Use Of Nano-Silica To Increase Early Strength And Reduce Setting Time Of Concretes With High Volumes Of Slag Or Fly Ash

Abstract

Ground granulated blast-furnace slag and fly ash have been used in concrete for many years as mineral admixtures to replace Portland cement. Concretes with high volumes of slag or fly ash (>50%) have been used for applications where durability is of prime concern. These high-volume slag or fly ash concretes can develop good strengths over time if properly cured, exceeding those of similar concretes without slag or fly ash. However, early strengths of such concretes are often lower than Portland cement concrete without slag or fly ash which may affect construction progress. The objectives of this research project are to study (1) Effects of nano-silica (NS) dosages, particle sizes, and dispersion methods on rate of heat development in cement pastes at early age, porosity and pore size distribution of cement pastes and compressive strength development of mortars; and (2) Effect of NS on setting time, compressive strength development and chloride-ion penetrability of concretes in comparison to those of silica fume. Most cement pastes, mortars, and concretes contained high volumes of slag or fly ash of about 50%. Control Portland cement paste and mortar without any mineral admixture were also included for comparison. Two types of NS with specific surfaces of 200.1 and 321.6 m2/g (mean particle size 12 and 7 nm, respectively) were included in the study in comparison to silica fume with a specific surface of 21.3 m2/g. A constant water-to-cementitious materials ratio (w/cm) 0.45 was used for all mixtures. A superplasticizer was used to achieve targeted workability. Dosages of the NS varied from 0.5, 1.0 to 2% of the total cementitious material. The NS were dispersed by two different methods. In one method, NS was premixed with water using ultrasonic mixer before being mixed with other materials. In the second method, NS was dispersed using conventional mechanical mixing. Compressive strengths of mortars at 1, 3, 7, 28, and 91 days, concrete setting times and strengths at 3, 7, 28, and 91 days, heat development of cement pastes up to 30 hrs, pore structure of pastes cured for 28 days, and concrete resistance to chloride-ion penetration at 28 days were determined. The results indicate that length of dormant period was shortened, and rate of cement and slag hydration were accelerated with the incorporation of the NS in the high-volume slag or fly ash cement pastes. The incorporation of a small amount of NS reduced setting times, and increased 3- and 7-day compressive strengths of high-volume slag or fly ash concrete, significantly, in comparison to the corresponding reference concrete with 50% slag or fly ash. Compressive strength of the slag or fly ash mortars were increased with the increase in NS dosages from 0.5 to 2.0% by mass of cementitious material at various ages up to 91 days. The strengths of the slag or fly ash mortars were generally increased with the decrease in the particles size of silica inclusions at early age. Ultra-sonication of nano-silica with water is probably a better method for proper dispersion of nano-silica than mechanical mixing method. With the increasing dosage of NS, large capillary porosity was decreased, whereas medium capillary porosity was increased in the slag cement pastes at 28 days. The 28-day charge passed through the slag or fly ash concrete with NS was lower than that of corresponding reference concrete. Nano-silica with mean particle sizes of 7 and 12 nm appears to be more effective in increasing the rate of cement hydration and reaction compared with silica fume. The NS reduced the setting times and increased early strengths of the high-volume slag or fly ash concrete. However, the setting times and early strength of the high-volume slag or fly ash concrete were not affected by the silica fume significantly.