Surface of cement

FIGURE 14.21 An electron micrograph of the surface of cement, showing the growth of tiny interlocking crystals as carbon dioxide reacts with calcium oxide and silica. 

The pivotal role that superplasticizers play in the formulation of self-leveling mortars is due to the dramatic effects they produce on flow behavior. Such effects are believed to be derived by the adsorption of the admixture on the surfaces of cement grains, thereby providing surfaces of a similar or zero charge which are mutually repulsive. They thus fully disperse cement particles, freeing more water for lubrication and reducing interparticle attraction. Both yield stress and plastic viscosity are decreased and the decrease is greater for yield stress it may be completely eliminated if sufficient admixture is added so that Newtonian behavior is observed (Fig. 7.25) [75, 76]. 

Sorbed or precipitated onto the surfaces of cement minerals or... 

Blaine (1941) used the Carman method, modified by Lea and Nurse (1939), to determine the specific surface of cement and developed a con-... 

Figure 88. Blaine s Apparatus for Determining the Surface of Cement.

Fig. 4 shows the morphologies of fractured cement pastes with and without SBR dispersion or powder observed by ESEM. Irregular hydrates is observed on the surface of cement particles in the control paste after 10 minutes, and AFt or calcium aluminate hydrates with relative regular shape are also found (Fig. 4 (a)) while at this time a polymer-particle layer is seen on the surface of cement particles in the pastes with SBR dispersion or powder, and AFt or calcium aluminate hydrates appear among the polymer particles (Fig. 4 (b) (c)). 

The reaction of clinker phases with water, together with the crystalUzation of hydrates, brings about the substantial changes of rheological properties of the paste. The formation of hydrated phases and, according to Locher [28], the morphology of hydrates crystallized in the liquid phase or on the surface of cement grains, leads to the reduction of distance between them and the viscosity of paste is enhanced. At... 

The mechanism of retardation by soluble phosphates is explained by the precipitation of insoluble tricalcium phosphate lay er on the surface of cement grains, impermeable for water [96]. For example the calcium dihydrophosphate Ca(H2P04)2 11,0 reacts with Ca(OH)2 in the strongly supersaturated layer surrounding the alite grains and the Ca3(P04)2 is produced. 

Similarly influence the soluble fluorides for example the F ions released from NaF to the solution form the insoluble CaFj which covers the cement grains. The complex fluorosilicates Na2SiFg and NajAlFg can additionally result in the precipitation of siUcate or aluminate gel on the surface of cement grain poor in calcium. Phosphogypsum, soluble calcium phosphates and sodium fluoroaluminates containing by-products exhibits the retarding effect too [96]. 

Fig. 4.50 Specific surface of cement vs. time of grinding. The continuous line—real curve the dashed lines show the beginning of agglomeration and the end of cement specific surface area increase during grinding...

InFig. 4.51 the effect of different grinding aids on the specific surface of cement is presented. As it is shown the poly-glycol-phenol ether and the heptane-phos-phonate acid have the strongest impact. The effectiveness of saturated fatty acids and alcohols action grows with the rednction of chains length, the undersaturated acids are more active than the saturated ones [152,153]. This can be explained by... 

The application of plasticizers leads to the complete dispersion of cement grains in water, without conglomerates formation. It is assumed that the molecules of superplasticizer are adsorbed on the surface of cement grains which is neutralizing the... 

The application of classic double layer model to the cement pastes is questionable because the surface in this case is not in thermodynamic equilibrium. The surface of cement grains reacts continuously with water and, as a result, the releasing of different ions into the liquid phase occurs and the surface charge varies all the time. Therefore opposite to the classic double layer its irmer part changes continuously. For this reason appeared the concept to replace the classic potential by the dynamic potential, which is changing continuously dining the hydration process [26]. However, the potential of hydrating cement is often measured and an example of these measurements results is shown in Fig. 5.18 [27]. 

Locher [2] investigated the effect of cement fineness, determined by the parameters of RRSB equation, such as x — particle size (determining such a diameter that 63.2% particles are smaller than x ), n—slope of the straight line determining the half of width of normal distribution, on the water demand of cement. At constant n, the reduction of x parameter from 20 to 13 pm in the n-x system leads to the significant growth of water demand (Fig. 6.1). The C3A reactivity, as well as the specific surface of cement are shown in Fig. 6.1 too. 

In these polymers, apart from the electrostatic repulsion, resulting from like charges occurring on the surface of cement grains, the so-called steric hindrance is produced, due to the presence of side chains. This steric repulsion is very important as a dispersive interaction in cement water suspension (see Fig. 6.99). 

Fig. 6.107 Distribution of air entraining agent molecules in concrete the non-polar chain is attached to the air bubble, the anionic group is adsorbed on the surface of cement grains and aggregate. (According to [358])...

The too high addition of air entraining agent is disadvantageorrs because primarily the Itydration is slowed down (the surface of cement grains becomes hydrophobic), and secondly too high air content is introdrrced to the paste. 

The term impregnation is sometimes wrongly used, to define the addition of water repellents (sueh as oleie, lauryl, stearie aeids or their salts added as about 0.5 % by mass), whieh are adsorbed on the surface of cement grains. This cement is hence hydrophobic and can be exposed to the humid air, e.g. during transport or storage, with no detrimental effects. 

Taking into account their stoichiometry, both reactions are similar and produce the same products in different proportions [30, 65, 66]. Hydrated calcium silicate (3Ca0.2Si0, AY[ O) is the principal hydration product and is the component that provides resistance to cement matrix and is formed on the surface of cement particles. The calcium hydroxide (Ca(OH)2) is responsible for the pH 12 of the cement matrix and is critical in protecting the steel reinforcement against corrosion however it deg-radates the hemicellulose and lignin of natural fibers decreasing the properties of the composite material. 

Bonding EPDM membrane with a cold adhesive is appropriate when structures are too light or roofs are steep with curves or sharp corners. Before application of the adhesive, the surface should be made smooth, clean, free from dust, oil, grease or other contaminates which may affect the adherence. This method is economical to cover all types of complex structures even on vertical walls. The sheets can be adhered to smooth surfaces of cement concrete, wood and metallic surfaces made of steel, aluminium, etc. An adhesive bonding system is also suitable for use on rigid insulation and old concrete roofing. 

It is essential that the concrete surface be properly prepared prior to application of the coating. The surfaces of cement containing materials may contain defects that require repair before the application of the coating. In general, the surface must be thoroughly cleaned and all cracks repaired. 

Fig. 16. ESEM image and EDS analysis of surface of cement paste...

Because the active surface of cement is increased, higher early and final strength of concrete is obtained as well as better durability. 

The irregular shape of cracks and fracture surfaces of cement-based composites is at the origin of the question as to whether they may be considered as fractal objects. The application of fractal analysis should be aimed at a better understanding of the fracture mechanics of these materials, by possible quantification of the fracture surface roughness, length and surface of cracks and all other irregular elements that may characterize the materials and their behaviour. 

Winslow, D. N. (1985) The fractal nature of the surface of cement paste, Cement and Concrete Research, 15(5) 817-24. 

The specific surface of cement in cm /g is calculated from the air permeability of a bed of cement, its porosity, the density of the cement and the viscosity of air. The measure of the permeability is the time it takes for a certain quantity of air to flow through the bed under specified conditions" (DIN 1164, Part4). 

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