S.A. Abo-El-Enein

Effect of temperature on physical and mechanical properties of concrete containing silica fume

Abstract Fire is one of the natural hazards which attack building construction. The damage in buildings which continuously exposed to fire are caused due to its high temperature. Thus the effect of high temperatures on physical and mechanical properties of concrete was investigated. In this study ordinary Portland cement has been partially replaced by ratios of silica fume. The heat treatment temperature varied from 100 to 600°C by increments of 100°C for three hours without any load. Concrete specimens were treated at each temperature level. The specimens were heated under the same condition for each temperature level. Comparison between physical and mechanical properties during heat treatment were investigated. All specimens were moist-cured for 28 days after casting. Tests were carried out on specimens cooled slowly to room temperature after heating. Results of this investigation indicated that the replacement of ordinary Portland cement by 10% silica fume by weight improved the compressive strength by about 64.6%, but replacment of ordinary portland cement by silica fume by ratios 20 and 30% improved the compressive strength by only 28% at 600°C. This could be attributed to the additional tobermorite gel (CSH phase) which formed due to the reaction of silica fume with Ca(OH)2.

Effect of temperature on phase composition and microstructure of artificial pozzolana-cement pastes containing burnt kaolinite clay

Abstract The influence of high temperature on the phase composition and microstructure of cement paste is vital for fire-resistance studies. Pozzolana tends to improve the microstructure of cement paste, due to its reaction with hydrated products. This study aimed to investigate experimentally the change occurring in the phase composition and microstructure of burnt kaolinite cement pastes after being exposed to high temperatures. The kaolinite clays were thermally activated by firing at 850°C for 2 h. The ordinary Portland cement (OPC) was partially substituted for by 0, 10, 20, and 30% of activated kaolinite clay by weight. The treatment temperature varied from 100 to 600°C by increment of 100°C a period of 3 h. The phase composition and microstructure measures were performed by means of differential thermal analysis and scanning electron microscopy. The results of this investigation show recrystallization and carbonation of Ca(OH) 2 ; they also show a deformation of C-S-H and C 4 AH 13 phases.

Surface properties of silicas produced by thermal treatment of rice-husk ash

Abstract Silica samples were prepared by firing rice-husk ash at various temperatures ranging from 500 to 1400°C for a period of 3 h. Adsorption of nitrogen was used to study the surface properties of the silica obtained at each firing temperature. The parameters derived were related to the pore structure and the degree of crystallinity of the silica samples. X-Ray diffraction analysis was also used for the identification of the silica phases produced by thermal treatment of rice-husk ash.

Studies on expansive cement II. Hydration kinetics, surface properties and microstructure

Abstract Hydration kinetics were followed by measuring non-evaporable water and free sulphate contents. Surface areas, total pore volumes and the microstructure of the hardened expansive cement pastes are discussed. Nitrogen and water vapour were used as adsorbates in the measurement of the surface areas and pore volumes and the results obtained are compared with each other. Scanning electron microscopy was employed to study the microstructures of the hardened pastes.

Hydration of low porosity slag-lime pastes

Abstract Slag-lime pastes of low porosity (water/solid ratio of 0.20) were hydrated from 6 hours to 180 days at 20°C. The kinetics and mechanisms of the hydration process were studied from the results obtained in this investigation. The depth of the hydrated layer on the slag particles is found to be thin indicating that the hydration reaction is very slow. The molar compositions of the formed hydrates could also be calculated from the free lime, nonevaporable water and uncombined slag contents. A high lime product (molar C/S+A ratio of 2.5–2.6) is formed during the early stage of the hydration process, then the molar C/S+A ratio drops to a value of 1.5 and finally rises to a value of 1.7 at 180 days. The surface areas and pore volumes of hydrates were determined from water and nitrogen adsorption measurements. For water vapor adsorption, the water molecules in the adsorbed phase seem to be highly oriented in an ordered array. This effect might be associated with the polar character of water molecule, when adsorbed on an ionic surface like high lime hydrate. The results of x-ray diffraction and SEM observations indicate only the formation of ill-cyrstallised hydration products.

