Mitsunori Kawamura

Effects of fly ash and silica fume on the resistance of mortar to sulfuric acid and sulfate attack

Changes in physical and chemical properties in the mortars with different replacements by fly ash and silica fume when immersed in 2 % H[sub 2]SO[sub 4], 10 % Na[sub 2]SO[sub 4] and 10% MgSO[sub 4] solutions for 3 years were investigated. The long-term exposure test data showed that the replacement of portland cement by fly ash and silica fume effectively improved the resistance of the mortar to the sulfuric acid and sulfate solution attack. However, the replacement percentage by fly ash and silica fume necessary to prevent the sulfuric acid and sulfate solution attack varied significantly depending on the type of sulfuric acid and sulfate solutions.

ELECTRICAL DOUBLE LAYER, ION TRANSPORT AND REACTIONS IN HARDENED CEMENT PASTE

Abstract In this paper positive ion concentrations in the Gouy-Chapman part of electrical double layers have been estimated both for calcium silicate hydrate and reactive silica grains when each of which is embedded in a hardened cement paste. The estimated positive ion concentrations, especially that of calcium ions, are much higher than that in corresponding pore water. These estimates together with some experimental results indicate that in low water/cement ratio pastes most, if not all, positive ions are transported through these overlapping double layers. This preferential accumulation of calcium ions in the electrical double layers together with their normal mobility can explain a number of unexplained observations, e.g. rapid growth of calcium hydroxide crystals in cement paste, calcium and alkali ion transport to reactive silica grains etc. of cement chemistry. This high concentration of calcium ions in the double layer also explains why calcium ions co-diffuse together with other ions in any diffusion of salts through hardened cement pastes. Attention has also been drawn to the implications of the double layer characteristics to other aspects of cement chemistry. These estimations show that pore water analyses could not be used directly, without further processing, for any interpretive work.

ASR gel composition and expansive pressure in mortars under restraint

It is significant for deeper understanding of features of ASR damages of concrete structures to reveal relations between expansive pressure under restraint and free expansions of mortars in the laboratory. One of the purposes of this study is to estimate the amount of ASR gel produced within mortars by the combination of EDS analysis for the gels and pore solution analysis. In addition, this study aims at elucidating relations between expansive pressure measured under a restraint condition, and the amount and composition of gels. The expansive pressure was approximately proportional to the amount of ASR gel formed, when alkali contents of ASR gels formed were less than a critical value. However, mortars containing ASR gels with higher alkali content than the critical value showed extremely low expansive pressure, even when they greatly expanded in expansion tests without restraint. These results suggest that, in existing ASR affected concrete structures containing gels with higher alkali content than a critical value, damages due to the secondary stresses caused by restraint might not be so significant, even if reactive aggregates used in the concrete have showed great expansions in mortar bar test in the laboratory.

Sulfate resistance of high fly ash content concrete

Changes in length, compressive strength, dynamic modulus of elasticity and pulse velocity of high fly ash content concrete with replacement levels of up to 50 % when completely immersed in the 10% Na{sub 2}SO{sub 4} solution were periodically measured for 2 years. It was found from the measurements of mechanical properties that the 50% replacement by fly ash was very effective in the improvement of the sulfate resistance of concrete. During 2 years of exposure to the 10% Na{sub 2}SO{sub 4} solution, high fly ash content concrete with the binder content of 400 kg/m{sup 3} and with replacement level of 50% was steadily gaining the compressive strength, and no detectable deterioration was observed. Chemical analysis data also showed that the excellence of high fly ash content concrete in the sulfate resistance was attributed primarily to the prevention of ingress of sulfate ions into concrete, resulting in little formation of gypsum and/or ettringite in concrete.

Mechanisms of expansion of mortars containing reactive aggregate in NaCl solution

Abstract The combined method of microhardness measurements and EDXA analysis was applied to the elucidation of the characteristics of the alkali-silica reaction occurring within reactive aggregate grains in mortars immersed in 1 N NaCl solution. Changes of the pore solution composition with time in the mortars were also pursued. As far as deduced from the results obtained in this study, the promotion of expansion of reactive aggregate-bearing mortars in NaCl solution is responsible for a rise of OH−ion concentration in the pore solution brought about by the intrusion of NaCl. Both reaction products which have already been formed by the alkali-silica reaction with mortars and Cl−ions certainly play an important role in arising OH−ion concentration in the pore solution.

