Artificial neural networks (ANNs) constitute a class of flexible nonlinear models designed to mimic biological neural systems. In this entry, we introduce ANN using familiar econometric terminology and provide an overview of ANN modeling approach and its implementation methods. † Correspondence: Chung-Ming Kuan, Institute of Economics, Academia Sinica, 128 Academia Road, Sec. 2, Taipei 115, Taiwan; ckuan@econ.sinica.edu.tw. †† I would like to express my sincere gratitude to the editor, Professor Steven Durlauf, for his patience and constructive comments on early drafts of this entry. I also thank Shih-Hsun Hsu and Yu-Lieh Huang for very helpful suggestions. The remaining errors are all mine.

Artificial neural networks

The capture and use of carbon dioxide to create valuable products might lower the net costs of reducing emissions or removing carbon dioxide from the atmosphere. Here we review ten pathways for the utilization of carbon dioxide. Pathways that involve chemicals, fuels and microalgae might reduce emissions of carbon dioxide but have limited potential for its removal, whereas pathways that involve construction materials can both utilize and remove carbon dioxide. Land-based pathways can increase agricultural output and remove carbon dioxide. Our assessment suggests that each pathway could scale to over 0.5 gigatonnes of carbon dioxide utilization annually. However, barriers to implementation remain substantial and resource constraints prevent the simultaneous deployment of all pathways. Ten pathways for the utilization of carbon dioxide are reviewed, considering their potential scale, economics and barriers to implementation.

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The technological and economic prospects for CO2 utilization and removal

Carbonation of cement-based materials: Challenges and opportunities

Carbonation for the curing of cement-based materials has been gaining increased attention in recent years, especially in light of emerging initiatives regarding carbon emissions. This article reviews the status quo of the carbonation curing process with insight into the approach’s scientific premise, industrial scalability, and commercial spin-offs. Calcium-silicate-based binders, such as Portland cement and its lower energy alternatives, experience very rapid hardening when adequately moistened and exposed to high concentrations of carbon dioxide. Concretes processed as such display improved physical performance and better overall resistance to freeze-thaw cycles, sulfate salts, and acids. In addition to binder activation, carbonation’s valorizing potential can also be exploited to recycle suitable industrial wastes into raw building materials. The perpetual fixation of carbon dioxide in building products conduces a more sustainable stance for the concrete industry as it fulfills mandates for lower carbon footprint. In this article, topics relevant to carbonation curing including reaction mechanisms, processing and impacts on material performance and sustainability are comprehensively reviewed. Further laboratory and industrial research are also proposed.

Review on carbonation curing of cement-based materials

Fresh and hardened properties of one-part fly ash-based geopolymer binders cured at room temperature: Effect of slag and alkali activators

Alkali activation of fly ash and ground-granulated blast-furnace slag (GGBS) is a sustainable technology that promotes recycling of industrial by-products in the form of geopolymer composites. In t...

Effect of process parameters on the performance of fly ash/GGBS blended geopolymer composites

The effect of carbonation curing on the mechanical properties and microstructure of concrete masonry units (CMU) with Portland limestone cement (PLC) as binder was examined. Slab samples, representing the web of a CMU, were initially cured at 25 °C and 50% relative humidity for durations up to 18 h. Carbonation was then carried out for 4 h in a chamber at a pressure of 0.1 MPa. Based on Portland limestone cement content, CO2 uptake of PLC concrete after 18 h of initial curing reached 18%. Carbonated and hydrated concretes showed comparable compressive strength at both early and late ages due to the 18-h initial curing. Carbonation reaction converted early hydration products to a crystalline microstructure and subsequent hydration transformed amorphous carbonates into more crystalline calcite. Portland limestone cement could replace Ordinary Portland Cement (OPC) in making equivalent CMUs which have shown similar carbon sequestration potential.

Early carbonation curing of concrete masonry units with Portland limestone cement

Properties of pervious concrete incorporating recycled concrete aggregates and slag

Effect of steel fibers on the performance of concrete made with recycled concrete aggregates and dune sand

The effect of initial curing on carbonation curing of lightweight concrete masonry units (CMUs) was examined. Initial curing was performed from 4 to 18 hours at a relative humidity (RH) of 50% and temperature of 25°C (77°F). Based on cement content, 4-hour carbonation curing allowed concretes to uptake 22 to 24% CO2 with initial curing and 8.5% without initial curing, while prolonged 4-day carbonation recorded an uptake of 35%. Carbonated CMUs exhibited higher early strength but lower 28-day strength due to water loss by initial curing in comparison to hydrated and steam-cured references. A water-spray mechanism was thus devised to compensate the water loss and, ultimately, made the late strength of carbonated CMUs comparable to references. Carbonation curing can replace steam in CMU production to accelerate hydration, improve durability, and recycle cement kiln CO2 in a beneficial manner.

Hilal El-Hassan

Papers

Effect of process parameters on the performance of fly ash/GGBS blended geopolymer composites

Early carbonation curing of concrete masonry units with Portland limestone cement

Properties of pervious concrete incorporating recycled concrete aggregates and slag

Effect of steel fibers on the performance of concrete made with recycled concrete aggregates and dune sand

Effect of Initial Curing on Carbonation of Lightweight Concrete Masonry Units