Eisa E. Hekal

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.

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.

Hydration characteristics of Portland cement – Electric arc furnace slag blends

Abstract Utilization of electric arc furnace slag (EAF slag) as blending material for Portland cement has been examined. This was done via the investigation of the hydration characteristic of EAF slag – Portland cement blended mixtures. Various ratios of EAF slag were used namely; 5, 10 and 20 wt% of solid mix. The hydration properties investigated for the various mixtures were; compressive strength, chemically combined water and free lime contents as a function of hydration times. The hydration ages were; 1, 3, 7, 28 and 90 days. In addition, phase composition of the formed hydrates was examined using XRD technique as well as differential thermal analysis (DTA) for some selected samples. The results showed that as the ratio of EAF slag increases the values of compressive strength decrease at all the hydration ages. Hydration kinetics of the investigated mixes was followed by determining the variation of free lime and chemically combined water contents with time of hydration. It was observed that hydration proceeds in four different stages. The values of chemically combined water of the cement pastes blended with EAF slag were less than those of the neat Portland cement paste at all hydration ages. The mode of variation of free lime content with time was nearly similar to that of combined water content. The results of chemically combined water, free lime, XRD analysis as well as thermal analysis were correlated well with those of compressive strength. All these results indicate that the used EAF slag has no significant pozzolanic reactivity.

Utilization of electric arc furnace dust as an admixture to Portland cement pastes

Electric arc furnace dust (EAFD) is termed as a hazardous waste due to its contamination with heavy metals. Inertization of such very fine dust can be occurred via stabilization and solidification process within the hydrated Portland cement matrix. In this paper, the effect of the addition of various ratios of EAFD on the properties of the hardened Portland cement paste was investigated. Compressive strength, chemically combine water and free lime contents were determined. In addition, phase composition using XRD; DTA analysis; as well as microstructure of the formed hydrates for some selected samples were investigated using SEM. The obtained results showed that the paste containing 1/mass% EAFD give the highest compressive strength values at most hydration ages, specially the later ages, compared to the neat Portland cement blank paste. Whileas, the pastes containing 3 and 5/mass% EAFD showed lower values of compressive strength compared to those of the blank paste.

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.

A new method to create one-part non-Portland cement powder

Non-Portland cement or alkali-activated slag is regarded as non-friendly for users, due to the corrosive nature of alkaline solution. This negatively affects the mass production and commercial viability of this cement. In this work, user- and eco-friendly one-part non-Portland cement (NPC) was prepared by blending ground-granulated blast-furnace slag (GGBFS) with stable dry activator (SDA). SDA was synthesized by mixing two moles of NaOH with one mole of MgCO3 or dolomite, followed by drying and pulverizing to a fixed particle size (namely, SDA-M and SDA-D, respectively). The Mg source was chosen to prepare active magnesia with no firing. So, this process was considered as eco-friendly method for this purpose. Ordinary Portland cement (OPC) was used for comparison. The SDA-M content plays a circular role on the reduction of drying shrinkage of one-part NPC. The SDA-M has a potential impact on the compressive strength development of one-part NPC compared to SDA-D. NPC containing 12 mass% SDA-M (NPC-12) gave compressive strength values superior to those of OPC at all curing ages, suggesting that the NPC-12 can be beneficially used as an alternative to OPC as confirmed by X-ray diffraction (XRD), thermogravimetric analysis (TGA/DTG), and scanning electron microscopy (SEM).

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.