International Concrete Abstracts Portal

Penetration of chloride ions from the environment into reinforced concrete is important in relation to corrosion behavior of embedded steel. Blended cements containing ground granulated blast furnace slag (GGBFS) are expected to offer a greater degree of protection compared to that of OPC. The kinetics of chloride ion diffusion through hardened cement pastes made from SRPC, OPC, and OPC/GGBFS blends have been determined by a steady-state (thin disc) method. Concrete slabs containing similar cements and quartzite aggregate have been made and ponded regularly with 5 percent NaCl solution. Material taken from various depths within these slabs has been subjected to pore solution expression and analysis, and the concentration profiles of free and total chloride have been determined. Values of chloride diffusivity obtained by the steady-state method have been used to calculate the chloride concentration profiles expected when penetration is into a semi-infinite medium. Comparison between the two techniques shows the same general trends in relative performance of the various cements, but actual chloride concentrations, at a given depth, are greater in the concrete slabs. Results from total and free chloride measurements indicate that the chloride-binding capacity of slag cements exceeds that of OPC and SRPC.

During hydration of portland cement clinker and granulated slag in portland blast furnace slag cement, finely dispersed calcium silicate hydrates are formed as the major constituent of hydrated cement paste. With increasing slag content of cement, more C-S-H phases are formed, contributing to the well-known dense pore structure of pastes made of PBFS cements. Upon carbonation of the hydrated cement paste, all alkaline compounds are decomposed to form carbonates. Furthermore, the decomposition of CSH results in the formation of a porous silica gel. In an experimental investigation, different types of hydrated cement paste, mortars, and concretes manufactured with portland cement and portland blast furnace slag cements with different slag contents were subjected to carbonation and the resulting changes in the pore structure monitored. These tests demonstrated that the silica gel formed during carbonation shows pores in the range of approximately 300 nm pore radius. Where large quantities of silica gel are formed, carbonation leads to a coarser pore structure compared to the original structure. Permeability of these systems then increases significantly. The porous silica gel, however, proved to be reactive. Upon access of alkalies, new C-S-H phases may be rebuilt with a very fine port size distribution with pore radii ó 10 nm.

Gives results of an investigation on the chloride ion permeability of concretes incorporating supplementary cementing materials, using the Rapid Determination of Chloride Permeability Test (AASHTO T277-83). A total of 18 concrete mixtures were made. These included mixtures incorporating silica fume (8 percent replacement or addition to the cement by mass) or ground granulated blast-furnace slags (50 percent replacement by mass), or fly ash (25 percent replacement by mass). The w/c of the mixtures investigated ranged from 0.21 to 0.71. From each mixture, a number of 152 x 305 mm cylinders for compressive strength testing and 102 x 203 mm cylinders for determining the chloride permeability were made. Porosity measurements were also performed on some of the concrete specimens. The test results showed that the use of supplementary cementing materials significantly reduced the chloride ion permeability of concrete. Silica fume and blast furnace slags investigated seem to be particularly efficient for producing concrete almost impermeable to chloride ions.

An extensive research program was performed on the rate of carbonation of concrete produced with ordinary portland cement, portland blast furnace slag cement, and portland fly ash cement, both with replacement of cement by fly ash and without. After various periods of wet curing, concrete samples were exposed to various exposure conditions. The wet curing ranged from 1 to 90 days, the exposure concerns outdoor sheltered from rain and at 20 C, 65 percent relative humidity in the laboratory. For the various exposure conditions, a relation has been found with respect to the carbonation rate as a function of the compressive strength at 7 days or at 28 days per type of cement and for all types of cement when the lime content is involved. Results of measurements over a period up to 2 years are presented.

Chemical characteristics of the cement paste-aggregate interfacial zone have been considered to influence the durability and mechanical properties of concrete. Particularly, effects of mineral admixtures such as fly ash and slag on the microstructure of the interfacial zone deserve attention. An x-ray diffraction technique was used to evaluate the amounts of Ca(OH)2, ettringite, and the orientation of Ca(OH)2 crystals in the interfacial zone. Composite specimens with several types of rocks were broken to produce a fracture surface on the cement paste prism to which the x-ray diffraction analysis was applied. The analyses showed that the addition of fly ash or slag considerably affected the peak height and orientation of Ca(OH)2 crystals in the interfacial zone, which normally extends up to 50 to 100 æm from the interface. The formation of ettringite in the vicinity of the aggregate surface was restricted by the addition of the admixtures. These results also suggest that the addition of the mineral admixtures favorably affects the resistance of the interfacial zone against aggressive agents from the surroundings. The x-ray fluorescence analysis was conducted to quantify calcium and silicon in the zone. The results obtained complemented the conclusions described previously. 137-389