Progress in developing self-consolidating concrete(SCC)constituting recycled concrete aggregates:A review

Recycled concrete aggregate(RCA)derived from demolition waste has been widely explored for use in civil engineering applications.One of the promising strategies globally is to incorporate RCA into concrete products.However,the use of RCA in high-performance concrete,such as self-consolidating concrete(SCC),has only been studied in the past decade.This paper summarizes recent publications on the use of coarse and/or fine RCA in SCC.As expected,the high-water absorption and porous structure of RCA have posed challenges in producing a high-fluidity mixture.According to an analysis of published data,a lower strength reduction(within 23%regardless of coarse RCA content)is observed in SCC compared with vibrated concrete,possibly due to the higher paste content in the SCC matrix,which enhances the weak surface layer of RCA and interfacial transition zone.Similarly,SCC tends to become less durable with RCA substitution although the deterioration can be minimized by using treated RCA through removing or strengthening the adhered mortar.To date,the information reported on the role of RCA in the long-term performance of SCC is still limited;thus,a wide range of research is needed to demonstrate the feasibility of RCA–SCC in field applications.

Recycling Bayer and sintering red muds in brick production:a review

Red mud(RM)is a solid waste generated from the extraction of alumina from bauxite.It is estimated that more than1.5 billion tonnes of RM are produced annually worldwide.Stockpiling and landfilling of RM is toxic for the environment due to the presence of high quantities of alkali-generating minerals and metal ions.The high cost of RM treatment and limited land availability encourage the recycling and reuse of RM as construction materials.In many developing countries,brick remains the ubiquitous building material.Thus,it is reasonable to use brick as a medium to accommodate a large amount of RM.In this review,the properties of RM are analyzed and their applications in bricks are discussed.Two common types of RM,namely Bayer red mud(BRM)and sintering red mud(SRM),are introduced along with the methods to produce bricks from them through firing,cementing,and geopolymerization.High alkaline-based BRM is particularly useful as a raw material to produce fired brick and geopolymer brick,whereas SRM,which contains more dicalcium silicate(C_(2)S)and some cementitious phases,can be favorably used to produce cementing brick.RM geopolymer brick normally has more strength(up to 51 MPa)than fired and cementing bricks,and good durability(no efflorescence)with low energy consumption and CO_(2)emissions.Finally,several solutions have been suggested to resolve the issue of the radioactivity of RM brick,and strict regulation of its application has been imposed in some countries,such as China.

Effect of CO_(2)-mixing dose and prolonged mixing time on fresh and hardened properties of cement pastes

This study aims to investigate the influence of CO_(2)-mixing dose(mass fractions of 0.3%,0.6%,and 0.9%)and prolonged mixing time on the fresh and hardened properties of cement pastes.The CO_(2)-mixing can act as coagulant in fresh cement mixtures,resulting in a significant reduction in workability associated with the formation of a rich calcium carbonate network on the surface of cement particles.The CO_(2)-mixing cement pastes were found to be much stiffer and more difficult to handle,place,and compact than the control mixture,which had a negative effect on the mechanical strength performance of the hardened pastes.However,prolonging the mixing time for 1 min(immediately after CO_(2)-mixing)can effectively improve the workability(by~53%–85%)by breaking up the flocculation network of deposited calcium carbonates.As a result,the presence of detached calcium carbonate accelerated early cement hydration and densified the microstructure;this improved early-age compressive strength by~6%–32%,depending on the CO_(2)-mixing dose used.Therefore,it seems that the CO_(2)-mixing dose should be controlled at≤0.6%with the mixing time prolonged in order to attain satisfactory workability and mechanical strength.