Unhydrated cement

In these early reactions the reactivities of the individual phases are important in determining the overall reaction rate. However, as the cement particles become more densely coated with reaction products, diffusion of water and ions in solution becomes increasingly impeded. The reactions then become diffusion-controUed at some time depending on various factors such as temperature and water—cement ratio. After about 1 or 2 days, ie, at ca 40% of complete reaction, the remaining unhydrated cement phases react more nearly uniformly. 

Mixtures of cement gel and unhydrated cement pa rtides enveloped with a close-packed layer of polymer partides... 

Unhydrated cement particles a Polymer particles jSP Aggregate B... 

Bensted (B38) described the use of TG in the study of clinkers or unhydrated cements. At 4°Cmin in Nj with cements, losses normally occur at I00-200"C from gypsum or hemihydrate, 400-500°C from CH and 500-800°C from CaCOj. If a normal, open sample cup is used, the losses from conversion of gypsum to hemihydrate and from dehydration of hemihydrate are poorly resolved, making interpretation difficult. Better separation can be achieved by using a sample cup with a top that is closed except for a narrow exit (S32). 

Applications of IR and Raman spectroscopy to the study of clinkers and unhydrated cements have been reviewed (B39,B40). The laser Raman microprobe, with which regions of micrometre dimensions on a polished surface may be examined, has been used to investigate structure and crystallinity, especially of the alite and belite (Cl9). Spectroscopic methods for studying the surface structures and compositions of cements are considered in Section 5.6.2. 

Infrared absorption spectra (B97) of unhydrated cements typically show moderate to strong bands at 525 and 925 cm due to alite, and at 1120 and 1145 cm from S-O stretching vibrations, and weak bands in the 1650 and 3500 cm regions due to H2O molecules. The early formation of ettringite... 

The experimental considerations applying to calcium silicate pastes (Sections 5.1 and 5.2) are equally relevant to cement pastes. Of the methods so far used in attempts to determine the degrees of reaction of the individual clinker phases as a function of time, QXDA (C39,D12,T34,P28) has proved much the most satisfactory. Procedures are essentially as for the analysis of a clinker or unreacted cement (Section 4.3.2), but it is necessary to take account of overlaps with peaks from the hydration products, and especially, with the C-S-H band at 0.27-0.31 nm. The water content of the sample must be known, so that the results can be referred to the weight of anhydrous material. If a sample of the unhydrated cement is available, and its quantitative phase composition has been determined, it may be used as the reference standard for the individual clinker phases in the paste. 

Copeland et al. (C38) reviewed the energetics of cement hydration. After the first few days, the rate of heat liberation is too low for conduction calorimetry to be a practicable means of investigation, but the total amount of heat liberated after any desired time can be determined from the heat of solution in acid, which is compared with that of the unhydrated cement. Fig. 7.9 gives average results thus obtained for different US types of Portland cements. 

In general, acid solutions attack concrete in any combination of four ways (a) by dissolving both hydrated and unhydrated cement compounds, (b) by dissolving calcareous aggregates present in the mix, (c) through physical stresses induced by the deposition of... 

First Step. When polymer latexes are mixed with fresh cement mortar or concrete, the polymer partides are uniformly dispersed in the cement paste phase. In this polymer-cement paste, flie cement gel is gradually formed by the cement hydration and the water phase is saturated with calcium hydroxide formed during the hydration, whereas the polymer partides dqrosit partially on the surfaces of the cement-gel-unhydrated-cement partide mixtures. It is likely that the calcium hydroxide in the water phase reacts witit a silica surface of the aggre tes to form a calcium silicate layer.I It is confirmed that tire formation of the calcium hydroxide and ettringite in the contad zone between tire cement hydrates and aggregates is attributed to the bond between them.I lPl... 

Abstract. The presence of water-soluble polymers affects the microstructure of polymer-modified cement mortar. Such effects are studied by means of SEM investigation. Polyvinyl alcohol-acetate (PVAA), Methylcellulose (MC) and Hydroxyethylcellulose (HEC) are applied in a 1 % polymer-cement ratio. The polymers provide an improved dispersion of the cement particles in the mixing water. The tendency of certain water-soluble polymers to retard the flocculation of the cement particles minimizes the formation of a water-rich layer around the aggregate surfaces. They also provide a more uniform distribution of unhydrated cement particles in the matrix, without significant depletion near aggregate surfaces. Both effects enable to reduce the interfacial transition zone (ITZ). The polymers also provide a more cohesive microstructure, with a reduced amount of microcracks. 

General characteristics of the interfacial transition zone. Fig. 5 illustrates the microstrueture of an unmodified eement mortar on SEM image in the backseattered mode (BSE). The large dark areas represent the sand particles. Unhydrated cement particles are responsible for white areas. The epoxy-impregnated pores have a low backscatter intensity and appear as black spots. 

It should be noted that significantly more cement and hydration phases are known in literature [12]. For example, dicalcium silicate (2 CaO Si02 C2S), an important clinker phase which accounts for approx. 5-10 wt-% of the cement used to prepare the samples, was deliberately left out of the refinement. Although dicalcium silicate could be positively identified in the diffraction patterns of the unhydrated cement, its quantification in the Rietveld refinement was not reliably possible after the samples had been exposed to water and the phase was, therefore, excluded. 

The composition of the pure, unhydrated cement before addition of the polymer was 58% C3S, 9% C3A, 3% bassanite, and 30% amorphous content. Upon addition of 20 wt-% polymers, the amorphous content should increase to a maximum of 44% provided that the polymers are fully amorphous. The observed contents of 48% in the c/PVA sample and 52 % in the c/PVAc sample (cf. Fig. 5) agree within experimental error with the expected 44 %. 

These discussions and doubts result first of all from the complexity of reaction of unhydrous cement phases with water. This can be illustrated taking into account a simple, on a first appearance, example of gypsum hemihydrate hydration. This reaction served as a basis of Le Chatelier s crystallization theory. The uncompUcated, at first sight, reaction ... 

Fig. 5.41 Schematic presentation of general relationship fracture energy-porosity (after [94]) high strength caused by low porosity and high ratio of unhydrated cement, B higher porosity and Ca(OH)j content, C local maximum of strength caused by fracture propagation inhibition on the pores (at porosity=22.5 %), D reduced strength by increased porosity and reduced number of intergranular contacts...

Cement paste can be considered as a concrete with micro—aggregate in the form of unhydrated cement grains. Its high strength is attributed to the occurrence of strong bonds between these unhydrated cement grains and C-S-H. 

Fig. 6.63 Appearance of normal features not subjected to sulphate attack A—unhydrated cement, B—dense inner C-S-H gel surrounding unhydrated cement, C—inner C-S-H gel constituting fully hydrated cement grain, D— region of small hollow shell hydration grains, E—groundmass or outer product C-S-H gel, F—Ca(OH)j surrounding a sand grain chip, G—deposit of calcium hydroxide within the groundmass (After [261]) Diamond S., Lee R.J. in Materials Science and Concrete, Special volume Sulfate Attack Mechanisms (J. Marchand amd J. Skalny eds.), p. 138, Fig. 1, 1999, published by The American Ceramic Society, 735 Ceramic Place, Westerville, Ohio 43081, 2001, reproduced with the permission of The American Ceramic Society...

Unhydrated cement grains lose calcium mostly, due to leaching process and the siliceous acid gel as residue remains. Therefore the strong bonds, found previously between these grains and inner C-S-H gel, are lost. The substitution of calcium with the magnesium ions with the formation of magnesium silicates can also occur [256], Occasionally, the brownmillerite relics, as a marker of cement grains remains [261]. 

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