H. El-Didamony

Metakaolin—lime hydration products

Abstract Four mixtures were prepared of weight ratios 80:20, 70:30, 60:40 and 50:50 of china kaolin/pure Ca(OH) 2 . Each mix was activated for 2 h at 800°C and then paste hydrated at room temperature and 100% RH for up to 28 days. The hydration products were studied by XRD as well as DTA and TG techniques. The chemically-combined water and Ca(OH) 2 contents were quantitatively determined from the TG curves. The results illustrate the formation of gehlenite hydrate (C 2 ASH 8 ) as the main hydration product; its amount increases with the curing time. Hydrogarnet is also formed at the early stages of hydration in the mixes of lower lime content (e.g., mix 80:20) and increases on prolonged hydration; this is probably due to the lower pH of the reaction medium. CSH gel could not be easily detected by XRD because of its low degree of crystallinity. However, this phase was identified by DTA and gives the characteristic losses on the TG curves. Ca(OH) 2 is consumed gradually as the hydration proceeds and disappears nearly completely after long periods (after 28 days).

Physico-chemical and Thermal Characteristics of Lime-silica Fume Pastes

Five lime-silica fume pastes were investigated using initial CaO/SiO2 molar ratios of 0.80, 1.0, 1.30, 1.70 and 2.0. The kinetics and mechanism of hydration interaction between lime and silica fume were studied on the basis of the phase composition and the physical state of the formed hydration products. The developed strength could be related to the lime content of the lime-silica mixture and the formed hydrates.

Application of differential thermogravimetry to the hydration of expansive cement pastes

Abstract Differential thermogravimetric analysis was used to determine the hydration kinetics of expansive cement and its products at various ages of hydration. Analytical grade reagents, kaolin and Portland cement were used to prepare an expansive cement on the basis of calcium sulphoaluminate. Two mix compositions having the stoichiometric composition of trisulphate and monosulphate were synthesized from pure reagents. Three clinkers were also prepared from kaolin, gypsum and calcium carbonate with different compositions. The hydration of expansive cement prepared from the stoichiometric composition of trisulphate and Portland cement gives ettringite as the stable phase after seven days of hydration. The presence of more CaO than the stoichiometric composition of trisulphate favours the conversion of some ettringite to the monosulphate hydrate. The hydration of expansive cement prepared from the stoichiometric composition of monosulphate and Portland cement shows the presence of ettringite and the monosulphate phase. Ettringite is formed initially, and then transformed to the monosulphate form.

Physico-chemical and surface characteristics of some granulated slag-fired drinking water sludge composite cement pastes

Abstract This investigation aims to study the physico-chemical and surface characteristics of some composite cement pastes. Granulated slag (GBFS) was substituted with fired drinking water sludge (FDWTS) with 5, 10 and 15 wt% to prepare composite cements. The hydration characteristics of composite cement pastes were studied by the determination of portlandite and chemically combined water contents at different curing ages of hydration as well as the phase composition of hydration products. The surface properties were studied using the nitrogen adsorption technique. The results showed that the chemically combined water content increased by using FDWTS instead of GBFS. The free portlandite content increased up to 7 days and then decreased up to 90 days. Compressive strength increases with FDWTS up to 5% and then decreases with its increase up to 15% by weight. The specific surface areas (SBET, m2/g) and pore volumes (VP, ml/g) increased with FDWTS content in composite cement pastes.

The hydration of alite in the presence of concrete admixtures

Abstract The role of three concrete admixtures on the hydration of alite clinker is studied. The effects of the three admixtures are (i) acts as a superplasticizer; (2) acts as a retarder; and (3) acts as an accelerator. The hydration products of alite with and without admixtures, added at different dosages, is examined by DTA and TG methods. The kinetics of hydration were followed by the estimation of free Ca(OH)2 liberated during hydration as well as the chemically-combined water contents of the hydration products. The results reveal that the superplasticizer did not alter the rate of hydration of alite to any considerable extent, though the workability of the paste was greatly increased. The addition of 2–4% retarder delays the hydration of alite by prolongation of the dormant period from 6 h to about 7 days. The chemically-combined water contents suggest that the suitable dosage of this retarding agent is 1.0–2.0%. When 0.15–0.25% accelerator was used, the reaction rate of alite slowed down until the end of the first 24 h and was then enhanced. The chemically-combined water content was found to go parallel with the reaction rate.

