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

A multi-variable study of factors affecting the complex resistivity of conductive mortar

Twenty-four mortar mixes were tested to assess the effects of mix design factors on complex electrical resistivity. Of these mixes, six were conventional and 18 were conductive mixes, containing varying quantities of either graphite or carbon fibre powder additions, which have been shown in previous studies to reduce the resistivity of cementitous materials. Complex resistance measurements from 20 Hz to 10 MHz taken between 7 and 35 d after casting were analysed. Comparisons were made between the effects on complex resistivity of varying quantities of additions, water/cement ratio and grit/sand ratio. In conventional mixes, the water/cement ratio was found to have more significance for complex resistance than the grit/sand ratio. Conductive additions were found to reduce the magnitude of complex resistivity as well as the influence of other mix factors. This novel and comprehensive comparison of the effects of three elements of mix design on complex resistance will be of use to anyone wishing to produce mortars or concrete for use in self-heating, self-sensing, or electrical property imaging contexts.</p

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

The pozzolanic properties of inorganic and organomodified nano-montmorillonite dispersions

The pozzolanic potentials of three non-thermally treated, nano-montmorillonite (NMt) dispersions were investigated by a new method involving the analysis of NMt/lime putty pastes via TGA/dTG and XRD crystallographic and semi-quantitative analysis. The criterion conceived was validated at eight days and eight months and was additionally verified via the Chapelle method. The inorganic NMt dispersion showed the most pronounced pozzolanic behaviour promoting Ca(OH) 2 consumption towards calcium silicate/aluminate hydrates formation and binding behaviour. The two organomodified NMt dispersions exhibited pozzolanicity increasing with time. The results can pave the way for advances in cement science and restoration mortars development for historical structures rehabilitation, where low CO 2-footprint, natural inorganic materials are a prerequisite. </p

Olivine as a reactive aggregate in lime mortars

This paper presents the first investigation into the use of olivine as an aggregate material for calcium lime mortars. Lime binders provide many advantages when compared to cement binders such as higher vapour permeability and the ability to accommodate movement. They are undergoing a resurgence in their use in the conservation of historic buildings and in combination with environmentally friendly natural materials where these attributes are particularly important. Their ability to mitigate against global warming through the sequestration of CO 2 by carbonation is a further advantage which will bring impact. The equilibrium reaction products between non-hydraulic lime and olivine were calculated using the thermodynamic software GEMS3 Selektor. Experimental mortar mixes were modelled with varying ratios of quartz sand aggregate and olivine sand aggregate. The software predicted phase assemblage at equilibrium comprising calcite, dolomite, magnesite and quartz, with mass percentages depending on the ratio of quartz to olivine. The mortars morphological, chemical and mechanical properties were evaluated using Scanning electron microscopy, X-ray diffraction (XRD), Raman spectroscopy, thermogravimetric analysis (TGA) and compressive strength testing. Significantly, this study has shown that the use of olivine based aggregates in finely divided form can enhance carbonation, and hence the CO 2 absorption capacity of these mortars. Dolomite formed within the mortar from the reaction of olivine aggregate with lime and carbon dioxide in the presence of moisture is attributed to the superior mechanical properties observed increasing from 0.5 to 2.5 MPa. </p

Sensing of damage and repair of cement mortar using electromechanical impedance

Lead zirconium titanate (PZT) has recently emerged as a low-cost material for non-destructive monitoring for civil structures. Despite the numerous studies employing PZT transducers for structural health monitoring, no studies have assessed the effects of both damage and repair on the electromechanical impedance response in cementitious materials. To this end, this study was conducted to assess the effects of the damage and repair of mortar samples on the electromechanical response of a surface-mounted PZT transducer. When damage was introduced to the specimen in stages, the resonance frequencies of the admittance signature were shifted to lower frequencies as the damage increased, and an increase in the peak amplitude was detected, indicating an increase in the damping and a reduction in the material stiffness properties. Also, increasing the damage in the material has been shown to decrease the sensitivity of the PZT to further damage. During the repair process, a noticeable difference between the after-damage and the after-repair admittance signatures was noted. The root-mean-square deviation (RMSD) showed a decreasing trend during the repair process, when compared to the before repair RMSD response which indicated a partial recovery for the material properties by decreasing the damping property in the material

Bacteria-based self-healing concrete: Effects of environment, exposure and crack size

The effect of water-borne contaminants on the durability of concrete is well-known and cracked concrete is more susceptible to permeation of these contaminants. An approach to autonomic self-healing of such concretes is the utilization of microbiologically-induced calcite-precipitation. This approach uses the metabolic activity of bacteria and biomineral precursors embedded within the concrete to form an inorganic material, usually calcite, as the healing compound. However, bacteria-based healing of concrete creates a number of scientific and engineering challenges at the biology-concrete technology interface. This paper provides a review of previous and on-going research on the use of bacteria-based self-healing of concrete in relation to the problems associated with the setting, hardening and carbonation of concrete and the problems associated with healing large cracks

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