Setting times cement mortar

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. 

Figure 19. DSC curves (at times of set) of cement mortars containing various admixtures.

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 damage to the building in Bavaria is a typical exception here, since the unheated church, notorious for its humid walls, had been plastered with cement mortar, which is known to remain alkaline for many months, only a few weeks before. These are exactly the conditions which in my view were favorable to the formation of Iron Blue. With increasing setting of the cement plaster over the course of months, the pH value of the masonry in the church finally dropped, so that the final reaction led to the formation of Iron Blue, which is stable for long periods of time. This final reaction of the adsorbed cyanide into Iron Blue was only completed after approximately two years. The prior stage of this reaction, the formation of considerably paler iron cyanides, could already have been completed or well progressed prior to this.520... 

Setting time of an SBR modified cement composition is delayed. However, in actual use, such mortar toppings may often be walked on in 12 to 18 hours, and may take light rubber tire traffic in 48 hours. These times will vary, depending upon the temperature, humidity and the percentage of latex in the mix. Table 2 shows the setting time for SBR latex/cement compositions. ... 

The addition of small amounts of epoxy emulsion to a cementitious mixture improves trowelability of the resultant product tremendously, in contrast to a typical latex admixture mortar where the high addition rate creates difficulty in troweling, because the latex solids are brought to the surface, exerting a drag on the trowel. The initial set time of an epoxy admixtured mortar is considerably reduced, and the final set time is also accelerated. This rapid setting of an epoxy emulsion admixtured cement mortar may be attributed to the polymerization of the epoxy resin, as well as to the acceleration in the rate of hydration of the cement. 

An addition of 0.1% CLS may extend the initial and final setting times of cement mortar by two and three hours, respectively. The influence of 0.3% CLS on the hydration of cement is shown in Fig. 9. Thermograms indicate that the reference cement containing no admixture exhibits a broad endothermal peak below 200°C, representing the formation of both ettringite and C-S-H phase. These peaks increase in intensity as the hydration period is increased. The effect between 450 and 500°C is caused by the dehydration of Ca(OH)2 and its intensity indicates the extent to which the hydration of the C3S component has progressed. The cement hydration, in the presence of lignosulfonate, is retarded as seen by the lower intensity of the Ca(OH)2 decomposition peak. The low temperature effect below 300°C in the presence of CLS is not sharp as that obtained in the reference sample. 

Pumice stone, brick, and tile dust, are also recommended for witer cements, but their only advantage seem to be an abso bent quality, which causes the mor-ter maue with them to set sooner, and therefore acquire a greater hardness in the s..me time, than mortar composed of sand and lime alone, for they have no power of Jiardening under water. 

The cement and sand are first mixed as for cement mortar the broken stone is added and the whole well mixed and immediately applied before it has time to set. Both concrete and cement mortar should be made as required for use, and in no case applied after standing over three hours. 

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