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Study examines the microstructure properties of cement-based grout consisting of Type II rapid-hardening portland cement, Saskatchewan fly ash, and brine. The liquid brine is composed mainly of salts of sodium, calcium, potassium, and magnesium obtained from an underground potash mine. A scanning electron microscope (SEM), with an electron probe x-ray microanalyzer, was used to study the mechanism by which fly ash and brine alters the microstructure characteristics of cement grouts under confining pressures of 0, 3.4, and 6.9 MPa (0, 500, and 1000 psi). The SEM examination was conducted at 7, 14, and 365 days. This examination revealed that grout mixes containing brine had a gel-like substance covering the entire surface of the hydrated products. The probe x-ray microanalyzer identified the gel-like substance as consisting mainly of sodium chloride salt. Fly ash cement particles were also found to be encapsulated by the sodium chloride gel-like substance. This encapsulation may decrease the rate of pozzolanic reaction between fly ash particles and the lime available in the cement. Microscopic examination of specimens mixed with brine also showed the presence of long fibrous crystals with diameters ranging from 3 to 20 æm growing on the surface of the gel-like substance. Generally, at 7 and 14 days, the fly ash-cement grouts were found to have more such fibers than the grout containing no fly ash. This trend reversed at 365 days.

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.

Heavy damage due to alkali-aggregate reaction has been observed in concrete structure in and along the sea. An accelerated test is performed on mortar to evaluate effectiveness of fly ash for controlling alkali-aggregate reaction in the marine environment. Mortar bars using Pyrex as aggregate and cements with 0.6 and 1.1% of equivalent sodium oxide are made. The alkali content in the mixture is adjusted by adding NaOH or NaCl. Specimens are stored in distilled water, NaCl solution, and under more than 95% of relative humidity. The controlling effect of fly ash and the effect of internal and intruded chloride ion in mortar on alkali-aggregate reaction is studied by measuring the expansion of mortar. Expansion of mortar depends on the type of cement and chemical reagents used for alkali adjustment, the amount of fly ash used and the exposure condition. Even with the same equivalent sodium oxide in the mixture, mortar using NaCl for alkali adjustment shows higher expansion than mortar using NaOH. The highest expansion is revealed for mortar cured in NaCl solution. The controlling effect of fly ash also depends on the type of cement and the exposure condition.