Kraiwood Kiattikomol

A study of ground coarse fly ashes with different finenesses from various sources as pozzolanic materials

Abstract The aim of this study is to evaluate the properties of ground coarse fly ashes, from five sources in Thailand, the shapes, sizes, and chemical compositions of which are completely different. Coarse fly ash was fractionated by an air classifier and ground into three different finenesses ranging from median particle sizes of 1.9–17.2 μm. Physical and chemical properties of the Portland cement and the fly ashes were investigated. Mortar cubes of 5 cm were cast with 20% replacement by weight of Portland cement with ground coarse fly ash. The compressive strengths of the fly ash–cement mortars were determined and compared with the control mortar. The results revealed that the degree of pozzolanic reaction, as determined using compressive strength, of coarse fly ash increased when its fineness was increased by grinding. The strength activity indices of the original fly ash–cement mortars at the curing ages of 7 and 28 days were in the range of 69–82% and 76–90%, respectively. When the particle size smaller than 9 μm of ground coarse fly ash was used, the strength activity index achieved was over 100% of that of the control within 28 days. The results also showed that the fineness of fly ash, not the chemical composition, was the major factor affecting the strength activity index of ground coarse fly ash–cement mortar.

USE OF GROUND COARSE FLY ASH AS A REPLACEMENT OF CONDENSED SILICA FUME IN PRODUCING HIGH-STRENGTH CONCRETE

Abstract This paper presents a method of improving coarse fly ash in order to replace condensed silica fume in making high-strength concrete. The coarse fly ash, having the average median diameter about 90–100 μm, yields a very low pozzolanic reaction and should not be used in concrete. In order to improve its quality, the coarse fly ash was ground until the average particle size was reduced to 3.8 μm. Then, it was used to replace Portland cement type I by weights of 0%, 15%, 25%, 35%, and 50% to produce high-strength concrete. It was found that concrete containing the ground coarse fly ash (FAG) replacement between 15% and 50% can produce high-strength concrete and 25% cement replacement gave the highest compressive strength. In addition, the concrete containing FAG of 15–35% as cement replacement exhibited equal or higher compressive strengths after 60 days than those of condensed silica fume concretes. The results, therefore, suggest that the FAG with high fineness is suitable to use to replace condensed silica fume in producing high-strength concrete.

Effect of insoluble residue on properties of Portland cement

Abstract Insoluble residue is a non-cementing material which is present in Portland cement. This residue material affects the properties of cement, especially its compressive strength. To control the non-cementing material in Portland cement, ASTM standard allows the insoluble residue to be not higher than 0.75%. This limitation is much lower than the allowance provided by the British standard which is 1.5%. To verify the effect of insoluble residue on the properties of Portland cement, artificial insoluble residue was prepared and replaced in Portland cement type I. Finely crushed sand was extracted to represent artificial insoluble residue. Setting times and compressive strengths of cement mortar mixed with insoluble residue were investigated. The Portland cement was replaced by insoluble residue which varied in amounts of 0%, 0.5%, 1.0%, 1.5%, 2.0%, 3.0%, 5.0% and 7.0% by weight. The results showed that the addition of the insoluble residue from 0.0% to 7.0% by weight in Portland cement did not affect the normal consistency or setting times of cement. However, the compressive strength of cement mortar was affected during the early age, but the figure reduced as the cement mortar was older. With 7.28% of insoluble residue in the mortar at 1 day, the compressive strength was reduced by 11.5%, but after 60 days, the strength of the same mortar was only reduced by 5.5% as compared to the control mortar. It was also found that the compressive strength of Portland cement mortar with insoluble residue provided by ASTM standard or British standard was still higher than the compressive strength of Portland cement mortar type I allowed by the standards. The limit of insoluble residue given by ASTM standard as 0.75 is rather low and can possibly be increased to 1.5% according to British standard, or even slightly higher, without significantly reducing the compressive strength of cement.