Calcium carbonate cement paste

Supercritical C02 treatment affects the microstructure of the cement paste. In the first stage of the sc C02 treatment, free water in the cement pores is extracted. As a consequence of this dehydration process, channels of about 50-pm diameter develop. Dissolved calcium in the free water reacts with the C02 and crystallizes with the C02 as calcite along the channel walls. In the second stage, the structural water of the hydrated cement phases is extracted. The carbonation of the portlandite to form more calcite takes place. Water, bound to the CSH surrounding the partially hydrated cement clinker particles, is partially replaced by a carbonate formation. The short fibers of the CSH-cement framework, which are responsible for the physical properties of the cement, are not affected (Hartmann et al., 1999). 

Calcium occurs mainly as calcium carbonate and calcium silicate on earth s crust both are found in limestone. By heating the limestone, carbon dioxide is driven away to obtain calcium oxide. Because of its abundance in nature, it is an inexpensive raw material and is used in various industries including in cement manufacture to tooth pastes. It is available in dilferent grades based on the particle size, purity, and reactivity. 

They reacted a mixture of monocalcium phosphate monohydrate (Ca(H2P04)2-H20), a-tricalcium phosphate (Ca3(P04)2), and calcium carbonate (CaC03) with a solution of trisodium phosphate (Na3P04) to produce this rapid-setting cement. They have been able to apply this cement as a paste that sets within minutes under physiological conditions. The mixing time is = 5 min, and the paste sets by crystallizing into dahllite in another 10 min. 

The slaked lime was mixed to a paste with water and used to cement sand or stone. The paste slowly reacted with carbon dioxide in the air to give calcium carbonate again ... 

The type of cement also influences the carbonation rate. In fact, concrete s capacity to fix CO2 is proportional to the alkahnity in its cement paste. In Portland cement, about 64% of the mass of the original cement is composed of CaO (mainly converted to solid portlandite) and about 0.5-1.5% of Na20 and K2O (mainly in solution as NaOH and KOH). If we only consider calcium oxide, the quantity of... 

Carbonation is the result of the interaction of carbon dioxide gas in the atmosphere with the alkaline hydroxides in the concrete. Like many other gases carbon dioxide dissolves in water to form an acid. Unlike most other acids the carbonic acid does not attack the cement paste, but just neutralizes the alkalies in the pore water, mainly forming calcium carbonate that lines the pores ... 

Bonin discussed the new results explaining the reasons of paste or concrete stiffening [37]. Apart of false set, occurring as the result of alkalis carbonation, which in turn react with calcium hydroxide with CaCOj precipitation, Bonin considers the case of cement paste stiffening, in which under moisture influence the rate of ettringite formation had changed [37]. 

There are controversial opinions about the corrosion of reinforcing steel in the calcium aluminate cement concretes. This is linked with the less basic paste in comparison with the Portland cement one. However, it appeared in practice, that in the good quality concrete there was no difference related to the stability of steel reirrforcement in comparison with Portland cement. The destmction of reinforced concrete was linked with high w/c, in which the conversion of hexagonal hydrates into cubic caused the porosity and rapid carbonation increase [12]. 

The main difference in the composition of cement pastes made from cements with increased belite and reduced alite contents is their lower calcium hydroxide content. This may affect positively the resistance of such hardened pastes to chemical corrosion. At the same time the depth of carbonation increases with declining C3S content in cement (Kelham and Moir, 1992). 

In cements with a sufficiently high tricalcium aluminate content the calcium carbonate of limestone may react with tlus phase, yielding calcium carboaluminate hydrate (C A CHjj). Up to about 6 wt% of calcium carbonate may be consumed in this reaction within 28 days (Ingram and Daugherty, 1992). Whether the formation of calcium carboaluminate also contributes to a strength increase of the hardened cement paste remains uncertain. 

As tricalcium aluminate may react to a limited extent with calcium carbonate, the limestone that is present may partially replace the gypsirm present in the cement to control setting. As, however, such substitution may adversely affect the mechanical properties of the hardened paste, it shoirld be avoided, except in cases of a limited... 

At low fly ash additions— that is, below 15%—the extent of carbonation in mature fly ash concrete tends to be equal to or lower than that in similar concrete mixes with no ash, in spite of the lower calcium hydroxide content of the formed hydrated cement paste (Buttler et al., 1983 Hobbs, 1988 Goni et al, 1997). This is due mainly to the reduced permeability of the paste to CO2. However, at higher fly ash contents the resistance to carbonation is significantly reduced (Goni et al, 1997). 

