Cement paste Particles

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. 

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 rheological characteristics of cement pastes are related to the nature of the attractive and repulsive forces which exist between cement and cement hydration product particles and can be categorized as follows ... 

Thus in normal cement pastes where particles come into close contact with each other there is a tendency for cement pastes to form large floes due to the attractive van der Waals forces holding particles together. 

A study of the foaming capacities and stabilities [10] of a variety of air-entraining agents in a solution of cement extracts showed that commonly used anionic air-entraining agents, such as sodium dodecyl sulfate and sodium resinate (1) were visually precipitated from solution, (2) retained their ability to form stable foams after precipitation with only minor amounts of admixture left in solution, and (3) lost the major part of their ability to form stable foams after filtration. It was further shown from studies in cement pastes firstly that the admixture should be adsorbed on the solid particles of the paste with the non-polar ends of the molecule pointed towards the water phase, imparting a hydrophobic character to the cement... 

Air-entraining agents are predominantly anionic surfactants which, on addition to cement pastes, are adsorbed on to the cement particles with their polar groups orientated towards the particles. This sheath is of limited solubility and only a minor, but finite, proportion remains in solution as the calcium salt. 

C. additional superfine particles in the cement paste. These are materials consisting of acrylic emulsions and aqueous clay dispersions. 

Supercritical C02 treatment affects the microstructure of the cement paste. In the first stage of the sc C02 treatment, free water in the cement pores is extracted. As a consequence of this dehydration process, channels of about 50-pm diameter develop. Dissolved calcium in the free water reacts with the C02 and crystallizes with the C02 as calcite along the channel walls. In the second stage, the structural water of the hydrated cement phases is extracted. The carbonation of the portlandite to form more calcite takes place. Water, bound to the CSH surrounding the partially hydrated cement clinker particles, is partially replaced by a carbonate formation. The short fibers of the CSH-cement framework, which are responsible for the physical properties of the cement, are not affected (Hartmann et al., 1999). 

Fig. 5.6 shows the XRD powder pattern of a 23-year-old paste of (f-CjS. Patterns of fully reacted CjS pastes are similar, except that the CH peaks are relatively more intense. The only effects definitely attributable to C-S-H are the diffuse peak at 0.27-0.31 nm and the somewhat sharper one at 0.182 nm. Attempts to obtain selected area electron diffraction patterns from the C-S- H of calcium silicate or cement pastes have usually failed, but, occasionally,. particles present in ground and redispersed samples have yielded poorly defined patterns (G41,C25) (Section 5.4.6). A later study by this method (M48) has been severely, and in the writer s opinion justifiably, criticized (G45).. 1... 

Studies of ground and redispersed cement pastes by analytical electron microscopy (L28-L30,T17) showed wide variations between individual analyses, even within single particles of micrometre dimensions, and gave mean Ca/Si ratios of 1.5-2.0 for the C-S-H. The mean ratios of minor elements... 

In the rate equations relating a to / for a single particle, the (irst did not involve r. In the second, a function of a was proportional to (/ - / )/r, where / is the time at which the process became rate determining. In the third, a function of a was proportional to (/ — to)/r. Kinetics represented by equations of the second and third of these types are described as linear and parabolic, respectively. It was shown that the kinetic curves of a number of alite and cement pastes, some of which contained added alkali sulphates, could be satisfactorily explained (BIOS). For the cements, diffision became virtually the sole rate-controlling process at values of a varying between about 30% and 60%. This appears to agree broadly with the evidence from apparent energies of activation noted in the previous section. 

To an extent that increases with the w/c ratio, fresh cement pastes exhibit the phenomenon of bleeding, i.e. settlement of the solid particles. The interparticle attractions are sufficiently strong that particles of all sizes settle at the same rate, typically about 2pms . Settlement also tends to increase the w/c ratio at the top and to decrease it at the bottom of the sample. It decreases with increased fineness or increased early hydration rate of the cement. In a concrete, it can produce layers of water beneath aggregate particles or reinforcing bars. 

Brunauer and co-workers (B55,BI08) considered that the gel particles of the Powers-Brownyard model consisted of either two or three layers of C S-H, which could roll into fibres. D-drying caused irreversible loss of interlayer water, and the specific surface area could be calculated from water vapour sorption isotherms, which gave values in the region of 200m g for cement paste. Sorption isotherms using N2 give lower values of the specific surface area this was attributed to failure of this sorbate to enter all the pore spaces. 

The hydrated material has been analysed by X-ray microanalysis and analytical electron microscopy. In a 3-day old paste, that formed in situ from alite or belite did not differ significantly in composition from the corresponding product in pure Portland cement pastes (H4). but at later ages Ca/ Si is lower and Al/Ca higher (R25,R26,T44,U 17,U 18,R42). Ca/Si is typically about 1.55, but the value decreases with age and ratio of pfa to clinker. Uchikawa (U20,U17) reported a value of 1.01 for a 4-year-old paste with 40% replacement of cement by pfa. Several of the studies (R25,T44,U20,U 17) showed that the C-S-H was higher in alkalis if pfa was present, but one cannot tell to what extent potassium or sodium apparently present in the C-S-H has been deposited from the pore solution on drying. For material close to the pfa particles in a 10-year-old mortar. Sato and Furuhashi (S92) found a Ca/Si ratio of 1.1-1.2. 

Halse and Pratt (H57) reported SEM observations on pastes hydrated at various temperatures. In those hydrated at 8°C or 23 C, the main feature was fibrous material that was considered to be hydrous alumina, but which could also have been partly dehydrated CAH,q. The hydrating grains of cement were surrounded by shells of hydration products, from w hich they tended to become separated in a manner similar to that observed with Portland cement pastes (Section 7.4.2) though the authors recognized that this could have been partly due to dehydration. Two-day-old pastes hydrated at 40"C showed spheroidal particles of CjAH and thin, flaky plates of gibbsite. In pastes mixed with sea water, hydration took place more slowly, but no other effects on microstructural development were observed. 

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