Concrete is the most used building material. Due to the scale of use, Portland cement and concrete production drive a large portion of the global greenhouse (GHG) emissions. High-GHG-emitting industries are under increased pressure to decrease their GHG impacts to minimize the impacts of climate change and avoid the worst-case climate scenarios. As Portland cement production is the primary driver for the GHG emissions of cement-based materials, partially replacing Portland cement with supplementary cementitious materials (SCMs) and/or mineral fillers is one of the primary strategies for reducing the clinker content of binder materials. However, the supply of common SCMs is already regionally restricted with constrained supplies of coal fly ash (fly ash) and ground blast furnace slag (GBFS) available for cement-based material production. Notably, as the high-GHG emitting electricity and metal industries work to decrease their own GHG impacts, the generation of fly ash and GBFS will decrease and further restrict the availability of SCMs that decrease the GHG emissions of Portland cement-based binders. Alternative mineral admixtures are needed to meet the continued demand. In this work, alternative mineral admixtures are investigated. Specifically, regionally available flows from agricultural rice hull and rice straw residues and post-consumer flows from waste carpet in Northern California are evaluated using experimental characterization coupled with material flow analysis and environmental impact assessment. Insights from these efforts are then used to present a national-level analysis of material availability and identify promising alternatives.
Post-consumer carpet calcium carbonate (PC4), from waste carpeting, was investigated as a filler material (like limestone). Material flow analysis is used to evaluate the potential annual flow of PC4 materials from post-consumer carpeting, the performance of PC4 in cement-based materials is characterized, and the environmental impacts of Portland cement-PC4 mixtures was quantified. Results showed a loss of performance when PC4 is used, but the potential to decrease GHG impacts in Portland cement-based materials. To address performance loss, PC4 materials was treated to improve mechanical performance, leading to strengths similar to mixtures made with limestone and Portland cement. Rice hull ash (RHA) and rice straw ash (RSA) were investigated as reactive SCMs. Rice hull and rice straw are lower-value residues from rice cultivation. Rice hulls are already a well investigated bioderived SCM. A material flow analysis was used to model the generation of rice hull and rice straw from rice cultivation and the potential ash generation. One challenge with rice straw combustion is the higher levels of alkali-metals (K, Na) and Cl which can cause slagging and fouling in biomass combustion reactors. Leaching that reduces slagging was performed and the effect on compressive strength of Portland cement-ash mortars and the environmental impacts of these ashes was investigated. Results indicate that the use of ashes from leached biomass may be best coupled with energy production systems. Importantly, ash production in these systems is optimized for energy generation, not material properties. Thus, additional investigations were performed into post-combustion processing of RHA and RSA.
These insights from material flow analysis, material performance, and environmental impacts were coupled together to evaluate alternative mineral flows in comparison to conventional mineral admixtures (i.e., fly ash, GBFS, limestone, metakaolin, and silica fumes). Recent trends in material generation are compared to the potential production of alternatives. Projections are made for the future generation of fly ash, GBFS, total coal combustion products (CCPs) and electric arc furnace (EAF) slags under shifting production technologies. By coupling performance needs, environmental impact assessment, and modeled material supply, the potential masses of binary Portland cement-RSA, -RHA, or -PC4 blends are shown to be smaller even compared to decreasing generation of fly ash and GBFS. Notably, EAF slag and CCPs remain larger flows compared to fly ash and GBFS. Demonstrating a coupled assessment of environmental impacts, supply, and performance can be used to identify alternative flows to meet the growing demand for Portland cement-replacing mineral admixtures.