A step by step methodology for building sustainable cementitious matrices

In an effort to produce cost-effective and environmentally friendly cementitious binders. mainly ternary (Portland cement + limestone + pozzolanas) formulations have been investigated so far. Various proportions of constituents have been suggested, all, however, employing typical Portland cement (PC) substitution rates, as prescribed by the current codes. With the current paper a step by step methodology on developing low carbon footprint binary, ternary and quaternary cementitious binders is presented (PC replacement up to 57%). Best performing binary (60% PC and 40% LS (limestone)) and ternary formulations (60% PC, 20% LS, 20% FA (fly ash) or 43% PC, 20% LS 37% FA) were selected on the grounds of sustainability and strength development and were further optimized with the addition of silica fume. For the first time a protocol for successfully selecting and testing binders was discussed and the combined effect of highly pozzolanic constituents in low PC content formulations was assessed and a number of successful matrices were recommended. The present paper enriched the current state of the art in composite low carbon footprint cementitious binders and can serve as a basis for further enhancements by other researchers in the field

Developments in bacteria-based self-healing of cementitious composites

The paper reports on work being carried out on bacteria-based self-healing of cementitious composites as part of the UK’s Resilient Materials for Life (RM4L) programme grant. RM4L is a follow-up to a previous project that developed bacteria- based self-healing using non-ureolytic bacteria encapsulated in lightweight aggregates and microcapsules. This led to the UK’s first full-scale trial of bacteria-based self- healing concrete. In RM4L, research has been undertaken to overcome a number of application issues with use of bacteria-based self-healing in practice. In particular, a large number of environmental bacteria have been screened that are potentially capable of working in cold and saline conditions. It has been shown that different microbial metabolisms can result in different mechanisms of precipitation, possibly impacting on performance in application. Further work has shown that it is possible to get healing to occur in aged concretes provided the healing agents are included appropriately. Work has also demonstrated to what extent wet/dry conditions are necessary for healing. Finally, the paper reports on work to genetically engineer bacteria to obtain more effective healing

Recycling of fly ash-slag Geopolymer binder in mortar mixes

© 2022 Imperial College London.Fly ash-slag based Geopolymer cement (GPC) has demonstrated mechanical properties and environmental advantages that make it one of predominant sustainable alternatives to Portland cement (PC). Despite the fact that numerous environmental analyses about geopolymers are being published, their environmental impact after the end of service-life has barely been explored. Given that construction-waste management is a major sustainability issue, the present study is investigating the potential of recycling fly ash-slag GPC as a fine aggregate in mortar mixes. The major physical properties of the fine recycled aggregates (FRA) were tested and compared to those of PC FRA and natural sand of similar fineness. The effect of incorporating FRA in low (25%) and high (50%) percentage in PC or GPC matrix mortars was investigated. The 28day compressive and flexural strength of mortars were tested. Also the 28day water absorption and flow of mixes incorporating GPC FRA were recorded. GPC FRA exhibited properties similar to those of PC FRA and poorer than those of natural sand. The results of compressive and flexural strength proved that FRA addition had a negligible effect in all cases. The influence of the high water absorption of GPC FRA, relatively to that of natural sand, was prominent on the workability of fresh mixes and possibly affected the water absorption of mortar prisms. The effect of GPC FRA proved to be similar to that of PC FRA on compressive strength, while none of the tested mortar properties appeared to be jeopardised by the incorporation of the GPC FRA in the mixPeer reviewedFinal Published versio

Steel fibre reinforced concrete for prestressed hollow core slabs

An investigation of prestressed concrete containing steel fibres as secondary reinforcement to improve performance in shear, flexure and bond is reported. Emphasis is placed on the use of steel fibres in prestresssed extruded hollow core slabs, since these common precast elements have intrinsic difficulty in incorporating traditional secondary reinforcement due to their unique shape and manufacturing method. Two separate studies were carried out. The first study involved laboratory investigations into the bond between fibre reinforced concrete (FRC) and the prestressing strand, and the shear behaviour of laboratory-cast prestressed fibre reinforced concrete (PFRC) beams. The second part involved the factory production of fibre reinforced hollow core slabs in co-operation with a local manufacturer. The fibre reinforced hollow core slabs were subjected to conventional full-width shear tests, concentrated load shear tests, and to transverse flexure. For all laboratory cast elements, cubes, cylinders and prisms were cast to investigate compressive, tensile and flexural properties, respectively. Two types of steel fibre were investigated: hooked-end steel fibres at fibre volume fractions (Vf) of 0.5%, 1.0% and 1.5%; and amorphous metal fibres at Vf‘s of 0.28% and 0.56%. The trial production of fibre reinforced hollow core slabs necessitated the investigation of the effect of steel fibres on the extrusion manufacturing process. It was shown that fibre reinforced hollow core slabs could be adequately compacted with only slight increases in mixing water. Fibres were found to distribute randomly throughout the cross-section. However, the rotation of the augers affected the orientation of fibres, with fibres tending to align vertically in the web. It was shown that the addition of steel fibres to prestressed concrete has a negative effect on the bond between matrix and tendon, leading to longer transfer lengths. The effect of the increase in transfer length was to reduce cracking shear strengths by 4%. Shear tests showed that the incorporation of steel fibres could increase shear strength by as much as 45% for Vf = 1.5%. This increase in shear strength, known as the fibre contribution, was shown to be due to fibres bridging across the crack and an increased compressive resistance due to fibres arresting the propagation of cracks into the compressive zone. A semi-empirical equation for shear strength of PFRC elements is developed. It is given in two forms, one compatible with the present equations for prestressed concrete given in BS 8110 and Eurocode 2, and a second form compatible with that advocated for fibres in reinforced concrete. The equation makes use of equivalent flexural strength which is recognised as the most useful material property for design of FRC. The equation was found to give good correlation with the shear strength of single web beams cast both in the laboratory and under factory conditions. However, a overall strength reduction factor is required for full-width hollow core slabs to account for uneven load distribution and inconsistent web widths. This is consistent with tests on plain hollow core slabs found in the literature

Chemical aspects related to using recycled geopolymers as aggregates

This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.Despite extensive research into sustainability of geopolymers, end-of-life aspects have been largely overlooked. A recycling scenario is examined in this study. This requires an investigation of alkali leaching potential from a geopolymeric matrix. To study the feasibility of geopolymer cement (GPC) recycling, the migration of alkalis was evaluated for the first time on a microstructural level through energy dispersive X-ray (EDX) scanning electron microscopy (SEM) elemental mapping and leaching tests. Macroscale impacts were assessed through an investigation of Portland cement (PC) mortar properties affected by alkali concentration. Leaching tests indicated that alkalis immediately become available in aqueous environments, but the majority remain chemically or physically bound in the matrix. This type of leaching accelerates the initial setting of PC paste. Elemental mapping and EDX/SEM analysis showed a complex paste-aggregate interfacial transition zone. Exchange of calcium and sodium, revealed by the maps, resulted in the migration of sodium into the PC paste and the formation of additional calcium-silicon-based phases in the geopolymeric matrix. Strength values of mortars with 25% and 50% recycled aggregates (RA) showed negligible differences compared with the reference sample. Screening tests indicated a low potential for GPC RA inducing alkali-silica reaction. Transport of GPC RA alkalis and the underlying mechanisms were observed. This transport phenomenon was found to have minor effects on the properties of the PC mortar, indicating that recycling of geopolymers is a viable reuse practice.Peer reviewedFinal Published versio