Fly ashes that do not meet the ASTM C618 specifications are considered “off-spec” and are not used as SCMs. In this research, four off-spec fly ashes (OFAs) were sourced from different parts of the U.S. and the characteristics of these OFA concretes were measured to compare and contrast their performance with that of the mixtures containing 0% OFA. The first objective of this study is to assess the influence of OFA reactivity and replacement levels on concrete characteristics. The second objective of this study is to assess the influence of constituent material characteristics, such as shape and size of coarse aggregate, fineness modulus of fine aggregate, and cementitious content of the concrete mixture on the fresh and hardened characteristics of concretes containing OFAs. Results indicate that at sufficient degrees of reactivity and replacement levels, OFAs can provide characteristics comparable to that of conventional OPC concrete while improving the consistency of the concrete. Findings from sensitivity analysis revealed that the degree of reactivity (DoR) of the OFA has a high influence on the hardened characteristics of concrete. Finally, the life cycle assessment of concrete mixtures containing OFAs indicates that greenhouse gas emissions can be reduced by up to 45% when compared to conventional mixtures.

This study aims to investigate the possibility of using Linz-Donawitz (LD) slag as one of the cementitious materials for preparation of composite slag (having 8 and 15% LD slag), which will subsequently be used for manufacturing portland slag cement (PSC). PSC samples (having overall 4 to 9% LD slag) were prepared using LD slag from two sources in a laboratory ball mill. PSC samples were analyzed for various chemical characteristics and physical properties. Studies were conducted on concrete mixtures prepared at water-cement ratios (w/c) of 0.65 and 0.40. Fresh, hardened, and durability properties of concrete mixtures prepared using PSCs made with composite slag having up to 15% LD slag were found to be comparable to their corresponding control mixtures. Based on results, it was observed that composite slag having LD slag up to 15% of total slag can be used up to 60% for manufacturing PSC along with clinker and gypsum. The 3-, 7-, and 28-day compressive strength of PSC samples containing LD slags in different proportions were found to be comparable to control PSC samples and meeting the requirements of IS 455:2015. Even though the free lime content in LD slags was significantly higher (free lime content of 3.03 and 3.48%) in comparison to granulated blast-furnace slag (GBFS), it had almost a negligible effect on the PSC prepared using LD slag and soundness of experimental and control PSC was comparable because the maximum amount of LD slag added in overall PSC was restricted to 9%. The addition of LD slag in different proportions up to 9% in overall PSC does not seem to have any detrimental effect on performance of concrete in terms of sorptivity, carbonation depth, chloride penetration, and diffusion, which indicates its suitability for application in reinforced concrete structures.

Post-fire rehydration is an interesting method to recover the structural performance of fire-damaged concrete. This paper evaluated the viability of using cementitious materials containing carbon nanotubes (CNTs) or carbon-black nanoparticles (CBNs) for damage recovery detection and self-monitoring of strain and stress of fire-damaged structures subjected to post-fire curing. Nanomodified mortars were subjected to high temperatures, rehydration, and measurements of capacitive behavior, electrical resistivity, and self-sensing properties. After 600°C and rehydration, mortars with 9.00% of CBN presented the ability of self-detection of damage recovery, as also verified in mortars with 0.4 to 1.20% of CNT and 6.00% of CBN after 400°C and rehydration. The post-fire curing method filled the pores and microcracks of the cementitious matrix with nonconductive rehydration products, increasing their electrical resistivity. Mortars with 0.80 and 1.20% of CNT presented self-monitoring of strain and stress after 400°C and rehydration, as also observed in mortars with 9.00% of CBN after 600°C and rehydration. The post-fire curing process also increased the selfsensing properties because nonconductive rehydration products obstructed conductive stretches, improving tunneling conduction mechanisms rather than contacting conduction. These self-sensing materials are promising alternatives to evaluate post-fire curing processes and self-monitor the strain and stresses of next-generation smart structures.

An important part of improving the embodied carbon of the built environment is reducing carbon emissions associated with concrete. The long-term limitations around the availability of supplementary cementitious materials (SCMs) to replace portland cement have driven the search for additional innovative approaches. The beneficial use of carbon dioxide (CO2) in ready mixed concrete production has been developed and installed as retrofit technology with industrial users. An optimum dose of CO2 added to concrete as an admixture leads to the in-place formation of mineralized calcium carbonate (CaCO3) and can increase the concrete compressive strength. The improved performance can be leveraged to design concrete mixture proportions for a more efficient use of portland cement, along with the use of CO2 to reduce the carbon footprint of concrete. One producer has used the technology, starting in 2016, at over 50 plants. More than 3 million m3 of concrete have been shipped with an estimated net savings of 35,000 tonnes of CO2. The concrete produced with carbon dioxide is discussed in terms of the fresh and hardened performance, durability performance, and life cycle impacts.

The built environment is facing an increasing challenge of reducing emissions regarding both embodied and operational carbon. As an ultra-durable concrete, engineered cementitious composites (ECC) reduce the need for repair, thus resulting in a prominent reduction of life-cycle footprints. Herein, a new version of low-carbon ECC was developed for cast-in-place applications by sequestering CO2 through mineralization. Two waste streams were pre-carbonated and incorporated into ECC as fine aggregate and supplementary cementitious material, respectively. At 28 days, the CO2-sequestered ECC exhibited a compressive strength of 32.2 MPa (4670 psi), tensile strength of 3.5 MPa (508 psi), and strain capacity of 2.9%. Multiple fine cracks were distinctly identified, with a residual crack width of 38 μm (0.0015 in.) and a selfhealing behavior comparable to that of conventional ECC. The new ECC sequestered 97.7 kg/m3 (164.7 lb/yd3) CO2 (equivalent to 4.7 wt% of final mixture) and demonstrated a 42% reduction in cradle-to-gate emissions compared to conventional concrete at the same strength level. This study demonstrates the viability of turning waste CO2 gas into durable construction materials and proposes a potential path towards carbon neutrality.