Study of the hydration products of the system CaOAl2O3SO3SiO2 with varying CaO mole ratio

Abstract The hydrated phases of the system CaOAl 2 O 3 SO 3 SiO 2 were studied, and the effect of the presence of varying amounts of CaO on the hydrated phases was also investigated. These phases were assessed by the aid of differential thermal analysis (DTA) and X-ray diffractometry. They hydration kinetics of the pastes under study were followed by the determination of the chemically combined water. Results revealed that 1 mole of CaO in excess reacts with the available SiO 2 and initiates the formation of sulphoaluminate hydrates. As the CaO is increased, the amount of such hydraulic phases is increased and, accordingly, the rate of hydration enhanced. The addition of 2 moles of CaO results in the formation of appreciable amounts of ettringite among the sulphoaluminate hydrates, but some anhydrous forms still exist. On the other hand, 4 moles give better formation of the hydraulic phases and some excess Ca(OH) 2 is also detected.

Hydration characteristics of metakaolin-lime-gypsum

Abstract The hydration characteristics of some metakaolin—lime—gypsum mixtures were studied by the determination of their hydration products using XRD as well as DTA techniques. The effect of the phase constituents on the compressive strength of the hardened pastes was also investigated. The results revealed that at all ages of hydration the compressive strength increases with the increase in gypsum content up to 10% and then decreases. Generally, the compressive strength of these pastes increases with curing time. Pastes with 15 and 20% gypsum show an increase in strength for seven days and then deteriorate. The gehlenite hydrate is formed in pastes with small amounts of gypsum (5–10%), whereas the presence of enough gypsum (15–20%) prevents the formation of gehlenite with an increase in the ettringite phase.

Thermal studies of basalt and clay for the manufacture of building bricks

Abstract Abou-Zaabal basalt and Fayoum clay were investigated in order to replace the Nile silt which until now has been used for the manufacture of building bricks. The quantity of silt diminished after the construction of the Aswan High Dam in Egypt. Thermochemical studies were carried out on these materials to show their mineralogical construction. A mixture of basalt powder and clay (75%:25% by weight) was prepared as bricks to show the suitability of basalt as a substitute for clay. The results showed that the semi-altered basalt consists of some clay minerals as well as the minerals of fresh basalt. The resultant products increase with decrease in grain size of the basalt. The basalt:clay (75:25) mix can be used for the manufacture of building bricks.

Effect of thermal treatment of rice husk ash on the surface properties of hydrated lime-rice husk ash binders

Rice husk ash fired at different temperatures, 450, 700 and 1000°C, was mixed with different concentrations of lime (molar lime/silica ash ratio of 0.2, 0.5 and 1.0). Each dry mixture was first ground and hydrated in the suspension form (water/solid ratio = 10) for various time intervals within the range of 1 to 365 days. The surface properties of the unhydrated and hydrated samples were studied by means of nitrogen adsorption measurements. The results indicated that the surface areas and total pore volumes of unhydrated solid mixtures and hydrated lime-rice husk ash samples, prepared with lime/silica ash ratio of 1.0, decrease with increasing firing temperature of rice husk ash. The effect of varying the lime/silica ash ratio of the solid mixture on the surface area and pore structure was fully discussed. The results of surface area and pore volume measurements could also be related to the crystal structure of silica produced from rice husk ash.

Thermal investigation on electrostatic precipitator kiln dust

Abstract The electrostatic precipitator dust collected from a cement kiln was studied as received from the factory and when fired between 1000 and 1250°C. Two calcines (70:30 and 50:50) were preared by blending the raw dust and kaolin, fired between 1000 and 1250°C, and the resulting phases were also studied. These investigations were undertaken by chemical analysis, DTA and X-ray diffractometry, as well as weight losses. The results revealed that the dust consists of dolomitic limestone, minor and amounts of alkalies and quartz, together with 2 C 2 S·CaCO 3 and 2 C 2 CaSO 4 . The dissociation of these phases occurred at 1000°C while volatilization of alkalies occurred at ⩾ 1200°C. The 70:30 (dust:kaolin) calcine is found to be more suitable, being composed mainly of gehlnite, while the other calcine is mainly composed of β-dicalcium silicate. The appropriate calcination temperature was 1150°C for 2 h, as negligible variations in phase composition could be detected above this temperature.