Efficiency Assessment of Accelerating Admixtures and Cement Kiln Dust with Cooperation with Different Phase Composition Slag Blended Cements

Abstract

Concrete technology involves nowadays several types of chemical admixtures. One of them is set and hardening accelerating admixtures. Main benefits of their usage are enhancement of early compressive strength and shortening of initial setting time. Unfortunately, they may cause decrease of long-term compressive strength and adversely influence durability properties of concrete. Slag blended cements with 30% and 60% of slag content were used. Ground granulated blast furnace slag (GGBFS) that is main non-clinker constituent of those cements is beneficially changing its properties. It favourably affecting long-term compressive strength, consistency and durability properties of concrete. Main drawback of slag blended cements are decrease of early compressive strength and extension of initial setting time. Both accelerating admixtures and ground granulated blast furnace slag have advantages and disadvantages that may be at least partially balanced. Slag blended cements were composed in laboratory conditions of three Portland clinkers, differing in phase composition, anhydrite as set regulating constituent and ground granulated blast furnace slag in amount of 30% and 60% of Portland clinker mass. Cements was modified with accelerating admixtures with crystal seeds and calcium nitrate as active agent and cement kiln dust in amount of 10% of the cements mass. During previous research crystal seeds and calcium nitrate were examined with CEM I, CEM II/B-S and CEM III/A, B, manufactured by one of polish cement plants. Calcium nitrate acts mainly on dicalcium and tricalcium silicate (C2S and C3S). Crystal seeds are acting physically on all cements phases by means of acceleration of C-S-H phase formation by faster crystallisation. Their performance may be similar independently on cement phase composition. Aim of this paper is to describe effectiveness of those admixtures in cooperation with CEM II/B and CEM III/A, differing in phase composition. Additionally, cements were modified with Cement Kiln Dust (CKD) and their properties were tested. Such dusts may accelerate setting and hardening of cement because of chloride and carbonate content, which are constituents of accelerating admixtures. In addition, their form of very fine powder may allow them to act similarly to crystal seeds. Initial setting time, consistency and compressive strength of mortars were examined. Tests were conducted in temperature of 20°C. Compressive strength was tested after 12, 24, 48 hours and 7, 28 and 90 days of curing in temperature 20°C in water. In most works those properties are considered separately. In order to determine efficiency of substances mentioned above the multiple-criteria decision analysis (MCDA) was made for every kind of cement. This kind of analysis allows to take examined properties as set of properties and give information about the behaviour of admixture and cement cooperation in more general terms. Criteria were defined on the basis of conducted tests and their weights were evaluated with pair analysis method. Admixture with crystal seeds was most efficient one, while Cement Kiln Dust was the least efficient.