Hardened cement paste

Cement and Concrete Concrete is an aggregate of inert reinforcing particles in an amorphous matrix of hardened cement paste. Concrete made of portland cement has limited resistance to acids and bases and will fail mechanically following absorption of crystalforming solutions such as brines and various organics. Concretes made of corrosion-resistant cements (such as calcium aluminate) can be selected for specific chemical exposures. 

Fig. 20.5. Concrete is a particulate composite of aggregate (60% by volume) in a matrix of hardened cement paste.

The most important characteristic of cement is its pore structure and aqueous phase hence, the microstructure of the hardened cement paste via the pore system. It is highly alkaline (pH >13) due to rapid and almost quantitative dissolution of Na and K salts from the cement clinker. The porosity of the paste comprises interconnected and isolated pores, the pore sizes of which are important to the strength and dimensional stability of cement products. Different types of cement are used to meet different performance criteria. Properties can be estimated from compositions and fineness (i.e., particle size and size distribution). In the past, additives... 

The graphs given in Fig. 1.38 show the logarithmic relationship between the water-cement ratio and the permeability coefficient of hardened cement paste. Thus concrete with a paste water-cement ratio of 0.4 will be almost impermeable. Water-reducing agents can be used to reduce the water- cement ratio, so ensuring that the permeability is kept to a minimum. 

A typical distribution of pore radii in the hardened cement paste of concrete was shown in Fig. 1.40 which indicated that the majority of pores lie in the region of 0.05 and 1.0 pm diameter and it is through these pores that water passes by applied pressure or capillary rise, as shown in Fig. 4.5(a). 

BONHOWE, I., WlELAND, E., SHEIDEGGER, A. M., Ochs, M. Kunz, D. 2003. EXAFS study of Sn(IV) immobilization by hardened cement paste and calcium silicate hydrates. Environmental Science and Technology, 37, 2184-2191. 

Richardson, I. G. Groves, G. W. 1993. The incorporation of minor and trace elements into calcium silicate hydrate (C-S-H) gel in hardened cement pastes. Cement and Concrete Research, 23, 131-138. 

The type of data produced by H O and N adsorption is relevant to gel pores having radii up to about 50 A,2but larger pores and capillaries exist in the hardened cement paste and probably are more significant in determining the porosity or permeability of the hardened paste in concrete to gases and liquids. 

C-S-H = poorly crystalline or amorphous calcium silicate hydrate of unspecified composition. Ggbfs = ground granulated blast furnace slag. Hep = hardened cement paste. Pfa = pulverised fuel ash (fly ash). 

Jambor (42) has published a damage function describing sulfate attack in terms of the percent of SO3 bound in hardened cement paste, which he reports as being the prime cause of sulfate corrosion. The damage function (DC) which is expressed as... 

Cardenas HE, Struble, LJ. (2006). Electrokinetic nanoparticle treatment of hardened cement paste for reduction of permeability. Journal of Materials in Civil Engineering Jul/Aug 546-560. 

The cement paste formed by the hydration reactions always contains interconnected pores of different sizes, as shown in Figure 1.2. The pores can be divided into macropores, capillary pores and gel pores. The interlayer spacing within C-S-H gel pores) have a volume equal to about 28 % of the gel and dimensions ranging from a few fractions of a nm to several nm. These do not affect the durability of concrete and its protection of the reinforcement because they are too small to allow significant transport of aggressive species. The capillary pores are the voids not filled by the solid products of hydration of hardened cement paste. They have dimensions of 10 to 50 nm if the cement paste is well hydrated and produced... 

In cements containing pozzolanic additions, the Ume needed to react with pozzolana is provided by the hydration of Portland cement. The hardened cement paste (compared to that obtained with ordinary Portland cement) has a lower lime content and higher content of C-S-H. The amount of pozzolanic addition to Portland cement generally ranges from 20 to 40 % of the total cement content it should be adjusted to the amount of lime produced in the hydration of Portland cement. Any excess of the pozzolanic addition wiU not react and thus will behave as an inert addition. 

J. Tritthart, Chloride binding in cement - II. The influence of the hydroxide concentration in the pore solution of hardened cement paste on chloride binding . Cement and Concrete Research, 1989, 19,... 

D. Btirchler, B. Elsener, H. Bohni, Electrical resistivity and dielectrical properties of hardened cement paste and mortar , in Corrosion of Rein-... 

C. L. Page, N. R. Short, A. El Tarras, Diffusion of chloride ions in hardened cement paste . Cement and Concrete Research, 1981, 11, 395-406. 

R. Beddoe, M. J. Setzer, Phase transformations of water in hardened cement paste. A low temperature DSC investigation . Cement and Concrete Research, 1990, 20, [13 ... 

CO2 (molecular weight 44) that can react with a concrete produced with 300 kg/m Portland cement that we can suppose is composed by 64% of CaO (molecular weight 56) is 300 X 0.64 X 44/56 150 kg/m. In the case of blast furnace slag cement with 70 % of GGBS, the percentage of CaO is only 44%. For other blended cements, the quantity of CaO is somewhere between these two values [3]. For blended cement, hydration of pozzolanic materials or GGBS also leads to a lower Ca(OH)2 content in the hardened cement paste which may increase the carbonation... 

Relatively simple methods based on spraying the surface of split cores with silver nitrate have been proposed a colour change indicates the chloride penetration front [48]. The result may depend on the original colour of the concrete and its particular chemistry. More complex tests are based on cutting slices from cores and pressing out the pore solution for determination of the free chloride concentration this techniques was developed for hardened cement paste [49] but is also used for concrete [50]. 

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