Setting Portland cement paste

Fig. 5.7 Effect of various calcium salts on the time of setting of Portland cement pastes.

In order for normal set to occur in Portland cement paste, mortar, or concrete, calcium sulfate must be present in the cement-water system. In today s cements, most of the calcium sulfate introduced into the system as a component of the cement, can be present in one or more forms gypsum (CaS04.2H20), hemihydrate (CaS04.1/2... 

The workability of a concrete mix is by no means dependent only on the physical properties of the cement paste it contains, but an understanding of it requires one of those properties. For some specialist uses in which cement is used without an aggregate, the latter are directly relevant. The most important properties are concerned with rheology, and this section deals primarily with these properties in Portland cement pastes, free from admixtures, prior to setting. From the chemical standpoint, this period comprises that of initial reaction and induction period. From the practical standpoint, it includes those of mixing, placing and compaction. 

Fig. 6. Time dependences of radon release rate from Portland cement paste sample (w/c -0.3) labeled with The sample setting was characterized under in-situ...

The time for which the suspension preserves its flowabilily rrray vary between a few minutes and several hours, and may be controlled by the amoimt of added additives (Harma, 1977 Odler et al, 1978 Satava and Tyle, 1994). In general, the setting time is extended with an increasing amount of lignosirUbnate, whereas the amoimt of carbonate has little effect on this behavior. Unlike the behavior of ordinary Portland cement pastes, the transition from the plastic to the set state occurs very suddenly. 

Diamond, S., and Gomez-Toledo, C. (1978) Consistency, setting and strength gain characteristics of a low porosity Portland cement paste. Cement and Concrete Research 8,613-622. 

Microhardness has been correlated with compressive strength for several cementitious systems.Figure 13 is a plot of microhardness versus porosity forthe following cement systems hydrated Reg Set cement paste with 0,1,2, and 5% CaCl2 and hydrated portland cement paste. The... 

Modulus of elasticity versus porosity curves for Reg Set cement paste hydrated in the presence of CaCl2 are all higher than the curve for Reg Set cement paste with 0% CaCl2. All curves for Reg Set cement paste are below the curve for hydrated portland cement paste. 

Whilst the microhardness values of hydrated portland cement paste lie between those for Reg Set cement paste with 2 and 5% CaCl2, the modulus of elasticity values for hydrated portland cement paste are higher than for any of the Reg Set cement pastes. It is suggested that the processes of microstructural deformation occurring in microhardness and modulus of elasticity measurements are influenced in different ways by bond formation during the hydration of Reg Set cement in the presence of CaCl2. 

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. 

Preparation of Hydrated Silicates. The hydrated silicate specimens used were all in the paste form—that is, mixtures of one of the calcium silicates with a limited amount of water to form a slurry, which sets and hardens as portland cement itself does. These pastes were prepared by the vacuum mixing procedure described by Powers, Copeland, Hayes, and Mann (23), adapted so that the temperature of the mix upon removal from the mixer was the temperature at which the specimen was to be hydrated. The 5° specimens were made by starting with an ice-water mixture the-50° specimens by starting with preheated water. A manostat was incorporated into the pumping system to prevent the pressure from dropping below the vapor pressure of water at the desired final temperature. This was especially important for the 50° mixes, to prevent excessive cooling. 

Fig. 10.8 Setting times of neat pastes of mixtures of Ciment Fondu and Portland cement. These are only general indications, as the times depend on Portland cement composition, mixing time, temperature and other factors. After Robson (R47).

Hydraulic cements are another class of technologically important materials. Examples include Portland cement, calcium aluminate cement, and plaster of Paris. They harden at room temperature when their powder is mixed with water. The pastes formed this way set into a hard mass that has sufficient compression strength and can be used as stmctural materials. Their structure is generally noncrystalline. 

The sludges were formulated to contain 0.04 moles/L each of cadmium, chromium, and lead. Metals were added as cliromium chloride, cadmium nitrate, and lead nitrate and converted to their hydroxide precipitate forms in the sludge by neutralization of the solution to pH 8.5 with sodium hydroxide. Some samples al.so contained sodium arsenite, to evaluate the leaching of anionic metals. The sludges were mixed with type II portland cement to form cement pastes with water/cement (w/c) ratios of 0.5 and 1.0 u-sing ASTM procedure C30582. The freshly mixed pastes were placed in 3.8 cm diameter by 7.6 cm tall PCV cylinders and allowed to set for 24—48 hours at 100% humidity before they were removed from their molds and returned to the 100% humidity environment. 

Alumina cement is used in refractory pastes at temperatures up to 1700°C. Rapid setting cement consists of mixtures of alumina cement and Portland cement (used e.g. in sealing water leaks). 

Cement is a powder, which mixed with water forms plastic mass, easy to shape (paste), setting with time and hardening gradually with strength increase. In marty standards the more exact Portland cement definitions were introduced. It is necessary because the name of cement is given to other binding materials, for example to anhydrite or magnesium oxychloride (the last is called Sorel cement). 

Figure 6 shows the DSA results obtained during the hydration of Portland cement (PC-400) in water wjc = 0.3) under isothermal conditions at 35, 45 and 65°C, resp. The reactivity of the cement towards water was determined from the DSA results from the early stage of the interaction with water. Changes of surface and morphology taking place in the hydration products of cement have also been monitored by this method [36] under in-situ conditions of the setting of the cement paste. 

The Portland clinker used should contain a high amount of tricaldum silicate, preferably more than 45%. This is necessary as the hydration of this phase produces the calcium hydroxide needed for a pozzolanic reaction of the ash. The hydration of the clinker minerals is mainly responsible for the setting and initial strength development of the cement, as the reaction rate of the fly ash is rather slow. The lydration of the ash contributes to strength only at longer hydration times, but also affects other properties of the hardened material. The calcium sulfate added in the form of gypsum or anhydrite serves to control the setting of the fresh paste in a similar way as in plain Portland cement. 

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