International Concrete Abstracts Portal

Presents the results of a study to evaluate the potential of modified fly ash for use as a construction material. Areas of utilization investigated were: structural fill for embankments and dykes; lining material for canals, waste disposal sites, etc.; replacement of cement in concrete; and cement-stabilized base and subbase course. This modified fly ash was evaluated mainly on the basis of its compressive strength at various ages. Availability of only a limited quantity of fly ash necessitated use of miniature specimens for strength test. Results indicate that modified fly ash with and without cement developed adequate compressive strength to be used as fill material in embankments and dykes. Compacted modified ash also exhibited very low permeability characteristics. Replacement of 35 percent cement by modified fly ash produced 20 percent more strength than portland cement mix at 28 days.

High-strength and ultra high-strength cast-in-place concretes tend to contain excessive unit volumes of cement when compared with normal concrete, and since the improvement of workability relies largely on the efficiency of the air-entraining and high-range water-reducing admixtures, the properties of workability (or consistency) are different from normal concrete. With high-strength concrete, it was found that the method of mixing concrete influenced flowability, strength properties, and pore structure; details of this influence are given.

The mechanical and structural properties of mortars containing silica fume were studied. Mortars containing 5 and 10 percent active silica additive were made. Mortars without silica fume (standard mortars) were also prepared. A first set of mortar specimens was cured entirely in water. A second set of mortars was cured in air. The third was immersed in water and then subjected to alternating cycles of storage in water and air. The results show a very close relation between the conditions of the mortars' curing and their mechanical properties. The flexural strengths of mortars containing silica fume, subjected to variable curing conditions, show periodic increases and reductions. SEM observations confirmed the relations found in the flexural strength tests.

High-lime fly ashes obtained from the combustion of lignites or subbituminous coals are common by-products of thermal power plants in many countries, including Turkey. Chemical analyses, mineralogical analyses, and the formation of hydration and pozzolanic reaction products at different ages of three Turkish high-lime fly ashes were carried out using X-ray diffraction (XRD) techniques. The relationships between the mineral phases in the fly ashes and the hydration and pozzolanic reaction products were investigated. Fly ash is formed at combustion temperatures of approximately 1000 C, at which the clay impurities decompose. These fly ashes contained highly reactive silica and alumina. The reaction of these oxides with the free lime and anhydrite present in two of the fly ashes led to the formation of C-S-H gel and ettringite starting with the beginning of hydration. The third fly ash, having anhydrite as the only major calcium-bearing compound, produced gypsum upon hydration. However, introduction of Ca(OH)2 into the system resulted in similar reaction products. At later ages, beside the previously mentioned products, C4ACH11 and C4AH13 were also observed in all three cases. Interpretation of the results indicated that although all three fly ashes were of high-lime type, two of them were hydraulic and autopozzolanic, whereas the third was pozzolanic only.

The admixtures of condensed silica fumes (CSF) and phosphogypsum (neutralized and dehydrated at 400 C) were used together with fly ashes as blended cement components to improve early strengths and other properties. The cements with the initial 15 to 50 percent low-calcium PFA content (SiO2 + AL2O3 + Fe2O3 - 83.3 percent) or 15 to 70 percent high calcium PFA content (22.1 percent CaO) were mixed with the additional components just mentioned. Standard tests at normal curing were made, as well as measurements after the low-pressure steam treatment at 70 C. All cements mixed with CSF showed standard compressive strengths about 13 to 20 MPa higher than the reference mortars. More detailed studies of the hardening process were also carried out using calorimetry, DTA, TG, XRD, and porosimetry, which showed acceleration of the hydration process due to pozzolanic properties of CSF. Reduction of total porosity and pore size was also found. The same positive effect of CSF was observed in the case of mortars treated at 70 C. This additive improves significantly the pozzolanic properties of low-calcium PFA. At standard curing, activated phosphogysum addition brings about a decrease in the hydrated calcium silicates. A substantial amount of ettringite forms and partially inverts into monosulfate after 28 and 90 days of hardening. At accelerated curing, the mortars containing phosphogypsum show a significantly higher degree of hydration than the reference mortar. The results relating to pastes and mortars have been confirmed for concretes. Therefore, one can conclude that the admixtures studied, particularly CSF, have positive influence on the properties of PFA concretes and help to augment the effect of PFA content in these concretes.