The investigation of the effects of chemical admixtures on backfill with fly ash

In recent years, plasticizers have been widely used in construction industry due to having positive effects on properties of fresh and hardened concrete. Although the plasticizers show similar effects on properties of backfill, their usage in the area of mining is quite inadequate. In this study, effects of use of plasticizers in backfill applications of mechanized longwall were investigated. First, characteristics of underground backfill containing cement-fly ash ratio of 25-75% by weight (1st mix ratio) and accelerator (sodium silicate type) were revealed using experimental studies. Then, amount of cement was gradually reduced to 22,5% (2nd mix ratio) and 20% (3rd mix ratio). Addition %1 and %2 of normal plasticizers (NA) and superplasticizers (SA) were used in these new mixing ratios (2nd and 3rd). Two type cements (CEM I and CEM II) were used in all mixing ratios. The backfill samples with 20 cm slump value were molded in 5 cm cubic molds, cured at higher than %90 humidity and temperature 20°C (±2), then pastes were kept for different time intervals 2, 7, 28 and 90 days. Strengths, setting times, micro-structures and economies of samples were studied.Results of experimental studies showed that plasticizers reduced water content of mixtures by %3 to %7 at constant consistency, increased compressive strength of mixtures by %8 to %32. Micro structural investigations identified that the cumulative pore size and formation of ettringite gel of samples was reduced by addition plasticizers. It was also determined that plasticizers delayed in setting time of samples but accelerator declined in setting time of samples. However, it was observed that accelerator additive increased in early strength of samples but adverse effect in ultimate strength of samples. It was foreseen that mixtures prepared with plasticizers and accelerator at 2nd mix ratio can be use in underground mechanized longwall backfill applications in terms of strength and economic.