Portland cement paste

C. W. Lent2, in Special Keport90, Structure of Portland Cement Paste and Concrete Highway Research Board, NRC-NAS, Washington, D.C., 1966. 

Fig. 5.7 Effect of various calcium salts on the time of setting of Portland cement pastes.

In order for normal set to occur in Portland cement paste, mortar, or concrete, calcium sulfate must be present in the cement-water system. In today s cements, most of the calcium sulfate introduced into the system as a component of the cement, can be present in one or more forms gypsum (CaS04.2H20), hemihydrate (CaS04.1/2... 

Tishmack, J. K. 1999. Characterization of High-Calcium Fly Ash and Its Influence on Ettringite Formation in Portland Cement Pastes. PhD dissertation, School of Civil Engineering, Purdue University, West Lafayette, IN. 

Portland cement pastes. Influence of composition on volume constancy and salt resistance. Ind. Eng. Chem., 26 1049-1060. 

Previous investigations of these hydration reactions at room temperature have been reviewed recently (4). Research in this laboratory has included the stoichiometry of the hydration of both silicates, employing different methods of hydration (2, 3, 5, 21), and a determination of the surface energy of tobermorite, the calcium silicate hydrate produced in the hydration of both silicates under most experimental conditions (8). The surface area and the surface energy of tobermorite are briefly discussed by Brunauer (I). These properties play vital roles in determining the strength, dimensional stability, and other important engineering properties of hardened portland cement paste, concrete, and mortar. 

Fig. 5.4 (A.B) Types I and II C-S-H, respectively (SEM of fracture surfaces courtesy K. L. Scrivener). (C,D) SEM/STEM pair of ion beam thinned section, showing Type III C-S-H (top, right) and Type IV C-S-H (top, left and bottom, right Jennings et al. (HO)). (A) is of an ordinary Portland cement paste, w/c = 0.5, aged 10 h. (B) is of a paste of an oil well cement, w/c = 0.44, with 2.4% of CaClj on the weight of cement, aged 1 day. (C) and (D) are of a CjS paste, w c = 0.47, aged 330 days.

Fig. 7.2 Backscattered electron image of a mature Portland cement paste, aged 2 months. Successively darker areas are of unreacted cement grains (bright), sometimes with visible rims of hydration products, CafOH), other ( undesignated ) regions of hydration products, and pores (black). Scrivener and Pratt (S28).

Fig. 7.3 QXDA results for the fractions of the clinker phases reacted in Portland cement pastes. Filled circles Copeland and Kantro (C39), w/c = 0.65. Diamonds Bezjak et al. (B98), sample C2. Vertical lines Osbaeck and Jons (08), range for 7 samples. Open circles Dalziel and Gutteridge (DI2). Open squares Patel et al. (P28), samples cured at 100% RH. Filled squares Tang and Gartner (T34), clinker interground with gypsum.

Fig. 7.4 TG curve for a Portland cement paste (w/c = 0.5), moist cured at 25"C for 14 months full curve, with points calculated as described in the text shown by open circles. Heating rate, lOdegCmin , flow rate of dry, COj-free Nj, I5mlmin sample weight, 50 mg. Taylor (T5).

Table 7.1 Results of X-ray microanalyses by EPMA or SEM of the gel formed in situ from alite or belite in Portland cement pastes"... 

Fig. 7.5 Al/Ca atom ratios plotted against Si-Ca atom ratios for individual X-ray microanalyscs of typical Portland cement pastes. Modified from Harrisson el al. (H4).

Silicate anion structures in Portland cement pastes have been studied by the methods described in Section 5.3.2 for calcium silicate pastes. Trimethylsily- i lation (TMS) studies (L20,T12,S69,T36,L31,M43,M44) show that, as with C,S. the proportion of the silicon present as monomer decreases with age and that the hydration products contain dimer, which is later accompanied and eventually partly replaced by polymer (>5Si). Some results have i indicated that fully hydrated pastes of cement differ from those of CjS in that substantial proportions of the silicate occur as monomer (S69,L31), but the results of a study in which pastes of CjS, P-CjS and cement were compared (M44) suggest that the differences between the anion structures of cement and CjS pastes are probably within the considerable experimental errors inherent in the method. The recovery of monomer from unhydrated P-CjS was only 66% and results for cement pastes can only be considered semiquantitative. 

Copeland et al. (C39,C25) treated CjS pastes, and C-S-H prepared in other ways, with various sources of AP, Fe or S04 ions. The XRD peaks of the phases providing the ions disappeared, and changes occurred in the micromorphology of the gels and in the non-evaporable water contents and fractions of the CaO extractable by an organic solvent. It was concluded that the ions were taken up by the C-S-H and that up to about one silicon atom in six could be replaced by aluminium, iron or sulphur, and that aluminium and iron could also replace calcium. These results suggested that the principal hydration product in Portland cement pastes was a substituted C-S-H, a conclusion that later appeared to be supported by the results of the... 

