The Preparation And The Performance In Concrete Of Limestone Powder Composite Admixture

In recent years, the research and application of limestone powder used as concrete admixtures become a hotspot. The development of limestone powder composite admixtures(LPCA), which possess adequate technical supports and tremendous market potential, is the only way for stone processing companies to practice environmental protection and sustainable development. Fly ash and cinder are both mineral admixtures for increasing late strength of concrete. However, in practical applications, the usage of cinder is far less than that of high quality fly ash. Thus, the combination of limestone powder, fly ash and cinder to develop the LPCA is feasible. The present work aims at developing high-performance compound admixtures by taking advantages of the abundant raw materials of limestone, fly ash and cinder. Noting that the present work will provide the basis of the production and application for LPCA.Mortar tests were performed to determine the optimal composite ratio of the LPCA and the kind and usage amount of activators. Detailed discussion of mortar performance, hydration heat, microstructure of hydration products and interface for the LPCA are carried on the data received via mortar tests, calorimetry, SEM. Work performance, mechanical performance and drying shrinkage performance for concretes using two kinds of concrete composite admixtures(limestone powder-cinder system and limestone powder-fly ash-cinder system) are investigated. The conclusions are drawn as following.(1) The mortar activity index of limestone powder-cinder and limestone powder-fly ash is higher than the mortar activity index only mixed with limestone powder. The proportion of limestone powder in composite admixture system should not be more than 30%. The greater the proportion of limestone powder in the mortar, the lower the compressive strength of mortar which next to the compressive strength of mortar only mixed with limestone powder.(2) Lime as activator can stimulate the activity of coal powder and fly ash. When the proportion of limestone powder,cinder and fly ash is 2:6:2 or 3:4:3, either the fluidity or the compressive strength for 28 d is the highest. Mixing-grinding the limestone powder, cinder, fly ash and lime for the composite admixture, can improve the mechanical performance of mortar, reduce the hydration exothermic rate of cementitious material, Prolong the induction period, shorten the acceleration period, and reduce the hydration heat. Mixing-grinding limestone powder composite admixture can reflect good filling effect and dispersion effect,making good interface bonding between hardened cement paste and aggregate.(3) Limestone powder-cinder(5:5), limestone powder-cinder-fly ash(3:4:3), these two kinds of composite admixture can improve the adhesiveness of fresh concrete. The influence of composite admixture on slump of concrete is determined by the mix proportion and usage amount of superplasticizer. In the C30 concrete, concrete slump decreased with increasing dosage of compound admixtures. In the C50 concrete composite admixture dosage for less than 30% of concrete slump will be slightly higher.(4) Limestone powder-cinder, limestone powder-cinder-fly ash, these two kinds of composite admixture within 30% content will improve the later strength of concrete in different degrees. In C50 concrete, these two kinds of composite admixture can make more contribution to the 28 d compressive strength than cement.(5) Mixed within 30% limestone powder-cinder, limestone powder-cinder-fly ash these two kinds of composite admixture can help reduce the drying shrinkage of C30 and C50 concrete.