Pore structure and chloride ion permeability of mortars containing silica fume

Abstract The addition of silica fume in concrete causes a remarkable increase in strength and a drastic reduction in chloride ion permeability. These effects may be due primarily to microstructural changes both in the cement paste phase and in the interfacial zone around aggregates. The standard method of test for rapid determination of the chloride permeability of concrete, AASHTO T 277–831, has increasingly been used to evaluate the permeability of concrete. However, for the concrete containing silica fume, the results of the AASHTO T 277–831 test, which is expressed in terms of electrical charge passed, do not necessarily reflect the real diffusion index of chloride ion through the concrete. There seems to be factors other than the pore structure which govern the results of the AASHTO T 277–831 test in the concrete containing silica fume. In this study, the effects of silica fume to reduce the chloride ion permeability of the mortar were investigated based on the results of pore size distribution measurements, X-ray diffraction analysis, SEM observations and pore solution extraction. The application of the AASHTO T 277–831 test to the evaluation of the chloride ion permeability of the concrete containing silica fume was discussed.

Moisture diffusion in soil-cement mixtures and compacted lean concrete

Abstract The process of drying in lean concretes and soil-cement mixtures is investigated experimentally and the moisture dependent diffusion coefficient is calculated by the use of the method of Matano. Moisture losses predicted with the nonlinear diffusion theory are in better agreement with the test data, than those with the linear theory, especially after 50% of the evaporable water has been dried. The relation between drying rate and shrinkage strain is not linear. In the case of soil-cement mixtures, the shrinkage surpasses the drying, whereas in lean concrete there is an opposite trend, which is explained with the pore size distribution.

Correlation between pore solution composition and alkali silica expansion in mortars containing various fly ashes and blastfurnace slags

Abstract The effectiveness of various fly ashes and blastfurnace slags in preventing expansion due to the alkali silica reaction was investigated by preparing mortars with Beltane opal. Pore solutions extracted from the corresponding mortars were analysed in order to elucidate the significance of alkalis in fly ashes and slags in their effects on deleterious expansion due to the alkali silica reaction. Even in mortars containing a quickly reacting silica such as opal, the effectiveness of fly ash in preventing the deleterious expansion is controlled not only by the alkalinity of pore solution developed at early ages, but also by the effect of the fly ash on pore solution composition over long periods.

Resistance of the cement-aggregate interfacial zone to the propagation of cracks

Abstract A simple model concrete was used for evaluating the resistance of the cement-aggregate interfacial zone to the propagation of cracks. The characteristics of the interfacial zone for various types of rocks were estimated from the microhardness measurements conducted for each cement-rock combination. The load at which a crack formed first at a point on the interface reaches the prescribed point was regarded as a measure showing the extent of resistance of the interfacial zone to the propagation of cracks. Any interactions between cement paste and rock seem not to favorably contribute to the resistance of the interfacial zone to the propagation of cracks.

Alkali-silica reaction and pore solution composition in mortars in sea water

Abstract The promotion of expansion of mortars containing a reactive aggregate in 1N NaCl solution at 38 °C was attributed to a rise of OH − ion concentration in the pore solution in the mortars. However, it is ambiguous whether the promotion of expansion of mortars in sea water at a room temperature can be explained in the same way as in NaCl solution at an elevated temperature. This study aims at pursuing the expansion behavior of mortars containing a reactive aggregate relating it to their pore solution composition and the extent of alkalisilica reaction occurring within reactive grains. The alkali-silica reaction in mortars in sea water and 0.51N NaCl solution at 20 °C appears to progress differently from that in mortars in 1N NaCl solution at an elevated temperature of 38 °C. The promotion of expansion of mortars in sea water at 20 °C was found to be responsible for an effect of Cl − ions on the alkali-silica reaction at early stages of immersion. Only when OH − ion concentration in the pore solution was relatively high, NaCl and sea water could accelerate the alkali-silica reaction in mortars at 20 °C.