Setting and Hardening of Cement

In order to accelerate or retard the setting of cement, water-soluble admixtures may be used (see Section 5.3.1). Whilst many inorganic salts, such as calcium chloride, can act as accelerators, lead and zinc salts, borates and phosphates, as well as hydrocarbonic acids (e.g., citric acid or tartaric acid) and, most importantly, sugar derivatives such as raffinose or sucrose, will tend to retard the setting process. As the rate of setting can be determined by recording the heat evolved during hydration, a plot of (in Wkg ) versus 1/t will yield a close to linear relationship. 

This ranking pertains to chlorides and nitrates. For anions associated with calcium the ranking is  

As the rate of hydration of clinker grains appears to be determined by the rate of diffusion of water molecules through the colloidal C-S-H gel layer, the rate of reaction is a function of the permeability and cohesion of the layer. 

During the setting of a cement, a large amount of added water will be used to form hydrate phases such as C-S-H, portlandite, and AFt and AFm phases. Yet, even after complete hydration some free water will remain confined within the porous C-S-H gel and the capillary pore spaces. Notably, this occurs when surplus water is added so that the water/cement (W/C) ratio exceeds 0.6. A lack of water (W/C raho 0.2) results in an incomplete hydration of clinker minerals. The optimum W/C raho for Portland cement, leading to maximum strength, is 0.3 W/C 0.5. 

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Part 1 Chemieal analysis (a) chloride (b) aeid soluble sulfate (c) impurities affecting setting and hardening of cement (d) impurities that affect surface finish (e) water solubility, (f) loss on ignition (g) slag unsoundness (h) free lime (i) fulvo acid test (j) sodium hydroxide test . 

Table 4.3 The admixtures retarding setting and hardening of cement paste Group of Compounds Percentage by mass Effects compounds of cement, %...

The formation of compounds with cementing properties, just like the chemical reactions causing setting and hardening of cements, may be expressed by chemical equations. Here also, either conventional or abbreviated chemical formulas may be... 

Fig. 20.3. The setting and hardening of Portland cement. At the start (a) cement grains ore mixed with water, H. After 15 minutes (b) the setting reaction gives a weak bond. Real strength comes with the hardening reaction ( ), which takes some days.

A.A. Mirtchi, J. Lemaitre, E. Munting, Calcium phosphate cements Effect of fluorides on the setting and hardening of beta-tricalcium phosphate-dicalcium phosphate-calcite cements. Biomaterials 12 (1991) 505-510. 

Water used for concreting must be free of salts or impurities that can interfere with setting and hardening of the cement paste or negatively affect concrete properties. 

FIGURE 2.10 Heat evolution during the setting and hardening of Portland cement. 

In Chapter 2 we mentioned cement and the reactions that occur during the setting and hardening of this material. There is a class of cements known as calcium aluminate cements (CACs) or high-alumina cements (HACs). These ceramics are not used as widely as Portland cement, but their attraction is the rapid hardening reactions. In 1 day CAC achieves the same strength as Portland cement achieves in a month. 

Materials or constituents that react spontaneously with water, causing setting and hardening of the original mix, are considered to be hydraulicaUy reactive. They exhibit cementing or hydraulic properties. 

Kovler, K. (1998) Setting and hardening of gypsum— Portland cement—silica fume blends. Cement and Concrete Research 28,423-438. 

Zinc phosphate cement consists of finely powdered zinc oxide suspended in phosphoric acid. The setting and hardening of this cement results from a chemical reaction between these two constiments, in which zinc phosphate tetrahydrate is formed as the product of reaction ... 

Period of massive precipitation ofreaction products. In this stage the nucleation barrier is overcome, and a massive precipitation of the reaction products gets under way. The concentration of Cl in the liquid phase declines rapidly. At the same time the concentration of Mg + does not change significantly, as additional amounts of MgO dissolve, to supply the solution with Mg. A setting and hardening of the cement paste takes place. The overall rate of the reaction is controlled by the rate of MgO dissolution. At this state of hydration the process is accompanied by the release of distinct amounts of lydration heat. In mixes in which two different hydrates are formed successively, two distinct thermal peaks may be observed. 

