Retarder-based admixture

In ready-mix concrete trucks, the unused concrete is washed out and this poses environmental, waste, and other problems. A novel approach consists of adding a retarder-based admixture, such as carboxylic acid or phosphorous-based organic acids and salts, to the concrete mix in the truck, This stabilizes the concrete from further hydration. When an activator is added, subsequently normal hydration occurs. 

The silica fume is said to decrease the formation of ettringite by reducing the concentration of calcium (Ca" +) and hydroxyl (OH ) ions involved in the formation of ettringite [146]. The expansion of lime-based admixtures is not affected by water-reducing admixtures to the same extent as observed for CSA-based admixtures. However, set retardation may result due to the increased amounts of calcium hydroxide produced in the presence of the WRA. 

Regulated-set cement and jet cement are modified Portland cements in which the normal aluminate phase is replaced by CuA CaF, through the use of a raw mix containing CaF,. Uchikawa and Tsukiyama (U2I) gave chemical (Table 10.4) and phase compositions of two jet cements. Botli contained approximately 60% of alite, 20% of C, i A CaF,. 1% of belite and 5% of ferrite. Admixtures are required to control the rate of reaction of the C, 1 A CaFj and the nature of the products. One of the cements included a proprietary retarder based on citric acid, togetlier with 2"/o of CaC O, . The other contained 2.5% of hemihydrale. In each case, Na,S04 (T o) and anhydrite were also present. The specific surface areas were around 550 m kg . 

The lignosulfonate-based admixtures have been used more widely than other water redueers. They are capable of redueing water requirements and retarding the setting times of concrete. They influenee the dispersion and the hydration rate of the individual cement compounds, and, thus, the cement itself. Techniques such as XRD, DTA, DSC, TG, DTG, and conduction calorimetry have been used extensively to follow the hydration of cement and cement compounds containing different t5q)es and amounts of lignosulfonates (LS). 

An extensive work was carried out by Uchikawa l on the conduction calorimetry of superplasticizers, such as SNF (NS in the figure), lignosulfonate (LS), a co-polymer of acrylic acid with acrylic ester (PC), and a three dimensional polycondensate product of aromatic aminosulfonic acid with trimethyl phenol (AS) (Fig. 11). The first peak in the calorimetry corresponds to the heat of dissolution of alite, the heat of formation of the AFt phase, and the calcium hydroxide formation from free lime. The second peak corresponds to the heat of hydration of alite. The admixtures were found to accelerate the formation ofthe ettringite phase. At w/cratios ofO.3 and 0.5 and a later addition of the admixture, the appearance of the second peak was significantly delayed and the peaks were of lower intensity. Most retardation occurred with polycarboxylic acid and amninosulfonic acid-based admixtures (Fig. 11). DSC was used to determine the amount of lime formed at different times. The DSC results show that the addition of admixtures at different w/c ratios generally decreases the amounts of lime in the presence of superplasticizers (Fig. 12). 

This category of admixture is based mainly on the major raw materials, calcium chloride, calcium nitrate, calcium formate [2] and calcium thiocyanate, with minor amounts of other materials occasionally being included in the formulations, such as calcium thiosulfate [3] and triethanolamine (TEA). TEA is not normally used alone but because it is sometimes used in other categories of admixture to compensate for retarding influences it will be included in this section. 

Hydroxy carboxylic Acids The hydroxycarboxylic acids have hydroxyl (OH) and carboxyl (COOH) groups attached to a carbon chain. Glnconic, citric, tartaric, mucic, malic, salicylic, heptonic, saccharic, and tannic acids can be nsed as retarding and retarding water reducing admixtures. For use as normal WRA they are mixed with accelerating admixtures [23]. Hydroxycarboxylic acid-based WRA are mostly used as aqueous solutions of sodium salt. However, they may occasionally be found as salts of ammonia or triethanolamine. 

Chemical admixtures in small amounts are added to concrete to enhance the physical, mechanical, chemical, and durability characteristics of concrete. Superplasticizers are admixtures that have the ability to increase the workability of concrete (for easy placement), but also produce high strength concretes. The relative rates of hydration of cement containing superplasticizers at different temperatures are conveniently followed by conduction calorimetry. In Fig. 11 both the rates of hydration and the cumulative amounts of heat developed in cement pastes hydrated at temperatures of20,40, and 55" C are plotted as a function of time. In the figure, SMF refers to the superplasticizer based on sulfonated melamine formaldehyde. The addition of the superplasticizer retards the hydration of cement. The retardation increases with the dosage of the superplasticizer. Also, the retardation effect becomes less significant as the temperature of hydration is increased. 

Most water reducers and retarders are based on similar formulations, and they are treated together in this chapter. These admixtures can be divided into four categories, viz., normal water reducers, water-reduc-ing-retarding admixtures, water-reducing-accelerating admixtures, and retarders. 

One of the best known accelerators of the hydration of Portland cement is calcium chloride. Generally, up to 2 percent solid calcium chloride dihydrate based on the weight of cement is added. Chlorides may be contained in other admixtures such as some water-reducing admixtures where small amounts of chloride are sometimes added to offset the set-retarding effect of the water reducer. 

Figure 2.21 shows the heat flow development of cement pastes without and with 0.3% of superplasticiser in simultaneous and in delayed addition mode (Winnefeld 2008). Products based on polycarboxylate ether (PCE) and sulfonated naphthalene-formaldehyde polycondensate (SNF) were applied. Compared to the plain paste, the pastes with admixtures show a retardation of the setting and a delay of the main hydration peak. In... 

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