive material. There is a need to decrease the shrinkage and the water uptake, and to increase the strength and the insulation ability. At present, the foam structures can be enhanced by the use of various additives, such as microsilica or superplasticizers which reduce the cement/water ratio and improve the stability. Polyester fibre additives enhance the hardening and also improve the tensile properties. 

The contents of Ca(OH)2 in the pastes at various times of hydration were determined from the results of the TG studies. Also, the pozzolana nature of the additives has been found out. The ability of combining with Ca(OH)2 was similar in the spent catalyst and the microsilica. In the presence of the spent catalyst, the hydration process was strongly exothermic, which promoted the rapid setting of the cement paste. Calcium carbonate aluminates that are formed in the system, favorably affect the strength of the concrete materials (38). 

Silica-fume concretes incorporate sihca fume, or microsilica, as a partial replacement for Portland cement. Strength increases of 25 percent may result without use of any other admixtures. 

---