CEMENT CONTENT REDUCTION IN CONCRETE THROUGH AGGREGATE OPTIMIZATION AND PACKING : A SUSTAINABLE PRACTICE FOR PAVEMENT AND SEAPORT CONSTRUCTION

Based on the full-bodied scientiic consensus that climate warming is occurring on Earth, signiicant environmental and economic beneits may be obtained from cement content reduction in concrete, naming reduction in CO2 emissions, energy consumption, and construction costs. In contrast, standard limestone aggregate may experience an increase in mining rate. The objective of this study was to propose the reduction of cement content in concrete framed by aggregate optimization as a viable alternative to reduce environmental efects from the cement industry. The study concludes that the higher the concrete volume in construction, the more environmental beneit can be obtained by reducing cement content in concrete. A 25 % cement reduction in a concrete pavement led to a decrease of 80.000 tons in CO2 emission, while a seaport construction displayed a decrease of 17.000 tons of CO2. The higher the designed cement per volume of concrete, the more budget savings in case of reduction in cement content. Port construction presented a reduction of 20,10 USD per m3 of concrete in cement costs against 11,04 USD per m3 in pavements. An 11,5 % increase in aggregate mining is expected when concrete contains less cement in its composition, targeting South Florida (U.S.A.) quarries.

Nowadays, technological innovations have turned possible cement plants to operate at the 90 % eiciency level due to higher equipment availability, lower energy consumption, and higher nominal eiciency of the equipment.For instance, pre-heating towers have been included in modern plant layouts.Also, renewable sources of energy have replaced coal by about 8 % as a source of fuel for kilns.Companies have used recycled carpet, plastic, and paper to generate heat in kilns, as well as roughly 60 % of natural gas to run cement production (LACC, 2015).
Although companies have taken some eicient measures, more can be done to reduce environmental impacts.For this reason, this study investigates the inluence of cement content reduction in concrete as a sustainable practice to reduce CO2 emission.A combination of aggregate optimization and packing is emphasized in typical real-world cases of slip-formed concrete for pavement, and concrete structures for port construction since these are essential infrastructure projects that usually need large volumes of concrete for suitable completion.

Cement Production
Despite industry eforts to address CO2 emissions, singular initiatives have low environmental signiicance.Thus, a joint efort must be made involving academia, government, policymakers, industry, and communities to create a sustainable production environment in which all stakeholders can be beneited (Agopyan and John, 2011).
Limestone is the primary component in cement production and is blended to aluminum, iron, and silica to form the so-called raw meal further burnt into a kiln at about 1300 °C to allow chemical reactions needed to produce clinker (IEA, 2010).At the end of the kiln, formed clinker usually passes through ball mills to achieve the appropriate Particle Size Distribution (PSD), then blended to 5 % of gypsum.Figure 01 illustrates part of the cement production process.
One procedure largely adopted in modern cement plants to reduce energy consumption is the implementation of pre-heating towers into the production process.In general, raw materials pass through pre-heating towers for about 10 seconds, enough time to increase the raw material temperature from 100 °C to 870 °C, approximately.Within this temperature range, most chemical reactions occur, thus reducing the time within rotational kilns (IEA, 2010).

INTRODUCTION
Scientists through a robust scientiic consensus claim that warming of the climate system is occurring on Earth.The Intergovernmental Panel on Climate Change (IPCC), in the Fifth Assessment Report (AR5) released in 2013, airms that climate change is unequivocal.The report conirms that atmosphere and ocean have warmed up, the amounts of snow and ice have decreased, the mean of global sea level has increased, and the accumulation of greenhouse gases (GHG) has raised.Continued GHG emissions may generate more warming in the climate system, and a large reduction of GHG is needed to restrain climate changes, preventing drastic consequences to society and environment (NEC, 2016).
Infrastructure has a signiicant efect on climate change.It accounts for more than 60 % of the GHG emissions.Sustainable infrastructure is crucial for society to adapt to the climate efects.Technological advances in Civil Engineering materials lead to higher eiciency in the use of natural resources and energy, which is an urgent challenge (NEC, 2016).
Cement and concrete industry is a signiicant contributor to Carbon Dioxide (CO2) emissions and environmental degradation.As such, eforts have been made to cut the CO2 footprint of the inal products.One of the primary measures partially implemented by the industry, including the cement producers, ready-mix concrete companies, and construction irms, is the rational use of natural resources.The rationale is: the lower the cement content and natural aggregate consumption in concrete, the lower the negative impact on the environment is supposed to be (Khatibmasjedi, De Caso y Basalo, and Nanni, 2016;Mehta 2002).While demand for new infrastructure remains strong worldwide, inding technical and environmentally sound solutions is a priority for delivering Sustainable Development Goals (SDG) and reducing climate risk (NEC, 2016).
Materials, water, and energy consumption in concrete production became a crucial aspect of the entire supply chain.Thus, adopting sustainable techniques while keeping the concrete properties at high standard levels is essential towards better resources management (Agopyan and John, 2011;Khatibmasjedi, De Caso y Basalo, and Nanni, 2016).Portland cement production is not only energy-intensive but also responsible for signiicant emissions of CO2 (MEHTA, 2002).Consequently, lower CO2 emissions would also be observed not just due to the decrease in raw materials extraction, but also by reducing the energy required to process the resources (Agopyan and John, 2011;Aissoun, Hwang, and Khayat, 2016).There are two conventional techniques to develop optimized aggregate gradations: the percent retained chart, and the Modiied Coarseness Factor Chart (MCFC).The irst step of the process is developing a target gradation that plots as a haystack on the percent retained chart (Lindquist et al., 2015).