Application of microbial biocementation to improve the physico-mechanical properties of cement mortar

Abstract Calcite is one of the most common and wide spread mineral on Earth constituting 4 wt% of the Earth’s crust. It is naturally found in extensive sedimentary rock masses, as lime stone marble and calcareous sandstone in marine, fresh water and terrestrial environments. Calcium carbonate is one of the most well known mineral that bacteria deposit by the phenomenon called biocementation or microbiologically induced calcite precipitation (MICP). Such deposits have recently emerged as promising binders for protecting and consolidating various building materials. Microbially enhanced calcite precipitation on concrete or mortar has become an important area of research regarding construction materials. This study describes a method of strength and water absorption improvement of cement–sand mortar by the microbiologically induced calcium carbonate precipitation. A moderately alkalophilic aerobic Sporosarcina pasteurii was incorporated at different cell concentrations with the mixing water. The study showed that a 33% increase in 28 days compressive strength of cement mortar was achieved with the addition of about one optical density (1 OD) of bacterial cells with mixing water. The strength and water absorption improvement are due to the growth of calcite crystals within the pores of the cement–sand matrix as indicated from the microstructure obtained from scanning electron microscopy (SEM) examination.

Studies on water and nitrogen adsorption on hardened cement pastes I development of surface in low porosity pastes

Abstract Adsorption studies of water vapor at 35°C and nitrogen at −196°C were performed on low porosity portland cement pastes of ages between 1.5 hours and 3 months. Nitrogen areas and total pore volumes were found not only to be much smaller than water areas as previously published, but their changes with paste age are basically different. Calculations of hydraulic radii refute the hypothesis that the areas inaccessible to nitrogen are interlayer areas. From both water and nitrogen results, it was possible to speculate qualitatively on the mechanism of early hydration in low porosity pastes.

Utilization of microbial induced calcite precipitation for sand consolidation and mortar crack remediation

Abstract The microbes can hydrolyze urea by urease enzyme to produce ammonium as well as carbonate ions and in the presence of calcium ions which can precipitate calcium carbonate; this process is called “biocalcification” or microbial induced calcite precipitation (MICP).This technology is environmentally friendly not only because it gives strength to sand body, but also it allows water to penetrate to sand body, which is unlike silicate cement that will destroy the ecosystem of the earth. Calcium carbonate precipitated by bacteria acts as a binding material to sand particles, so incompact sand will be consolidated. Calcium chloride, calcium acetate and calcium nitrate (1 M) as calcium sources were tested for their ability to consolidate sand by mixing with urea (1 M) and bacteria cells (one optical density, 1 OD). The key point of this study aimed to choose the suitable calcium source which produces higher compressive strength and lower water absorption. The results showed that the degree of crystallinity and amount of precipitated calcium carbonate, as well as the consequent increase in strength of consolidated sand, in case of calcium chloride medium are higher than those precipitated in case of calcium acetate as well as calcium nitrate media. In addition, consolidated sand by calcium chloride was also used for cement mortar crack remediation.

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.

Effect of silica fume on the phase composition and microstructure of thermally treated concrete

Abstract The influence of elevated temperature on the microstructure of concrete is vital for fire-resistance studies. Silica fume tends to improve ordinary concrete properties, due to its reaction with hydrated products. This study aimed to investigate experimentally the change occurring in the phase composition and microstructure of concrete pastes containing silica fume after heat treatment. The temperature varied from 100 to 600 °C by increment of 100 °C for three hours period. The investigation was performed by means of the X-ray diffraction analysis, differential thermal analysis and scanning electron microscope. The results of this investigation show that, additional hydration of unhydrated cement grains, recrystallization deformation and transformation of CSH phases were occurred.