International Concrete Abstracts Portal

A study was made of the effects of Saskatchewan lignite fly ash on the resistance of concrete to freezing and thawing. Concrete was made with either ASTM Types I or V cement and different percentages of fly ash with an air content of 4 to 6 percent. Performance of the concrete was evaluated by measuring the changes in its dynamic modulus and its mass. A scanning electron microscope was also used to examine the changes in the microstructure of the cement paste due to exposure to freezing and thawing. Results show that the use of high percentages of fly ash in concrete (35 and 50 percent) reduced its resistance to freezing and thawing even though it contained about 6 percent air and was cured in water for 80 days. However, concrete containing 20 percent fly ash gave satisfactory performance, provided its air content and strength were comparable to control concrete that contained no fly ash. Results from the SEM examination show that the decrease in resistance of fly ash concrete to freezing and thawing may be due to the slow migration of portlandite and ettringite crystals from the dense C-S-H zones to the air voids. Concrete with fly ash was less susceptible to the migration of portlandite, but its air voids contained more fibrous hydrates, which may have led to an increase in the past porosity.

The use of high-volume fly ash as a supplementary cementing material in controlling alkali-aggregate reactivity is an attractive solution. Fly ashes are often used in reducing the expansions due to alkali-aggregate reaction in concrete. However, in the past, only smaller quantities of fly ash, less than 30 percent by weight of cement, have been used. This paper presents the results of a study to determine the influence of very high quantities of fly ash in reducing the expansion due to alkali-aggregate reactions. Ten samples of sands collected from various locations in South Dakota were tested for alkali-aggregate reactivity using both standard ASTM C 227 and accelerated test methods. Five of the sands that caused greater expansions than permitted were tested with high fly ash contents, using the accelerated test method. Cements satisfying ASTM Type I and a low-calcium fly ash (ASTM Class F) were used for the entire investigation. The water/fly ash + cement ratio was 0.44 and the fly ash/fly ash + cement ratios expressed as percentages were 40, 50, 60, and 70. Control mortar specimens containing the same Type I cement and alkali content were used for comparison. An accelerated test method proposed by the Canadian Standards Association was used for the detection of potentially deleterious expansion of mortar bars. The test results had shown that high fly ash replacement levels were very effective in reducing the expansion due to alkali-aggregate reaction. The expansions of the mortar bars made with the highly reactive sands and high volumes of fly ash were negligible as measured in the accelerated test method.

A considerable amount of natural zeolite has been used as an admixture for portland cement in the People's Republic of China. Paper first deals with a comprehensive characterization of inorganic admixtures such as natural zeolites with different mineralogical compositions, a fly ash, a fine blast furnace slag, and a silica fume. Binders, such as ordinary portland cement and a quick lime for the substitution of portland cement, were also subjected to the characterization. Next, bending and compressive strength and drying shrinkage of the test mortars were measured under the constant flow value. Standard test mortars were prepared by making use of the ordinary portland cement and quick lime-substituted portland cement, and blended cement mortars were also tested with the inorganic admixtures previously mentioned. As a result, natural zeolite was proven to be of sufficient applicability as an admixture for cement.

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.

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.