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]. 

On the basis of these observations we applied LRC to the setting of hydraulic mortars. The LRC-active salt was dissolved in water which was then mixed with the powdered mortar. Unfortunately, Portland-type cements could not be examined because this mortar gives a strongly alkaline mixture which leads to a facile decomposition of the emitter [4], In distinction, the setting and hardening of gypsum plaster takes place in a neutral medium. 

Some indication of the respectively contributions of the clinker phases to the strength development of cement is given in Fig. 9. However, these results obtained for individual phases cannot be directly applied to the conditions actually occurring in cement paste, as is apparent also from the heat of hydration values given in Table 10. Fig. 17 schematically shows the sequence of formation of the hydrate phases and the structure development in the setting and hardening of Portland cement. 

Surveys of other proposed uses can be found in references (5, 6). Not included there is its use as an inhibitor for the setting and hardening of Portland cement (39). 

The setting and hardening of concrete are accelerated in the presence of calcium chloride. These are related to the effect of CaCl2 on the rate of hydration of cement in concrete. The hydration of cement is exothermic, resulting in the production of heat, and if the heat is produced at a faster rate, larger amounts of the hydrates are formed at earlier times in the presence of accelerators. This is particularly significant in the first 10-12 hours. The influence of different amounts of calcium chloride on the rate of heat development is depicted by conduction calorimetric curves (Fig. The position of the peak corresponding to the... 

Kovler, K., Setting and Hardening of Gypsnm-Portland Cement-Silica Blends, Part 2 Early Strength, DTA, XRD and SEM Observations, Cement Conor. Res., 28 523-531(1998)... 

Several different constituents are found in Portland cement, the principal ones being tricalcium silicate (3Ca0-Si02) and dicalcium silicate (2Ca0-Si02). The setting and hardening of this material result from relatively complicated hydration reactions... 

Frieficements (83), manufactured in Belgium, are produced as a wet slurry of finely ground slag. When activators such as Pordand cement, lime, or sodium hydroxide are added in a concrete mixer, the slurry sets and hardens to produce concretes with good strength and durabiUty. 

In what way would you expect the setting and hardening reactions in cement paste to change with temperature Indicate the practical significance of your result. 

Figure 5.11 (Crisp Wilson, 1974b) shows the time-dependent variation of the concentration of soluble ions in setting and hardening cements. Note that the concentrations of aluminium, calcium and fluoride rise to maxima as they are released from the glass. After the maximum is reached the concentration of soluble ions decreases as they are precipitated. Note that this process is much more rapid for calcium than for aluminium and the sharp decline in soluble calcium corresponds to gelation. This indication is supported by information from infrared spectroscopy which showed that gelation (initial set) was caused by the precipitation of calcium polyacrylate. This finding was later confirmed by Nicholson et al. (1988b) who, using Fourier transform infrared spectroscopy (FTIR), found that calcium polyacrylate could be detected in the cement paste within one minute of mixing the cement. There was no evidence for the formation of any aluminium polyacrylate within nine minutes and substantial amounts are not formed for about one hour (Crisp et al, 1974).

Figure 5.11 The time-dependent variation, in setting and hardening cements, of the concentration of soluble ions Al ", Ca, F and PO (expressed as P Oj). These ions are released from the glass powder into the cement matrix (Crisp Wilson, 1974b).

Consistency, working time, setting time and hardening of an AB cement can be assessed only imperfectly in the laboratory. These properties are important to the clinician but are very difficult to define in terms of laboratory tests. The consistency or workability of a cement paste relates to internal forces of cohesion, represented by the yield stress, rather than to viscosity, since cements behave as plastic bodies and not as Newtonian liquids. The optimum stiffness or consistency required of a cement paste depends upon its application. 

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