AN INVESTIGATION OF THE USE OF LIME STONE FILLER IN SELF COMPACTING CONCRETE

Abstract

Self-compacting concrete, a recent innovation in the concrete industry, is a concrete which can flow under its own weight, fill the formwork completely and pass through dense reinforcement. The conflicting fresh state properties like flowability and stability of SCC have to be reconciled with the help of powder materials. Limestone powder is a non-pozzolanic mineral filler which can be used as a powder for improving the stability of fresh SCC. The fine limestone powder particles significantly improve the flowability and also reduce the decay in fluidity of fresh concrete, due to improved particle packing and water retention of the fresh SCC mixes. Improvement in the strength development in the SCC mixes occurs due to pore filling effect of the fine particles of limestone powder and the participation of CaCO3, in the hydration reactions. Unlike pozzolanic fillers like fly ash, the strength enhancement due to limestone powders is concurrent with strength development due to the hydration reactions. The present investigation has been carried out to examine the possibility of using commercially available finely ground limestone, limestone dust and chalk powder as replacement fillers in SCC and to evaluate their effects on the rheology of the fresh mix, the superplasticiser demand and the strength characteristics of the hardened concrete. In the parametric study related to powder replacement, two levels of replacement of fly ash as a fraction of the total powder content by mass with two types of limestone powder and one type of chalk powder viz. 25% and 45% were chosen with respect to the control self-compacting concrete mixture designed to have a 28-days target compressive strength of 30 MPa. Characterization studies on the powder materials were carried out to indentify the chemical compositions and physical morphology of each of the limestone/chalk powder used in SCC mixes. For the given materials and mix compositions, it was possible to develop self-compacting concrete satisfying requirements of flowability, passing ability and segregation resistance by using limestone powder/chalk powder as a filler material. Flowability and stability were significantly influenced by the fineness of the different types of powders used in the SCC mixes. Higher superplasticizer demand was observed in the SCC mixtures containing the coarser and compared to SCC mixes with the fine limestone powder. The higher strength gains in the SCC mixes having finely ground limestone powder as filler was obtained due to the increased particle packing and associated chemical activity. The higher early age strength gains indicate that addition of limestone powder in SCC accelerates cement hydration. The strength of the SCC mixes containing the finely ground limestone powder at 25% and 45% replacement levels was significantly greater than that of the control SCC for the same water/cement ratio. However, the strength of the SCC mix with 45% replacement using fine limestone powder was lesser than the mix with 25% replacement. Brittle stress-strain behaviour of all the SCC mixes was seen at 28 days age. However, non-brittle behavior at early ages of 1, 3 and 7 days has been observed. This investigation establishes the viability of using fine limestone powder as a non-pozzolanic mineral filler in SCC.