International Concrete Abstracts Portal

Mechanisms of stratlingite (C 2ASH 8) formation in high-alumina cement (HAC)-siliceous material systems were investigated. Different siliceous materials (silica fume, fly ash, ground granulated blast furnace slag) and chemical admixtures (sodium silicate, sodium sulfate) were employed. Reactions between CAH 10 or C 2AH 8 and dissolved silica occur. Acceleration of silica dissolution by addition of chemical admixtures promotes the formation of stratlingite. The pH value of the HAC-siliceous materials system was also studied. The intrinsic relationship between the pH value and stratlingite formation is discussed in this paper. Mechanisms of stratlingite formation in preference to hydrogarnet (C 3AH 6) in HAC products are postulated. A method for prevention of strength reduction of HAC products due to the conversion of thermodynamically unstable hexagonal calcium aluminates to cubic hydrogarnet is described.

As a result of the very low water-cement ratio in a high-performance concrete, the rate of cement hydration at early ages is significantly different from that in a normal strength concrete. The ultimate degree of cement hydration is lower in a high-performance concrete; the hydration process will terminate earlier because of the rapidly diminishing water supply. Another characteristic of high-performance concrete is caused by the relatively high dosage of superplasticizer which delays the onset of the cement hydration. This paper presents the extension of the research on temperature and strength development in hardening concrete from normal strength concrete to high- performance concrete. It models the development of heat of hydration in high-performance concrete, taking into account the effects of water-cement ratio, superplasticizers, and temperature changes. General formulations of the rate of heat of hydration as functions of concrete maturity (hydration stage) and current temperature are provided. Comparison with some test results verifies the theoretical model.

Presents results of expansion tests carried out on concretes immersed in 1/10-M and 1-M sodium chloride solutions. The concretes were prepared using two reactive aggregates, cristobalite and a natural aggregate from the southwest of the U. K. Tests were carried out both at alkali levels which were known to induce expansion due to alkali-silica reaction (ASR) and alkali levels which would not normally induce expansion due to ASR. The concretes were, at the ages of one, three, and six months, immersed in a sodium chloride solution. The concretes were stored at 38 C, 20 C, and externally. For the concretes containing the natural aggregate, it was shown that immersion in a 1-M salt solution had no major adverse effects upon long-term expansion. This is attributed to the low available reactive silica content within the concretes. In the case of concretes containing cristobalite, it was shown that the immersion in 1-M salt solution had an adverse effect upon long-term expansion. This is attributed to the high available reactive silica content of the concrete.

Presents the results of a study on the effect of curing conditions on the air-entrained, superplasticized high-volume fly ash concrete made with ASTM Types I and III cements and silica fume. For the four concrete mixtures made, the total cementitious materials content was about 370 kg/m 3, and the water- cementitious materials ratio was kept at 0.310.01. The proportion of ASTM Class F fly ash in all the mixtures was 58 percent by weight of the (cement + fly ash) content. Two mixtures incorporated silica fume at the dosage rate of 8.5 percent of the total cementitious materials content. The properties of fresh concrete including the time-of-set and autogenous temperature rise were determined. Specimens were cast and moist cured for the determination of compressive and flexural strengths, resistance to chloride ion penetration, and freezing and thawing cycling at various ages. Compressive strength and chloride ion penetration measurements were also performed at various ages on the specimens that were subjected to laboratory air curing after three days of moist curing. The use of ASTM Type III cement instead of Type I increased the early age strength significantly without affecting the long term strength development under moist curing conditions. Under air curing conditions, the concretes incorporating the Type III cement achieved significantly higher strengths at every test age up to one year. The use of silica fume resulted in only marginal improvement in the strength properties under the two curing regimes. The air curing resulted in a significantly lower resistance to chloride ion penetration (RCP) of all the concretes, but the drop in RCP was greater for those made with ASTM Type I cement. The use of silica fume increased the resistance to chloride ion penetration of concretes significantly under both curing conditions.

Presents a review of recent research focusing on the durability of concrete and mortar containing rice husk ash (RHA). The purpose of the investigation was to determine the effects of RHA in cement products exposed to hostile environments. Included are the results of laboratory research on hydrochloric acid attack, sulfate attack, alkali-silica reaction, and frost action on mortar or concrete mixtures containing RHA. The results showed that mortar or concrete containing RHA showed a substantial reduction in mass loss on exposure to a hydrochloric acid solution and considerable reduction in alkali-silica and sulfate expansions. Also, the frost resistance of non-air- entrained concrete containing RHA was found to be considerably higher than similar concrete mixtures containing silica fume.