In hot climates, it may be necessary to retemper the concrete to maintain the required workability. The retempering is done by additional dosages of superplasticizer. However, there is a paucity of information on the effects of retempering at higher temperatures. Therefore, an extensive investigation was undertaken to determine the slump loss characteristics and effect of repeated retempering with superplasticizer on the strength and elastic properties of concretes mixed at ambient temperatures ranging from 30 to 60 C (86 to 140 F). A total of 15 concrete mixes with water-to-cement ratios 0.40, 0.50, and 0.60 were made at ambient temperatures of 30, 40, 50, 55, and 60 C (86, 104, 122, 131, and 140 F) to give constant workability. Immediately after mixing and after two retemperings at 30 min intervals, fresh concrete properties (slump, air content, concrete temperature, and unit weight) were determined. Specimens were cast after initial mixing and after two retemperings and tested for hardened concrete properties at 7 and 28 days. The additional water and cement needed to achieve the same workability at higher temperatures is very high for low w/c concrete, whereas there is only a slight increase in the water demand for concretes with w/c 0.50 and 0.60. The increase in the rate of slump loss is not significantly higher at higher temperatures. When retempering with superplasticizer, there are no detrimental effects on fresh as well as hardened concrete properties even under the unfavorable high ambient temperature of 60 C (140 F). The performance characteristics are the same for concretes mixed at higher ambient temperatures and retempered with superplasticizer.

Except for degree of effectiveness, plasticizers and superplasticizers have very similar effects on the workability of hydraulic mixtures. In a microscopic scale, however, their action clearly differs by the effective deflocculation and the high dispersion of the cement particles obtained with superplasticizers. This behavior was the basis of development of two test procedures. The first one makes it possible to discriminate between both types of admixture. Furthermore, it reveals possible specific interactions with the cement used and makes it possible to assess the period of activity of a superplasticizer. The second procedure allows easy and quick assessment of the minimum active dosage required of a superplasticizer for a given cement. Though often higher than the percentages recommended by the manufacturers, the values agree well with those obtained by a vibratory compaction test on cement mortar. This test also allows a quick assessment of the relative effectiveness of various superplasticizers. From a study on 17 admixtures and two cements (one portland and one with granulated slag), it appeared that: an admixture supplied as a plasticizer may actually be a superplasticizer; an admixture may behave as a plasticizer or as a superplasticizer according to the nature of the cement; and the period of activity of the superplasticizers was generally longer with the slag cement.

Discusses a chemical approach to dealing with returned concrete that has the potential of reducing the amount of waste in concrete production to a level unmatched by other methods. This approach allows use of concrete up to 72 hr after batching and the reduction of wash-out slurry and reclaimer debris. This two-part chemical system is comprised of a stabilizer that strongly retards the hydration of all clinker minerals and a hydration initiator or activator added to stabilized concrete prior to its placement. Data presented on commercial portland cement and individual clinker minerals treated with this system show the effects of the stabilizer and initiator on the different clinker phases and were drawn from calorimetry, x-ray diffraction, and SEM studies. Control of nucleation and nuclei growth are proposed as the mechanism by which the chemical system works. Data from field-batched mixes demonstrate that the plastic and hardened properties of concrete made using this chemical system are no different than those of conventionally batched concrete.

Eight samples of sodium salts of naphthalenesulfonic acid-formaldehyde condensates with different molecular weight have been synthesized by stopping the polymerization process at different reaction times between zero (monomer) and 14 hr at 112 C using a molar H2SO4/naphthalene ratio of 1.07 and a molar HCHO/naphthalene ratio of 1.20. The longer the reaction time, the higher the molecular weight. The condensates have been analyzed by Gel Permeation Chromatography to determine the chemical composition and average molecular weight. The condensates have been used as superplasticizers (0.4 percent of dry product by weight of cement) for cement paste (water/cement ratio = 0.35) and the fluidifying effect has been determined by the mini slump test. It was found that the fluidifying effect increased by reducing the content of monomer and increasing the molecular weight of the condensate. To confirm that the fluidifying effect of the condensate substantially depends only on the content of the higher molecular weight fraction, two samples of the condensate, with different reaction times and then with a different condensation degree have been subjected to an ultrafiltration process. This technique allows removal of the monomer and the lower molecular weight fraction. The two samples of the condensate, which had different fluidifying effects before the ultrafiltration process, behaved similarly as superplasticizers after the ultrafiltration process.

The influence of alkali sulfate on the viscosity of cement paste containing a superplasticizer was studied by using rotational viscometer. The mechanism of the action of superplasticizer on the fluidity of cement paste was also investigated. A larger amount of superplasticizer was rapidly adsorbed onto C3A and C4AF compared to that on C3S and C2S. The presence of alkali sulfate inhibited the adsorption of superplasticizer on C3A and C4AF, thus permitting increased adsorption on C3S and C2S. The reduction of viscosity of cement paste by superplasticizer is dependent mainly on its adsorption onto C3S and C2S. Therefore, an increase in alkali sulfate leads to an increase in the adsorption of superplasticizer on C3S and C2S, and results in reduced viscosity of cement paste. Excessive amount of alkali sulfate, however, compressed the electric double layer, providing an increase in viscosity of cement paste. Based on the results, it was concluded that there was an optimum alkali sulfate level with respect to the fluidity of cement paste containing the superplasticizer.