Fig. 7.8 Concentrations in the pore solution (scaled as indicated in the cases of OH and SiOj) of a Portland cement paste of w/c ratio 0.5. After Lawrence (L. 2).

The rates of reaction of the clinker phases are greatly influenced by the RH of the atmosphere in which curing occurs. For a typical Portland cement paste of w/c ratio 0.59 cured at 20°C and 100% RH, Patel el al. (P28) found the fractions of the alite, belite, aluminate and ferrite phases hydrated after 90 days to be respectively 0.94, 0.85, 1.00 and 0.51. If the RH was lowered to 80%, the corresponding values were 0.77, 0.19, 0.83 and 0.32. The hydration rate of the belite thus appears to be especially sensitive to RH. On the basis of earlier data from the literature, Parrott and Killoh (P30) concluded that the effect of RH on the hydration rate (da/d/) of each of the phases could be represented by a factor (RH — 0.55)/0.45. ... 

Temperature has a large effect, especially in the earlier stages of hydration for example, from QXDA, Copeland and Kantro (C39) found that in a Portland cement paste of w/c = 0.57, the fraction of the alite hydrated at 2 days was 0.28 at 5°C, 0.63 at 25°C and 0.81 at 50°C. Apparent energies of activation calculated from such data were 41 kJ moC at a = 0.6 and 26 kJ mol " at a = 0.7 for belite, a value of 56 kJ mol at a = 0.4 was obtained. The decrease in the apparent energy of activation in the case of alite was attributed to a gradual change in rate control from a chemical process to diffusion. 

Structure and properties of fresh and hardened Portland cement pastes... 

The workability of a concrete mix is by no means dependent only on the physical properties of the cement paste it contains, but an understanding of it requires one of those properties. For some specialist uses in which cement is used without an aggregate, the latter are directly relevant. The most important properties are concerned with rheology, and this section deals primarily with these properties in Portland cement pastes, free from admixtures, prior to setting. From the chemical standpoint, this period comprises that of initial reaction and induction period. From the practical standpoint, it includes those of mixing, placing and compaction. 

The content of non-evaporable water, relative to that in a fully hydrated paste of the same cement, was used as a measure of the degree of hydration. Portland cement paste takes up additional water during wet curing, so that its total water content in a saturated, surface dry condition exceeds the initial w/c ratio. Evidence from water vapour sorption isotherms indicated that the properties of the hydration product that were treated by the model were substantially independent of w/c and degree of hydration, and only slightly dependent on the characteristics of the individual cement. The hydration product was thus considered to have a fixed content of non-evaporable water and a fixed volume fraction, around 0.28, of gel pores. 

Fig. 8.3 Diagram illustrating the Powers-Brownyard description of a fully hydrated and saturated Portland cement paste of initial water/cement ratio 0.5. All quantities refer to 1 kg of cement.

Fig. 8.4 Feldman-Sereda model of the structure of the C-S-H gel of Portland cement paste, showing C-S-H layers (lines), interlayer water molecules (crosses) and adsorbed water molecules (circles). After Ramachandran el al. (R32).

Fig. 8.5 Relations between porosities (volume percentages) and water/ccmcnt ratio for mature Portland cement pastes. The experimental data are for pastes at least 8 months old, and the calculated curves relate to a typical cement aged 18 months. Open symbols total water porosities. Filled or half-filled symbols mercury porosities. Curve A total water porosity. Curve B free water porosity. Curve C capillary porosity. References to data O (P20) O (S77) A (F33) V (M68) (S78) (F34) 9 (019) (M68) (D3I) 3 (H4I). In the last two cases, porosities by volume were estimated from data referred in the original sources to masses of dried paste, assuming the tatter to have contained 0.23 kg of water per kg of cement having a specific volume of 3.17 x 10 m kg h...

Studies on other materials show that MIP determines the width distribution of pore entrances and not of the pores themselves (D34). The intrusion of mercury may also coarsen the pore structure this need only imply that, at the higher pressures employed, some of the foils of the gel are displaced so that some pores are widened and entered while neighbouring ones are closed up. The combined result of these processes would be to produce a distribution narrower than that existing before the intrusion began, and a value for the porosity at maximum pressure that corresponded to a minimum pore width before intrusion of less than 3.5 nm. Experiments in which the mercury was removed and subsequently reintruded have indicated that the structure is usually not altered in the case of Portland cement pastes, though it is in that of pastes of composite cements (F35,D32), but cannot show whether an irreversible change occurred during the first intrusion. 

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