Accelerating admixtures

Thermal analysis techniques have been applied widely for the investigation of the role of admixtures, espeeially that related to the hydration of cement and cement components. AppUcation of thermal analysis permits determination of the heat of reaction, mechanism of reaction, kinetics of reactions, compatibility of admixtures with cements, prediction of some properties, durability problems, material characterization and selection, development of new admixtures, quick assessment of some physical properties, etc. In some instances, they 5deld results that are not possible to obtain with the use of other teehniques. 

In this chapter, typical examples of the application of various thermal techniques to the study of the effect of admixtures on the hydration of cement and cement compounds is emphasized. Where relevant, the results obtained with these techniques are compared with those derived from other tools. 

Collepardi, et al., studied the hydrates formed during the hydration of C3S in the presence of calcium chloride.In an autoclaved sample containing CaCl2, they detected an exothermic peak in the thermogram that was attributed to the chemisorption of chloride on the C-S-H surface and its penetration into the molecular layers of C-S-H. 

Compared to the extensive thermal investigations of C3S hydration in the presence of CaCl2, only a limited amount of work has been reported on the effect of calcium chloride on the hydration of C2S. Calcium chloride accelerates the hydration of dicalcium silicate. In the thermograms of C2S paste hydrated for 1-3 months, lower amounts of calcium hydroxide are formed in the presence of calcium chloride, compared to that hydrated 

The reaction of C3A with calcium chloride results in the formation of high and low forms of tricalcium chloroaluminates. Under normal conditions of hydration, the low form, viz., C3A CaCl2 XH20 is obtained. The DTA technique may be used to differentiate between the two forms. Endothermal effects at about 190 and 3 50°C are caused by the low form and the endotherm at about 160°C is exhibited by the high form. In the system C3A-Ca0-CaCl2-H20, at higher concentrations of calcium chloride, calcium hydroxychloride is formed that is identified by peaks at 130,145, and 485°C.[ ] 

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It is obtained either as a liquor or as flake material of approximately 20% moisture content, and for use as an accelerating admixture is normally supplied as a 33-35% solution. 

It is obtained as a fine powder and is supplied normally in this form as an accelerating admixture because of its limited solubility in water (about 15% at normal room temperature). 

It has already been shown that the presence of accelerating admixtures produces hydration products of a different type to those from a plain cement paste because of chemical involvement, predominantly with the C3A phase... 

Accelerating admixtures based on calcium chloride, formate, nitrate, and thiocyanate have no significant effect on the workability, air content, mix stability, or water-cement ratio of concretes into which they are incorporated. The only properties of plastic concrete which are affected are the heat evolution and setting time. 

Accelerating admixture type Stiffening time (h) for penetration resistance of 0.5 N mm- 3.5 N mm-2... 

Accelerating admixtures reduce both the initial and final setting time of mortar sieved from concrete mixes, determined by ASTM C403 68 or BS 5075 (1975). Typical results are given in Table 5.1 for a 300 kg m cement content mix with a compacting factor of 0.85 0.02 [24] at normal ambient temperature. 

There are only limited data available on the effect of other accelerating admixtures, although one comparative study [46] suggests that calcium formate... 

A variety of admixtures and additives are used to accelerate strength development, cohesiveness, bond, freeze-thaw and abrasion resistance, and to reduce rebound. Most of the accelerating admixtures used probably act by precipitating as insoluble hydroxides or other salts - a form of false set. Conduction calorimetric studies show that their main effect on early strength is due to the action on the C3 A fraction of the cement. The reaction of... 

Durability most proprietary accelerating admixtures adversely affect the concrete s resistance to freezing and thawing [114, 118]. More recently, the widespread use of silica fume has enabled the use of considerably low dosages of accelerators and also contributed to the improvement of porosity values, thereby dramatically improving durability of such concretes. 

Other fine materials which tend to inhibit air entrainment include pigments, particularly carbon black. This is of concern to the ready-mixed operator supplying integrally colored concrete for exterior exposure [6]. Accelerating admixtures which are used to reduce down time and in cold weather concreting can be used successfully in air-entrained concrete, but should be added separately and in solution form to the mix. Direct contact of these admixtures with some types of air-entraining agents mixed in the same water phase may adversely affect both admixtures. 

When high dosages of normal water-reducing and accelerating admixtures are used, to produce high water reduction and high workability. 

The main non-chloride, non-corrosive accelerating admixtures available on the market are of two types (1) accelerating admixtures which accelerate hydration but do not depress the freezing point of water and (2) accelerating admixtures for use in sub-freezing ambient temperatures which depress the freezing point of water. The former contain salts of formates, nitrates and nitrites and are effective for set acceleration and strength development. However, their effectiveness is dependent on the ambient temperature at the time of placement. 

Fig. 7.39 Effect of cement content and a non-chloride accelerating admixture (NCAA) on initial time of set, at concrete and ambient temperatures of 10 C (Nmai [117]).

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