The effect of the autoclaving process and addition of silica fume on Portland cement in shielding gamma radiation

An autoclaving treatment for disks made of neat Portland cement and cement with 5% silica fume hardened pastes was carried out to obtain suitable shield blocks for gamma radiation sources. The attenuation parameters such as the total linear attenuation coefficients, the mean-free-paths and the half-value thickness for different thicknesses at various periods of autoclaving were obtained. It was found that the attenuation by the neat cement disks autoclaved for 12 h and of 13.8 cm thickness and the disks of cement with 5% silica fume autoclaved for 6 h and of 12.9 cm thickness were the same and identical to that obtained by 3 cm thick sheets of lead. These results are interpreted by measuring the bulk density of the samples and are supported by measuring the compressive strength for the hardened samples.     

Thermal resistance,microstructure and mechanical properties of type I Portland cement pastes containing low-cost nanoparticles

This study aimed to utilize laboratory-prepared nano-silica (NS) and nano-alumina (NA) as low-cost nano-oxides additions for improving the mechanical properties and thermal resistance of hardened ordinary Portland cement (OPC) pastes. NS was synthesized from rice husk ash in the absence of any surfactant, while NA was synthesized from AlCl₃ in the presence of CTAB as a surfactant. The average particle sizes of synthesized NS and NA were 30 and 40 nm, respectively. Nano-silica or nano-alumina was added to OPC as a single phase with different ratios of 0.5, 1, 2 and 3 by mass % of OPC. The physico-chemical characteristics of different OPC-NS and OPC-NA hardened pastes were studied after 1, 3, 7, 14, 28 and 90 days of hydration. The resistance of the hardened composites for firing was evaluated for specimens cured for 28 days under tap water and then fired at 300, 600 and 800 °C for 3 h. The fired specimens were cooled by two methods: gradual cooling and rapid cooling. The compressive strength test was performed for all mixes at each firing temperature. The compressive strength results revealed that the optimum addition of NS is 1, whereas the optimum addition of NA is 0.5 by mass % of OPC. XRD, TG/DTG and SEM results indicated that ill-crystalline and nearly amorphous C–S–H, C–A–S–H and C–A–H were the main hydration products.     

Effect of thermal treatment of rice husk ash on surface properties of hydrated portland cement-rice husk ash pastes

Portland cement was mixed with rice husk ash (RHA) fired at 450, 700 and 1000C, in ratios of 5, 15 and 25% of RHA by mass. Cement-RHA pastes were made by using a water/solid mixture ratio of 0.30 by mass and then cured for various hydration periods within the range 3 to 90 days. The surface properties of the hydrated samples were studied by means of the nitrogen adsorption technique. The results indicated that the hydrated cement-RHA samples made from rice husks fired at 450 and 700C gave higher values of surface area than that for the hydrated cement-RHA sample made from rice husks fired at 1000C. The surface area and pore volume results could be related to the pore structure of the silica produced in the RHA, as controlled by its predicted degree of crystallinity.     

Characteristics of lime-silica fume mixtures

The hydration of two calcium hydroxide-silica fume mixtures was studied at 25°C. The mixtures were prepared at lime/silica molar ratios of 1.0 and 1.7. The free lime, free silica and chemically combined water contents were determined after various periods of hydration (0.5 h-90 days). Thus, the molar ratios CaO/SiO₂ and H₂O/SiO₂ molar in the calcium silicate hydrates (C-S-H) formed could be derived. The hydrates formed were identified by using differential thermal analysis. The mechanism of the hydration-gardening thermal analysis. The mechanism of the hydration-hardening reaction between lime and silica fumes was suggested. The changes in the molar ratios CaO/SiO₂ and H₂O/SiO₂ in the C-S-H formed with the time of hydration were found to follow the same trends as observed during the hydration course for the suggested mechanism.     

Thermal and Pore Structural Characteristics of Polymer-impregnated and Superplasticized Cement Pastes

Two sets of hardened cement pastes were prepared by (a) impregnation with polymethyl methacrylate and polystyrene or (b) admixing with water-soluble condensates(superplasticizers) such as Na-phenol sulphonate formaldehyde, Na-polystyrene sulphonateand Na-β-naphthol sulphonate formaldehyde. The pastes were hydrated for 180 days. The results of nitrogen adsorption indicated that polymer impregnation strongly affected the specific surface areas and the total pore volumes of the hardened cement pastes. XRD analysis, DTA and TG demonstrated (a) the formation of new phases as a result of the interactions involving the polymer within the pore system of the hydrated products of the impregnated cement pastes, and (b) no change in the phase composition of the hydrated products of the cement pastes admixed with superplasticizers. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.