Essam A. Kishar

Magnesium sulfate attack on hardened blended cement pastes under different circumstances

This paper describes the sulfate resistance of some hardened blended Portland cement pastes. The blending materials used were silica fume (SF), slag, and calcium carbonate (CaCO3, CC). The blended cement pastes were prepared by using W/S ratio of 0.3. The effects of immersion in 10% MgSO4 solution under different conditions (room temperature, 60 °C, and drying–immersion cycles at 60 °C) on the compressive strength of the various hardened blended cement pastes were studied. Slag and CC improve the sulfate resistance of ordinary Portland cement (OPC) paste. Mass change of the different mixes immersed in sulfate solution at 60 °C with drying–immersion cycles was determined. The drying–immersion cyclic process at 60 °C accelerates sulfate attacks. This process can be considered an accelerated method to evaluate sulfate resistance of hardened cement pastes, mortars, and concretes.

Studies on the stability of the calcium sulfoaluminate hydrates. Part 1: Effect of temperature on the stability of ettringite in pure water

Abstract The stability of the ettringite phase in relation to its solubility in pure water has been studied at 30°, 60° and 100° for a time interval of 14 days. Ettringite is stable up to 60° in a solution of a pH value around 11. Its solubility increases with temperature. It converts to the low sulfate form after one hour boiling which decomposes further to gypsum after 14 days. Gypsum appears first when the pH of the solution is lowered to 9 and the aluminium concentration clearly decreases.

Effect of sodium salt of naphthalene-formaldehyde polycondensate on ettringite formation

The suspension hydration of C3A with gypsum (in the molar ratio of 1:3) was investigated at room temperature and water/solid ratio of 4. The hydration was carried out in presence of 0, 1, and 3% sodium salt of naphthalene-formaldehyde polycondensate and the mixes were designated as I, II, and III, respectively. The only hydration product formed in the presence and absence of the superplasticizer was ettringite. The rate of ettringite formation was retarded by the presence of the superplasticizer. This effect was more pronounced at high dosage of the superplasticizer only during the first 24 h. The presence of sodium salt of naphthalene-formaldehyde polycondensate caused a decrease in the size of the formed ettringite crystals, and as the percentage of the admixture increased, the crystal size decreased. In addition, there was an interaction between the used superplasticizer and the formed ettringite as indicated from infrared analysis.

Studies on the Stability of the Calcium Sulfoaluminate Hydrates. Part II: Effect of Alite, Lime, and Monocarboaluminate Hydrate

The variation of the solubility values of ettringite in the presence of alite, lime, and monocarboaluminate hydrate at 30 and l00°C is studied. The solubility product of ettringite is found to be 1.98 × 10−37 in pure water, 1.11 × 10−40 in the presence of alite with a mole ratio of l0, and insoluble in the presence of saturated lime solution as well as in an excess amount of monocarboaluminate hydrate. The ettringite phase reacts with the calcium silicate hydrates with the weakening of its X-ray diffraction intensity; at the boiling water temperature the tobermorite phase forms, in the presence of lime or monocarboaluminate hydrate the hydrogarnet phase appears beside the monophase solid solution.

Studies on the stability of the calcium sulfoaluminate hydrates, Part III: The monophases

Abstract The effect of ettringite, monocarboaluminate hydrate, alite, and lime on the solubility of the monosulfate hydrate is studied at 30 and 100°C. At the same temperatures, the stability of the monocarboaluminate hydrate in presence of alite and lime is investigated. At 30°C both types of calcium sulfoaluminate hydrates exist with a total of 0.8 mol sulfate in equilibrium with 0.2 mold sulfate dissolved in solution. The ettringite phase appears with a sulfate content exceeding 3 mol. The monosulfate hydrate resists longer a boiling solution of its mixture with ettringite or lime than with alite or monocarboaluminate hydrate. The hydrogarnet phase appears preferentially in the monocarboaluminate hydrate-bearing systems. The solutions compositions are reported.

EFFECT OF MIXING AND CURING WATERS ON THE BEHAVIOUR OF CEMENT PASTES AND CONCRETE. PART 1: MICROSTRUCTURE OF CEMENT PASTES

Abstract The influence of Sea, ground, Nile and drinking waters on the microstructure of some hydraulic cements is investigated by means of X-ray diffraction analysis. The calcium sulfo- and carboaluminate hydrates form well with a water cement ratio of 2. Seawater leads to the precipitation of gypsum. The hydration products of ordinary portland cement (OPC) pastes hydrated for a year with the four water types are similar. A survey on the analysis of waters used is given.

