International Concrete Abstracts Portal

Concrete samples made with and without blast furnace slag (BFS) and stored in saturated gypsum solutions for 40 years have been studied. After this long period of time, there still remains small amounts of anhydrous cement and unreacted BFS grains. The reacted BFS are easily identified by EPMA analyses by considering the Mg concentration ; Mg does not diffuse away from the BFS grains. On the contrary Al diffuses away from the BFS grains towards the paste, whereas Ca tends to diffuse from the paste towards the BFS grains. The evolution of Ca, Al, Si and Mg concentrations from slag grains to the paste, or to cement grains, can be described by a succession of local equilibria defined in the CaO-Si02 -A12 03 -MgO-S03 -H2 0 system. The formation of different phase assemblages in the presence of BFS may account for the finer pore structure found despite similar total porosities for both concretes. Moreover a calcite layer has been formed : this layer limits sulfate ions ingress into the paste for both concretes.

Progressive utilisation of silica fume in concrete reactor containment walls that are susceptible to accident conditions, requires the study of leak tightness and the integrity of such structures, when exposing them to high temperatures. This paper deals with the influence of the silica fume dosage rate on the permeability coefficient for temperatures ranging from room temperature to 500°C. The use of silica fume reduces the oxygen permeability of concrete by 2 to 3.6 times at room temperature. When concrete, with or without silica fume, is exposed to heating up to 500°C free and bound water are released progressively; most hydrates are decomposed, the porous structure is modified and finally heating cracks are created, leading to concrete damage and an increase in the permeability coeffkient. However, the advantage of silica fume addition to decrease the loss of leak tightness integrity, depends strongly on the use of the optimum dosage rate of silica fume (i.e. 6-7% in these tests). The optimum dosage rate of silica fume can be estimated from 28 days compressive strength tests using silica fume dosage rates between 5 and 15%. A higher silica fume dosage rate leads to higher evaporable water content at T=lO5“C and for temperatures above 105’C leads to higher decomposing CSH, which causes the loss of more weight at any given temperature.

Metakaolin-blended cements were used to stabilize two kinds of wastes: municipal incinerator fly ash and latex waste coming from the carpet industry. The physical and chemical properties of wastes were investigated and mortars containing these wastes were cast. A blended cement containing 20 % metakaolin was utilized to immobilize the incinerator fly ash which was introduced as a sand substitute in mortars. Leaching tests and microstructural investigations were carried out as well as mechanical tests. The results obtained show that metakaolin plays a beneficial effect in the stabilization of chlorides contained in such fly ash. Cements containing from 10 to 30 % metakaolin were used to cast mortars, in which latex was introduced as a sand substitute. Metakaolin reacted with the calcite contained in the latex and the C3A of portland cement to form carboaluminate. Leaching tests showed a good immobilization of this waste.

This paper presents a research carried out to convert paper sludges from de-inking and water-treatment processing plants into a pozzolanic product usable in the cement and concrete industries. Paper sludges contain inorganic fillers like ground limestone, kaolinite, clay, and organics. The process consists in heat treatment in the range of 600 to 700°C in order to transform the kaolinite present in the sludge into metakaolinite, a very reactive pozzolan, without the formation of large amounts of free lime (due to the decarbonation of the calcite also present in the sludge). In addition, the organic compounds in the sludge must be burned off and their presence reduces the use of fossil fuels to reach the temperature needed for processing. The results of both laboratory and field tests show the feasibility of the process. A reactive pozzolan is obtained when the amount of kaolinite present in the sludge is higher than 20 % of the dry inorganic phase. This metakaolin is sometimes more reactive than that obtained by calcining pure kaolinic clays, and can be used to enhance the durability of concrete in severe environments.

Concrete in contact with a sulfate environment can severely degrade due to expansion accompanying the formation reaction products such as ettringite. The use of Portland-pozzolan cement has been successful in mitigating this expansion. However, it is important to study the effectiveness of natural pozzolans to improve the resistance to attack by sulfates. This paper reports results of different portland-pozzolan cements containing different natural pozzolans and ASTM Types I, II, and V portland cements. The pozzolanic activity and composition of each pozzolan was evaluated. The susceptibility to sulfate attack was studied by measuring the expansion in mortar bars at different ages according to ASTM Method 1012 for 78 weeks. It was found that cements containing pozzolans with high activity or low alumina content improved the sulfate resistance. Also, the pozzolan content in the cement was found to be important.