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Aging of Portland cement, due to its reaction with moisture and atmospheric CO 2 prior to regular use, has the potential to severely impair its hydration reactivity. The effectiveness of using metakaolin to improve the properties of aged Portland cement exposed to a laboratory conditions (RH=65 %, 20 °C) as well as the associated mechanism are studied. The incorporation of 5–10 % metakaolin changes the early-stage hydration kinetics of aged cement obviously by enhancing reactions of both C 3 S and C 3 A through the filler effect at the early hydration stage, thereby mitigating detrimental influences of aging on setting time and early-age strength development (up to 7 days). Metakaolin exhibits significant effectiveness on enhancing 28-day strength of aged cement, and the enhancement arises from the synergic reaction between metakaolin and CaCO 3 , which converts CaCO 3 resulting from aging into carboaluminate phases and produces secondary C-S-H. The incorporation of 5 % metakaolin increases the 28-day compressive strength of the sample with cement aged for one month to a comparable level as that of fresh cement. • Aging leads to relaxation and reconstruction of the cement surface can hinder the activity of the cement. • The filler effect caused by metakaolin promotes the reaction of C 3 S and C 3 A in aged cement. • The synergistic reaction between metakaolin and CaCO 3 formed in aged cement improves the mechanical property. [ABSTRACT FROM AUTHOR]

• Application of steel slag in cementitious materials based on dense packing can achieve high utilization efficiency of both steel slag and cement. • Early age hydration kinetics, fluidity of blends, and properties including strength and water absorption are investigated on paste specimens. • The appropriate amount of gypsum addition can enhance long-term strength through improving the early-age hydration kinetics SS-SPC cementitious systems. • The findings of this research help to valorize steel slag utilization in cement-based materials without sacrificing mechanical property and durability. Steel slag is an industrial solid waste with latent hydration nature, possessing a high potential use in cement and concrete but has not been fully utilized yet. This study reveals an implication approach based on dense packing to achieve high utilization efficiency of both steel slag and cement. Steel slag-based reverse filling approach utilizes higher volume of relatively coarser steel slag particles as micro fillers and a smaller quantity of fine cement grains as the major binding component. Thus, giving an initial compact packing structure and an enhanced strength of the resulting materials. Based on the packing density of various mixes, steel slag-based reverse filling cementitious materials are formulated. Early age hydration kinetics, fluidity of fresh pastes, and properties including mechanical strength and water absorption are further investigated on paste specimens. Results indicated that a maximum packing density of 0.6956 was attained for the blend containing 85 % steel slag with 15 % superfine Portland cement. The compressive strength of formulated mixes reached 96.4 MPa, 105.6 MPa and 123.8 MPa at 180 days for pastes containing 15 %, 25 % and 35 % of superfine Portland cement, respectively. The addition of appropriate amount of gypsum can enhance long-term strength by 8 ∼ 18 MPa through improving the early-age hydration kinetics of the steel slag based cementitious systems. The findings of this research help to valorize steel slag utilization in cement-based materials without sacrificing mechanical property and durability. [ABSTRACT FROM AUTHOR]

To understand the effect of elevated Al concentration on early hydration in Portland cement, hydration heat, the evolution of solid and aqueous composition in paste blended with reactive alumina were investigated in this study. The increase of Al concentration depletes gypsum rapidly with massive precipitation of ettringite during the pre-induction period, resulting in decreased Ca and S concentration in pore solution. The elevated Al concentration passivated the active sites on C 3 S surface, and the decreased Ca concentration inhibited the nucleation of C–S–H and precipitation of portlandite, hence prolonging the induction period. Meanwhile, the reaction of C 3 A has not been accelerated even after the complete depletion of gypsum, which is associated with the lowered undersaturation degree caused by the increased Al concentration. Additionally, the precipitation of large amounts of ettringite and foil-shaped C–S–H reduced available nucleation sites, thus hindering the reaction of both C 3 S and C 3 A during the acceleration period. [ABSTRACT FROM AUTHOR]

The incorporation of steel slag usually leads to retardation on the early-age hydration of cement, which limits its use as supplementary cementitious material in cement or concrete. Aiming to a better understanding of the retarding mechanism, the influences of two representative basic oxygen furnace steel slags on setting behaviour, hydration kinetics, aqueous and solid composition during early age were investigated. Different from the existing mechanisms, this study highlights the role of aluminate phase contained in steel slags in the retardation of hydration. The incorporation of steel slag with high aluminate phases content reduces C 3 S/aluminate phase ratio significantly. This reduction in C 3 S/aluminate phase ratio leads to undersulfation in the system, where aluminate phases react prior to C 3 S, resulting in flash setting, reduced C 3 S reaction, and thus the hindered early-age strength gain. This study also proposes an efficient way to alleviate the retarding effect of steel slag on early-age hydration by increasing the sulfate supply, which allows the system to react in a properly sulfated manner. [ABSTRACT FROM AUTHOR]