Aggregate Packing
Historical data on packing come from Scandinavia as early as 1896 for providing concrete durability in marine environments.Most of the literature on packing was published in the 1930s describing the optimization of packing (Mohammed et al., 2012).
Fundamentals on aggregate packing have been the focus of scientiic discussions for about a century (Moini et al., 2015).Figure 03a (Moini et al., 2015) shows monosized and random spherical particle shapes used in aggregate packing theoretical simulations.The well-known spherical simulations may generate fewer voids to ill with cement paste than empirical approaches, leading to optimistic results.Since spherical blends pack 12 % more than crushed blends (Figure 03b) (De Larrard, 1999), computer simulations may predict use of 12 % less cement paste content in concrete mixtures, which does not match to current practices regarding cement paste volume (De Larrard, 1999).

Aggregate Gradation
Aggregate gradation is deined as the distribution of particles of a granular material among the various size ranges, usually expressed in cumulative percentage passing through standard sieve series.The Maximum Aggregate Size (MS) is deined as the smallest sieve size that retains 15 % or more of the particles, and the ineness modulus (FM) of ine aggregates as the cumulative percentage of particles retained within the standard sieves 150 µm and 37.5 mm (MEHTA AND MONTEIRO, 2006).
Most concrete mixtures are composed of: ine aggregate, with particles smaller than 4.75 mm; and coarse aggregate, with particles larger than 4.75 mm.A well-graded distribution, which is, covering all particle sizes, often contains a higher percentage of intermediate-sized particles and smaller portions at the extremes.Such gradation is often described as a haystack when plotted on percent retained charts (MEHTA AND MONTEIRO, 2006).

Aggregate Optimization
Optimized aggregate gradations have been speciied and endorsed.In contrast, very few practical and comprehensive methods to perform the optimization are available to concrete practitioners (Lindquist et al., 2015).
The combination of coarse and ine aggregates is commonly not well-graded because of the absence of intermediate-sized particles.Figure 02 shows an example of a poorly-graded aggregate with a gap-graded or peak--valley-peak distribution (Lindquist et al., 2015), as well as haystack curve.
is slowing down in South America, Africa, and Asia.The expectation is that in 2050 the world's population will be stable at approximately 10 billion.However, natural resources are likely to decrease in availability due to current high demand for new infrastructure, including roads and ports.Indeed, extracting raw materials is causing environmental degradation and, signiicant energy is required to mine, crush, and process operations (Khatibmasjedi, De Caso y Basalo, and Nanni, 2016;mehta, 2002).
The GHG emission level has a direct impact to the Earth's atmospheric layer, contributing to an increase of its opacity, with the potential of diminishing its capacity to release energy generated by the surface (Agopyan and John, 2011).
In detail, the Earth's surface absorbs the energy provided by the sun, turning it into heat or infrared energy.The heat is relected on the Earth's surface, ascends and provides energy for the excitation of CO2 molecules in the atmosphere.Besides other molecules, CO2 acts as heat--relector, redirecting the heat back to the surface of the Earth.This process can create warming efects on the planet (Rifkin, 1989).
For instance, 1 g of limestone decomposed in high-temperature furnaces can generate 0.81 g of CO2.
Cement is composed of limestone and is the most used artiicial material worldwide, which is also responsible for about 5 % of the global CO2 emissions (Khatibmasjedi, De Caso y Basalo, and Nanni, 2016;mehta, 2002;Agopyan and John, 2011)

Materials
The Fine Aggregate (FA) considered in this study was silica sand, the Intermediate Aggregate (IA) was the Miami Oolite Limestone #89, and the Coarse Aggregate (CA) was the Miami Oolite Limestone #57, all according to the ASTM C33-16e1 (ASTM, 2016), Speciication for Concrete Aggregates.
The Portland cement Type I/II and Type V considered in this investigation had physical properties and chemical composition according to the ASTM C150-17 (ASTM, 2017), Standard Speciication for Portland Cement.

