Ferenc D. Tamás

Electrical conductivity of cement pastes

Abstract The electrical conductivity of cement- and clinker mineral pastes was measured during setting and hardening. Conductograms give interesting and structurally important data on the beginning period of cement hydration.

Chloride ion binding capacity of aluminoferrites

Abstract The hydration of aluminoferrites (C 4 AF, C 6 AF 2 and C 6 A 2 F) and their mixture with gypsum (up to 10/5 ratio) was investigated as well as their chloride binding capacity (after immersion in 10% NaCl solution) by thermal tests (DTG and TGA) and X-ray diffraction (XRD). Hydration products of these aluminoferrites are similar to that of C 3 A, but amorphous AH 3 and/or FH 3 are formed, and transformation of metastable (hexagonal hydrates) to the stable (hydrogarnet) phase is slower. Iron content of aluminoferrites determines the rate of hydration reactions: hydration rate increases in the order C 6 A 2 F>C 4 AF>C 6 AF 2 . In the case of C 4 AF+gypsum mixtures, monosulfate was found prior to the total exhaustion of gypsum. After salt treatment, iron-containing Friedels salt or Kuzels salt was found. Characteristics of DTG peaks are described and interpreted.

A Mössbauer study on the hydration of brownmillerite (4CaO.A12O3.Fe2O3)

Abstract The hydration of the iron-bearing phase of portland cement or brownmillerite, can be easily studied by Mossbauer-spectrometry, because a very significant difference exists between the spectra of anhydrous and hydrated brownmillerite. The Mossbauer spectrum of anhydrous brownmillerite shows a magnetic hyperfine structure and iron is present in a complex environment. During hydration the magnetic hyperfine structure disappears and only one sharp line remains showing that all iron atoms are surrounded by the same, very symmetric environment. The hydration is almost complete after 14 days. The hydration product is a solid solution of tricalcium aluminate hexahydrate and tricalcium ferrite hexahydrate (hydrogarnet). The full quantity of iron is bound in this hydrate and is present in an octahedral environment. Excess aluminum is present in the form of amorphous hydroxide.

Trace elements in clinker: II. Qualitative identification by fuzzy clustering

The trace element content of clinkers (and possibly of cements) can be used for the qualitative identification (i.e., manufacturing factory). This paper proposes a fuzzy classifier for the discrimination of clinkers produced in different factories based on their Mg, Sr, Ba, Mn, Ti, Zr, Zn and V content. The fuzzy classifier is identified by unsupervised fuzzy clustering. The most relevant trace elements were selected based on the obtained clusters by the modified version of the Fisher interclass separability method. The classification of a country from the European Community and South African clinkers is used as an illustrative example. The results show that the proposed method is useful to identify compact classifiers that are able to determine the origin of the clinker; the obtained classifier is easy to use and interpret for engineers and researchers, even when they are not familiar with the concept of fuzzy logic.

Burnability of cement raw materials at rapid calcination conditions

Abstract Cement raw materials (clay, limestone and pyrite cinders) were mixed, pressed into pellets, burnt at high temperatures, cooled and the resulting clinkers analysed for free CaO content. The effect of the following parameters and their interactions on the free CaO was studied: top temperature of burning (in three levels: 1300, 1350, 1400°C), soaking time at top temperature (in three levels: 5, 30 and in some cases 60 min), method of heating (in two levels: rapid and normal), method of cooling (in two levels, rapid and slow), lime saturation factor of the mix, LSF (in three levels, 0.95, 0.85, 0.76). Experiments were planned to enable a statistical evaluation (calculation of error variance at various levels of significance). Rapid calcination improves the burnability of cement raw materials, as contrasted to normal calcination. This indicates that rapid calcination enables the use of lower calcination temperatures. A long soaking at top temperature is unnecessary in case of rapid calcination. The free CaO content of the clinker is affected primarily by the LSF of the mix and the top temperature of firing; all other parameters are less effective.

Solid state difuusion of calcium into kaolinite and illite

Abstract Coarse marble was mixed with a high-kaolinite and a high-illite clay, pressed and fired. Contact zones occur around the CaO-grain after firing; these were studied by line-scanning electron probe micro-analysis. Solid state reactions during firing are diffusion-controlled. The contact zones are composed of several layers. The tricalcium silicate layer is formed through dicalcium silicate from kaolinite at temperatures higher than 1100°C; from illite it is formed directly at only 1000°C. Rhythmic diffusion occurs at higher temperatures. The diffusion constants and activation energies are calculated for the different layers.

The change in reactivity of silicate anions during the hydration of calcium silicates and cement

Abstract The change in reactivity of the silicate anion complex of tricalcium silicate, β-dicalcium silicate and portland cement during hydration was studied by measuring the rate constant ( k ) of the silicomolybdic acid formation. The three anhydrous starting materials have identical k values indicating their identical monosilicate anion structure. During hydration the value of k decreases continuously; the rate of decrease of k is highest in case of cement and lowest in case of β-dicalcium silicate. The decrease of k can be attributed to the formation of bridging oxygen ions in the silicate complex. k tends to reach a final, limiting value; beyond this k can still be decreased by special techniques as e.g. by regrinding and rehydrating the hardened samples.

A note on the preparation of clinker and cement paste samples for scanning electron microscopy

Abstract The samples for the SEM investigation of clinkers or cement pastes are usually made conductive by a carbon or gold layer prepared by vacuum deposition. This conventional method does not enable the study of the very same grain during hydration as vacuum deposited layers cannot be removed. This paper describes a technique where the sample surface is made conductive by the application of an antistatic spray; this can be dissolved after SEM examination and the same grain can be re-investigated after subsequent hydration.