Inertization of lead by using blended cement pastes

Abstract Inertization of lead nitrate Pb(NO3)2, a representative of soluble lead compound, with five cements pastes based on either ordinary Portland cement (OPC) or blended Portland cement with granulated blast-furnace slag (GBFS) or metakaolin (MK) was studied. Various mixes were prepared by using a water/solid ratio (W/S) of 0.28 (by weight). Two ratios of Pb ions (1.0% and 2.0% of the solid binder) was used by adding to the mixing water. Hydration characteristics of the different cement pastes were investigated via the examination of chemically combined water content, compressive strength, X-ray diffraction analysis and thermal analysis (DTA/TGA). Leaching of lead ions from various hardened cement pastes was examined. The results showed that lead nitrate retards cement hydration through the formation of plumbate salt CaPbO3. The obtained results of leaching showed a high degree of immobilization of Pb ions in the various cement pastes. Moreover, partial replacement of Portland cement by MK is more efficient than granulated blast-furnace slag GBFS.

Removal of copper ions by Friedel´s salt

Friedels salt (3CaO.Al2O3.CaCl2.10H2O) or (Ca4Al2(OH)12Cl2(H2O)4), a layered double hydroxide (LDHs), is a calcium aluminate hydrate and used as an adsorbent for copper (Cu 2+ ) ions from aqueous solution .In our study,we carefully examined the adsorption behaviors of Friedels salt toward different concentrations of copper solutionat pH = 4. Both experimental and modeled data indicate that Friedels salt can adsorb all copper ions from solution at pH=4 for total [Cu/mg] in solution= 724.8 mg/l (3mM/l) with high efficiency ˃99.9 % of adsorbent FS.The FS, at different pHs,shows a higherstability at pH=6 thanat pH=4 andpH=8. Finallythis study suggests that Friedels salt is a potential cost – effective adsorbent for Cu [ІІ] removal.

Effect of nano-SiO2 (NS) on dolomite concrete towards alkali silica reaction

Abstract The aim of this work was to study the effect of nano silica on the physico-mechanical properties of concrete containing dolomite coarse aggregate and its effect on alkali silica reaction. In this work, Portland cement is replaced by 1, 2 and 4 wt.% nano silica (NS). It is clear that nano silica up to 2 wt.% leads to increase the compressive strength and decrease the total porosity up to 180 days of curing in tap water. This is due to that nano silica content acts as pozzolanic material and nucleating sites for the hydration products, which enhances the cement hydration rate. A 4 wt.% NS led to reduce the mechanical properties but is still higher than dolomite control because it leads to coating of the cement grains, retarding its hydration rate and therefore, decreases the hydration products formation. The resistivity of dolomite concrete control against 1 N NaOH up to 6 months of immersion has been carried out. The results show that all concrete mixes have high resistivity towards 1 N NaOH up to 6 months of immersion. The compressive strength development rate of dolomite concrete control containing 2 wt.% of nano silica is higher than that of control. This is due to the continuous activation and accumulation of hydration products. This indicates that the NaOH has marginal compacts on the mechanical properties reduction, and this result confirmed that dolomite aggregate has higher resistivity to alkali silica reaction. XRD and SEM photograph are applied to investigate the formed hydration products.

Immobilization of Co(II) ions in cement pastes and their effects on the hydration characteristics

Abstract The immobilization of Co (II) in various cement matrices was investigated by using the solidification/stabilization (S/S) technique. The different cement pastes used in this study were ordinary Portland cement in absence and presence of water reducing- and water repelling-admixtures as well as blended cement with kaolin. Two ratios of Co (II) were used (0.5% and 1.0% by weight of the solid binder). The hydration characteristics of the used cement pastes were tested via the determination of the combined water content, phase composition and compressive strength at different time intervals up to 180 d. The degree of immobilization of the added heavy metal ions was evaluated by determining the leached ion concentration after time intervals extended up to 180 d. The leachability experiments were carried out by using two modes: the static and the semi-dynamic leaching processes. It was noticed that the concentration of the leached Co 2+ ions in the static mode of leachability was lower than the solubility of its hydroxide in all the investigated cement pastes.