Hardening Portland cement

When anhydrous cement mix is added to water, the silicates react, forming hydrates and calcium hydroxide. Hardened Portland cement contains about 70% cross-linked calcium silicate hydrate and 20% crystalline calcium hydroxide. 

Hardened Portland cement contains a distribution of pore and capillary sizes, depending on the initial water-cement ratio and the maturity of the paste. 

Hardened Portland cement contains a distribution of pore and capillary sizes, depending on the initial water-cement ratio and the maturity of the paste. 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 he 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). 

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. 

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

The most important contributor to the strength of hardened Portland cement is tricalcium silicate, (Ca0)3-Si02, for which the measured standard enthalpy of formation is — 2929.2 kj moUT Calculate the standard enthalpy change for the production of 1.00 mol tricalcium silicate from quartz and lime. 

T. C. Powers, Structure and physical properties of hardened Portland cement paste . Journal of American Ceramic Society, 1958, 41, 1-6. 

Under normal conditions on repair sites it seems likely that a more reliable bond can be achieved by using a bond coat rather than by relying on any natural bond between fresh and hardened Portland cement concretes(2). However, bonding agents for use on site must be tolerant of site conditions, be reliable and cannot be selected solely on the basis of slant shear results. The bonding performance of carefully applied SBR and acrylic latex/cement slurry coats appears to be similar to that of epoxy resin bonding aids(14). However, other factors such as open time, barrier coat effects and cost may affect the final choice. 

An essential characteristic of any hardened Portland cement paste is the presence of pores. Their radius may vaiy widely between about 1 and 1000 ran. The overall porosity of the hardened paste declines as the hydration progresses, and increases with increasing water/cement ratio. 

Table 13.1 Properties of hardened Portland cement pastes modified with a butadiene-styrene copolymer dispersion.

From among different inorganic binders, the resistance to chemical attack of Portland cement—the binder most widely used in concrete constmction—has been studied the most extensively. It has been found that hardened Portland cement pastes perform well in many environments, bnt there are applications in which the use of other binders is more appropriate. In this chapter the action of the most common chemical agents on selected cementitions systems and the response of them to the chemical attack will be discussed. 

In hardened Portland cement pastes alkali hydroxides are the species most loosely bound. They enter the liquid phase that is in contact with the paste surface first, followed by calcium hydroxide. In percolation experiments the alkali ion concentration in the liquid phase started to deeline after reaching a maximum, whereas the calcium ion concentration increased to a value that remained constant for the rest of the experiment (Unsworthetfl/., 1997). 

Solutions of alkali hydroxides generally do not exhibit a corrosive action towards pastes of hardened Portland cement and related binders. Only at very high concentrations may a moderate corrosion become apparent, probably owing to degradation of the hydrated aluminate phases. Also highly resistant to alkaline solution are alkali-activated slag binders (see section 8.5). In contrast, hydrated calcinm aluminate cement may be attacked by high-pH solutions. 

If hardened Portland cement paste or concrete is exposed to a solution of artrmorrirtm sulfate, this compormd decomposes in the highly alkaline envirorrment of the cement paste, and gaseous armnorria is liberated ... 

The way in which organic compounds are bound within the hardened Portland cement paste, and their effect on hydration and the stmcture of the Itydrates formed, may vary greatly. Compounds that are insoluble in water usually have no effect, whereas those that are at least partially soluble may or may not slow down the rate of hydration, alter the stmcture of the hydrated paste, and decrease its strength. Compounds that exhibit an adverse effect on Portland cement hydration include various phenols, chlorophenols, and ethylene glycol, whereas metlianol or ethanol have almost no effect (Gmtzek, 1992). 

It has been reported that the immobilization of waste in hardened Portland cement may be improved by adding sodium silicate to the system, as this additive accelerates the hydration reaction and lowers the amount of portlandite in the hydrated material (Scheetz and Hoffer, 1995). 

Sodium polyphosphate solutions when brought into contact with set and hardened portland cement concrete will cause severe disintegration. On the other hand, sodium phosphorofluoridate, NaPOsF, when added to the mixing water for concrete, will inhibit the corrosion of steel reinforcement [39]. 

Work at Courtaulds [22,23] in the early 1970s attempted to incorporate carbon fiber in a cement slurry, which was difficult due to the size of the cement particles. They tended to be filtered out by the fiber reinforcement, so a cement with a fine particle size (Swiftcrete, an ultra rapid hardening Portland cement with a maximum diameter of about 45 pm) was used and the fiber spread as thinly as possible, using either an air knife, or a water flume and then held in the spread position by sizing with a water based compatible size such as sodium carboxymethylcellulose [22,23]. These larger particles limit the carbon fiber content to about 5% v/v, but in practice, due to a non-uniform distribution, a value of some 12% v/v was attainable. 

Polymer concrete is a composite material formed by polymerising a monomer and aggregate mixture. There is no other cementitious material present in it. PPCC (or LMC) is a Portland cement concrete produced usually by replacing a specified portion of the mixing water with a latex (polymer emulsion). It can also be produced by adding a monomer to fresh concrete with subsequent in situ curing and polymerisation. PIC is a hardened Portland cement concrete with impregnated monomer which is polymerised in situ. 

Hardened portland cement concrete with impregnated monomer which is polymerised in situ. 

Type III Rapid-hardening Portland cement (RHPC) 52 Used when high strength is required after a short period of curing. 

Knhl Cement. A hydraulic cement introduced by H. Kuhl (Brit. Pat., 231,535 31/3/25). It contains less Si02 but more AI2O3 and Fe20 (about 7% of each) than does portland cement its strength properties are similar to those of rapid-hardening portland cement. 

Ordinary Portland cement Rapid hardening Portland cement... 

Concrete hardens as a result of chemical reaction between Portland cement and water. The hardened Portland cement is called cement paste. As the volume of the hydration products is greater than the volume of concrete grain, the accumulation of hydration products create a space filling effect. The paste contains two different forms of pores - capillary pores and gel pores. 

The compressive strengths for Reg Set cement during the first 24 hours are higher than those for ultra-rapid hardening portland cement. The strength characteristics of the cements are similar beyond 24 hours. 

Problem. In their paper Studies of the Physical Properties of Hardened Portland Cement (J.of ACI, Vol 18, No.3), T.C.Powers T.L.Brownyard mention an investigation of the heat development properties of cement using solution calorimetry. The paper specifies, among other things, the solution heat of a cement paste that is fuUy dissolved in an acid. 

Powers, T.C. Brownyard, T.L. Studies of the Physical Properties of Hardened Portland Cement Paste, Journal of ACI, Vol. 18, No.3 1946. 

Since mercury porosimetry has been used to a considerable extent in the study of hardened Portland cement, it seemed appropriate to apply it in the assessment of porosity of Ancient Egyptian Mortars. X-ray diffraction and simultaneous thermograviraetry and differential thermal analysis were used as additional techniques in the present study. 

Figure 3.12 Correlated values of water-cement ratio and sample size not causing an apparent change In kinetics for degree of hydration a < 0.85 of rapid-hardening Portland cement at 20°C. The solid line indicates the recommended combination of minimum w/c and maximum sample height to be used (Geiker 1983).

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