International Concrete Abstracts Portal

Cement pastes of interest for high-strength concrete technology were investigated by high-resolution solid state magic angle spinning (MAS) Si-nuclear magnetic resonance (NMR) in combination with thermal analysis (DTA/TG). NMR reveals the degree of hydration for C3S/C2S in cement, pozzolanic activity of condensed silica fume, and average chain length of the silicate anions in the CSH-gel. A combination of NMR and DTA/TC data gives the empirical formula of the CSH-gel. The binders investigated were made from blended portland cement containing 0, 8, and 16 percent cement replacement with condensed silica fume and water-binder ratios of 0.20, 0.30, and 0.40. The specimens were allowed to cure in sealed conditions for 1, 3, 7, 28, 126, and 442 days. The results confirmed that condensed silica fume is a very reactive pozzolan. The conversion rate of condensed silica fume to hydration products after 3 days of curing was, in fact, higher than for the neat cement at the same age. After 3 days of curing, condensed silica fume reduced the degree of hydration of the cement in the blended cement pastes when compared with pastes without it. The effect was enhanced at later ages when the cement hydration process stopped while the pozzolanic reaction continued to near completion. In regard to the composition of the CSH-gel, it was found that the average chain length for the linear polysilicate anions increased with decreasing w(c + s) and, in particular, with increasing dosages of condensed silica fume. Furthermore, the c/s of the gel decreased considerably with increasing dosages of condensed silica fume. The mechanism of the pozzolanic reaction of condensed silica fume is discussed.

The decrease in relative humidity during hydration and the chemical shrinkage have been measured for different cement paste compositions. The amount of nonevaporable water per degree of hydration as found by NMR, pore size distribution by mercury intrusion, and total porosity to water have also been determined. The cement pastes were made form portland cement with 0, 8, and 16 percent condensed silica fume, with w/c + s of 0.20, 0.30, and 0.40. The relative humidity (RH) was found to decrease rapidly during the first 2 weeks and reach about 78 percent RH after more than a year for the lowest w/c + s, independent of the CSF dosage. The highest ratio gave about 87 percent RH. The nonevaporable water per degree of hydration depends on the NMR-based estimate of the degree of cement hydration, but it is most consistent (i.e., independent of w/c + s and CSF dosage) when it is assumed that the CSF dosage does not consume any water. The water porosity was found to increase with increasing CSF dosage, while the mercury intrusion results showed both a finer pore structure and smaller total porosity with increasing CSF dosage. Mercury intrusion into miniconcretes (dmax = 8 mm) with the same binders gave a much coarser pore size distribution, indicating that the paste-aggregate interface region is more open than the bulk paste. No evidence was found that increased CSF dosage improved the interface pore structure. This is in contrast to other evidence in the literature, and may be caused by partial dehydration and/or microcrack formation during the drying at 105 C.

Condensed silica fume (CSF) greatly influences not only the mechanical but also the physical properties of concrete. The most striking effect is the reduced permeability, caused by a change in the pore structure. Another sign of this alteration, though not as evident, is the change in the form of the water vapor isotherm. Preliminary results from an investigation concerning the first desorption isotherms of mortar with CSF-cement ratio varying between 0 and 25 percent and a water-cement ratio varying from 0.3 to 0.6 are presented. The results show that CSF influences the pore size distribution not only in the mesopore range, as shown in earlier studies, but also in the micropore range. The drying courses were also recorded in the project and it is clear that CSF significantly prolongs the time in reaching equilibrium, especially in relative humidities below 80 percent. This indicates that the continuous pore system is much narrower when CSF is incorporated. The question of when the "true" equilibrium is attained is discussed.

Results of an investigation of cement paste structure, and strength, permeability, and frost resistance of concrete with admixtures of silica fume type are given. The admixtures are waste materials from metallic silicon, low-grade ferrosilicon, ferrosilicon chrome production, containing SiO2 in the amount of 92, 70, and 66 percent, and surface area of 25.0, 44.9, and 18.5 mý/g, respectively. The influence of the admixtures on the cement paste microstructure results in an increase of gel porosity volume, decrease of capillarity porosity, and in an increase of strength. Thus, concrete strength increases and its permeability decreases. Physical and chemical properties of the silica fume-type admixtures insignificantly affect gel pore volume, whereas they have significant influence on capillary porosity. An increased dosage of high-range water-reducing admixture (HRWR) is a beneficent factor in increasing hydration degree and gel porosity, decreasing capillary porosity volume, and, consequently, increasing strength. Concrete frost resistance with silica fume dosages up to 10 percent of cement mass is not lower than the reference concrete with the same amount of binder.

The interface between the cement matrix and aggregate is mostly regarded as a weak link in concrete with respect to durability and strength. It is shown that the positive effects of pozzolans on the permeability of concrete are partly related to a decrease in the thickness of the weak, lime-rich, interfacial zone. Results for various mineral admixtures, such as ground granulated blast furnace slag, powder coal fly ash, silica fume, a synthetic colloidal silica, and metakaolinite are presented. It is shown that in the presence of mineral admixtures, the calcium hydroxide content in the interfacial zone is reduced substantially.