In steel-reinforced concrete stractures made with calcium alununate cement and with a sufficiently low water/cement ratio, the reinforcement is sufficiently protected from corrosion. However, in mixes made with too much water, corrosion of the steel may take place, especially after conversion of the hardened paste has occurred, as the cement paste becomes too porous and too permeable for oxygen of the air. Carbonation of the paste, which progresses especially easily in porous mixes, enhances the corrosion process even further, as the pH of the pore solution drops from its original 10-12 to lower values, making the steel susceptible to corrosion. 

As a result of this process the calcium aluminate hydrates will convert to calcium carbonate and hydrous alumina. The process is associated with a lowering of the pH value of the pore solution, which in turn may result in corrosion of the steel reinforcement. Rocks that may include freeable alkalis include granite or mica. As well as alkalis derived from the aggregate, alkalis may also migrate into the paste from an outside soitrce, for example if placed in contact with Portland cement concrete. 

Carbonation is a process in which calcium oxide CaO and calcium hydroxide Ca0-2H20 in hardened cement paste are converted to calcium carbonate by carbon dioxide CO2 penetrating by diffusion from the atmosphere into the system of pores and microcracks ... 

The calcium hydroxide which is formed in accordance with equation (4) produces a strongly basic environment (pH > 12) in the freshly hardened cement paste (and therefore in mortar and concrete). This high pH value inhibits the corrosion of embedded steel and is indeed what makes reinforced concrete such a durable material in which the reinforcing bars are normally so well and lastingly protected by the concrete. However, as a result of carbonation and other influences, this protective action may diminish in course of time. 

The contents of Ca(OH)2 in the pastes at various times of hydration were determined from the results of the TG studies. Also, the pozzolana nature of the additives has been found out. The ability of combining with Ca(OH)2 was similar in the spent catalyst and the microsilica. In the presence of the spent catalyst, the hydration process was strongly exothermic, which promoted the rapid setting of the cement paste. Calcium carbonate aluminates that are formed in the system, favorably affect the strength of the concrete materials (38). 

Of these, because method i) requires a great deal of space and many days for drying, it is seldom used. Soil paste is not fluid enough to use pressurized pump transport or filtering and the mechanical dehydration remains rare. The use of additives is easy to use, needs little spaee, and continuous treatment is possible. Thus, it is more often used. The additives used for this purpose are cement, calcium carbonate, and polymers. Polymers do not require ineubation time and are effeetive nearly instantaneously. The soil ean be treated eontinuously. Henee, this method ean be a part of the soil pressure sealed method. In the ease of polymers, ehanging the soil to basic pH can be avoided. Table 4 shows the experimental results of soil improvement following inelusion of polymerie materials. In this table slump or flow values are used to evaluate the fluidity of eonciete or mortar. 

Thermal analysis data on the hydration of dicalcium silicate are sparse because it is time consuming to follow the reaction of this phase which is very slow. The characteristic products obtained during its hydration are not much different from those formed in C3S hydration. Also, the major strength development that occurs in cement in the first 28 days (a period of practical significance) is mainly due to the tricalcium silicate phase. TG, DTG, and DTA investigations of C2S were carried out by Tamas.t The sensitivity of the instrument had to be increased substantially to detect the peaks due to the decomposition of calcium hydroxide and calcium carbonate, especially at earlier times. In Fig. 21, the DTA, DTG, and TG curves of C2S hydrated for 21 days and 200 days are given. A comparison of these peaks with those obtained from C3S pastes shows substantial differences in the intensity value of the peaks. The 200 day C2S sample shows a weight loss of 4%, whereas C3S hydrated for 21 days indicates a loss of 13%. 

In addition to calcium carbonate, the formation of scawtite (Ca7Si502iH5 C03) and its decomposition with the evolution of CO2 at lower temperatures was reported through TG investigations It has also been reported that by applying DTA/TG techniques the calcium carbonate formed by exposure of cement paste to CO2 migrates to the surface of concrete in the form of layers. The rate of carbonation on low and high alkali cements has not been found to be different and that the C-S-H and ettringite phases are also carbonated. 

The addition of MK influences the hydration of cement significantly. The DTA curves of cement pastes treated with various amounts of MK are presented in Fig. 26The first peak at 135°C is due mainly to the dehydration of C-S-H (I). The endothermal effect at about 175°C is caused by the decomposition of C4AH13 and that at 480°C is due to the decomposition of CH. Two other endotherms at 740° and 765°C are ascribed to the decomposition of the amorphous and crystalline forms of calcium carbonate respectively. The peak effects of C-S-H (I) and C4AH13 increase as the amount of MK increases up to a 30% addition signifying an acceleration effect. The decrease in the peak area of CH is attributed to the reaction between MK and CH liberated by the hydration of the cement. 

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