Portland cement accelerator

M. Torabinejad, C. Hong, F. McDonald, T. Pitt Ford, The chemical constitution and biocompatibility of accelerated Portland cement for endodontic use, Int. Endod. J. 21 (1995) 349-353. 

In concrete, triethanolamine accelerates set time and increases early set strength (41—43). These ate often formulated as admixtures (44), for later addition to the concrete mixtures. Compared to calcium chloride, another common set accelerator, triethanolamine is less corrosive to steel-reinforcing materials, and gives a concrete that is more resistant to creep under stress (45). Triethanolamine can also neutralize any acid in the concrete and forms a salt with chlorides. Improvement of mechanical properties, whiteness, and more even distribution of iron impurities in the mixture of portland cements, can be effected by addition of 2% triethanolamine (46). Triethanolamine bottoms and alkanolamine soaps can also be used in these type appUcations. Waterproofing or sealing concrete can be accompUshed by using formulations containing triethanolamine (47,48). 

Concrete, Mortar, and Plaster. Citric acid and citrate salts are used as admixtures in concrete, mortar, and plaster formulations to retard setting times and reduce the amount of water requited to make a workable mixture (172—180). The citrate ion slows the hydration of Portland cement and acts as a dispersant, reducing the viscosity of the system (181). At levels below 0.1%, citrates accelerate the setting rate while at 0.2—0.4% the set rate is retarded. High early strength and improved frost resistance have been reported when adding citrate to concrete, mortar, and plaster. 

The addition of sodium sulfate to Portland cement accelerates the cement hardening and increases mobility of the solution. Chloride-free accelerators... 

Portland cement is susceptible to corrosion by CO2 and H2S. The chemical attack by CO2 is called carbonation. A microsample technique has been developed to study the CO2 corrosion in cements, because the corrosion is difficult to monitor with common test procedures [264]. This technique is also advantageous as an accelerated testing method. A polymer-modified cement has been tested in field studies [694]. The addition of silica also improves chemical resistance [146], in particular brine corrosion. 

As far as the final hydration products of ordinary Portland cement are concerned, there is an indication from isothermal calorimetry [57] that there is very little difference in the presence or absence of a calcium lignosulfonate water-reducing admixture. In this work, the heat evolved per unit of water incorporated into the hydrate has been determined for two cements, with the results shown in Fig. 1.25. It can be seen that the relationship between the amount of heat evolved and the amount of water combined with the cement is maintained whether the admixture is present or not. This work also indicated that the retardation in the early stages is compensated for at later times by an acceleration. 

All the materials accelerate reaction of C3S phase hydration, which is the main strength contributing component of Portland cement. 

Two types of calcium nitrite-based corrosion inhibitors are currently marketed, viz. a set- and strength-accelerating type and a normal-setting type. The former increases the early strength development in concrete. This effect increases with the dosage. Both admixtures are compatible with all types of Portland cements and... 

Reports of incompatibility between Portland cement and chemical admixtures have increased over the past 10 years. Rapid set, accelerated stiffening, increase in time of set, lack of water-reduction are some of the reported problems. Such effects produced by the interaction of the composition of the cement and that of the chemical admixtures has been often referred to as cement-admixture incompatibility . This is discussed below. 

Bensted, J. 1977. Chloroaluminates and the role of calcium chloride in accelerated hardening of Portland cement. World Cement Technology, 8, 171-175. 

Supercritical C02 (sc C02) is being used to accelerate the natural aging reactions (i.e., carbonation) of Portland cement. This treatment method alters the bulk properties of cement, producing profound changes in both structure and chemical composition. As a result of these changes, the mechanical and transport properties of these cements are also dramatically affected, and they display reduced porosity, permeability and pH, as well as increased density and compressive strength. 

Figure 15.1. Supercritical fluid carbonation of cast cement. (Top) Idealized chemical reactions occurring during the manufacture of a cast Portland cement. (Bottom) Idealized chemical reactions occurring during the accelerated carbonation using supercritical C02.

Fly ash increases the density, decreases the permeability, and increases the leaching resistance of Ordinary Portland Cement (OPC). It is a truism that The leach resistance of solidified cement-waste systems can be improved by any process which accelerates curing, limits porosity, or chemically bonds fission product or actinide elements. (Jantzen et al., 1984). Supercritical C02 treatment of a modified Portland cement is expected to further increase the density over the untreated material, so that a reduced porosity and improved leachability should result. In addition, the high silica content of fly ash, with its well-known sorbent properties toward actinides and certain other radionuclides, enhances the immobilization characteristics. 

We have demonstrated that supercritical C02 can be used to accelerate the natural carbonation reactions in unmodified Portland cements, and that this treatment enhances the physical properties of the cement. Further, it has been proven that the use of supercritical C02 allows the replacement of Portland cement powder with inexpensive, lower-grade pozzolans, such as fly ashes. 

The accelerated carbonation reactions have also been shown to improve the immobilization properties of Portland cements used as radioactive was-teforms. The formation of carbonates eliminates the potential problem of radiolysis while at the same time improving the leaching resistance. The resulting carbonate matrix, which is more thermodynamically stable relative to the untreated cement, would also be preferred for the underground disposal of the wasteforms. Incorportion of fly ash into the cement-type wasteform further enhances the favorable properties of this waste type. 

Wilding, C. R. The Performance of Cement Based Systems. Cem. Res. 1992, 22, 299. Wilk, C. M. Stabilization of Heavy Metals with Portland Cement Research Synopsis Portland Cement Association Report IS007 PC A Skokie, IL, 1997. Young, J. F. Berger, R. L. Breese, J. Accelerated Curing of Compacted Calcium Silicate Mortars on Exposure to C02. J. Am. Chem. Soc. 1974, 57, 394—397. 

Traetieberg (T47) showed that microsilica used as an addition with cement has considerable pozzolanic activity, mainly in the period 7-14 days after mixing, and that the reaction product formed with CH probably had a Ca/Si ratio of about 1.1. Several subsequent studies have shown that the pozzolanic reaction is detectable within hours and also that the early reaction of the alite is accelerated (H37,H54,H55). Huang and Feldman (H54,H55) studied the hydration reactions in some detail. In pastes with 10% or 30% replacement and w/s ratios of 0.25 or 0.45, the CH content passed through maxima usually within the first day before beginning to decrease in those with 30% replacement, it had reached zero by 14 days. Table 9.9 gives some of the results obtained for CH content and non-evaporable water in these pastes. As with pfa cements, and for the same reason, the non-evaporable water contents of mature pastes are considerably lower than those of comparable pastes of pure Portland cements. 

Jetset. A fast-setting cement developed by the Portland Cement Association. Reported to harden in 20 minutes after pouring. Accelerating agent has not been disclosed. 

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