Research Methodology
The objective of this study is to propose the reduction of cement content in concrete as a viable alternative to green concrete by illing voids in aggregate gradation with intermediate-sized particles.This study can add to current initiatives to reduce cement production environmental The deinition of Aggregate Packing (k) is the ratio between the actual Solids Volume (Pv) and the Compacted Bulk Volume (V).In any typical blend, packing depends on three main parameters: the size of the grains, the shape of the grains, and the packing method (De Larrard, 1999).The method depends on the energy applied to the particulate samples.

Green Concrete
Is it possible to reduce environmental damage caused by cement and concrete industries?In general, a great approach to minimize the environmental impact of using any natural mined resource is downgrading its consumption rate (Mehta, 2002).
Aggregates usually occupy 70 to 80 % of the volume of Portland cement concrete, signiicantly dominating its mechanical and physical properties (Tasi, Li, and Hwang, 2005).The properties of the aggregates, proportioning and packing signiicantly afect the performance of concrete.Optimized aggregate blends may improve concrete performance and be an essential advantage to design concrete at lower cementitious material content levels (Moini et al., 2015).
North America and Europe are experiencing a population growth stabilization, while population growth Agopyan and John (2011), 44 % of the limestone consists of CO2, and the ASTM C150 (2017) requires that the limestone used to produce cement contain at least 70 % of Calcium Carbonate (CaCO3) in its composition, although routine tests indicate a 79 % content of CaCO3 in limestone.To put it another way, making 100 g of Portland cement would require 62,9 g of CaO, 112,3 g of CaCO3, and likely 160,5 g of limestone.Reducing cement content in 25 % can result in a decrease of 80.000 and 17.000 tons of CO2 emission in pavement and seaport construction, respectively.

CONCLUSION
This study discussed the efects of the reduction of 25 % in cement content in concrete on CO2 emissions and cement budget, which has a signiicant impact on the current and future sustainability of cement production and concrete employed in business-as-usual construction such as pavements and seaports.Many advanced economies such as the U.S. must replace and upgrade long-neglected bridges, pavements, pipelines, transit systems, among other infrastructure.
The primary indings are: 1. Results show an 11,5 % increase in limestone mining for concrete production alone, targeting South Florida sources.
2. Typical cement costs in concrete can be reduced in 11,04 USD per m3 of concrete for pavement and 20,10 USD per m3 for seaport construction.
3. A decrease of 80.000 tons of CO2 emission in a 160km road segment (one direction), and 17.000 tons of CO2 in seaport construction might be obtained.
We have a timely opportunity to reduce the risk of climate change in infrastructure construction through innovations in aggregate optimization and packing techniques.These are inexpensive, signiicant and still not quite realized opportunities for advances in concrete design that can reduce the costs of built infrastructure.
On the one hand, the higher the concrete volume in construction, the more signiicant environmental beneit can be obtained by reducing cement content in concrete.On the other hand, the higher the designed cement per volume of concrete, the more budget savings in case of reduction in cement content.To conclude, an increase in aggregate mining is to be expected when concrete contains less cement in the composition, relation that merits further investigation.
Concrete is usually used in breakwater systems to protect channel entrances, in berths to increase vessels handling capacity, and in dry-dock bottom slabs to increase storage capacity.Tetrapods are required in breakwaters (Figure 06), comprising an estimated volume of 61.200 m3 of concrete.Pier structure is usually made of precast and reinforced concrete spanning for about 914 m.Approximately 536 m3 of concrete are needed to cast circular piles with an outer diameter of 81 cm, inner diameter of 51 cm, and 610 m depth (Figure 07) (Larrossa, Real, and Dias, 2014).The slab considered in the calculations is a concrete structure of 130 by 335 by 0,90 m (40.000 m3).Assuming tetrapods, pier, and slab use the same type of concrete, a total of 102.280 m3 would be required for this construction.Table 03 details that nearly two million USD would be saved in this project under a 25 % cement content reduction in the concrete, resulting in a reduction of 20.10 USD per m3 of concrete in cement costs.

Reduction in CO2 Emission
Cement is composed of about 60 % Calcium Oxide (CaO) from limestone.The ASTM C150 (2017) does not require a minimum or maximum limit of CaO in the chemical speciication of Portland cement.But, standard tests show that inal products contain 62,9 % of CaO.According to

Figure 02
Figure 02Haystack and a gap-graded peak-valley-peak gradations.

Figure 06
Figure 06 Tetrapods used in port breakwaters.Source: author.

Figure 07
Figure 07 Piles for the pier.Source: author.