The major environmental impact of concrete comes from the CO 2 emissions, produced during the cement manufacturing process. The main goal of this research project is to evaluate the efficiency of limestone powder as a partial cement replacement, in order to reduce energy consumption and CO 2 emissions. This study utilizes limestone powders, with different particle sizes, to replace a portion of Portland cement using various ratios. Due to the dilution effect when partially replacing cement,

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... e is a reduction in the concrete's physical properties. To assess the dilution effect, a modification to Féret's equation is used to calculate an efficiency factor for the limestone powder when compared to cement. To measure the environmental impact, a life cycle assessment is conducted on concrete made with limestone powder combined with cement. This allows for an evaluation of the various cement/limestone powder ratios that will maximize the environmental benefit, with minimal reduction in concrete strength. Additional microstructural analysis using petrographic examination was completed to provide a visual understanding of the distribution of the limestone particles within the cement paste. The results indicate that the efficiency of limestone powder in partially replacing cement can be achieved by particle packing and particle distribution in the concrete and the benefits of emission reductions exceed the loss in compressive strength when higher levels of limestone powder is used to replace cement. Sustainability 2018, 10, 550 2 of 24 than its influence on the chemistry, indicating that limestone powder has little effect on the temperature of the fresh properties of concrete. Limestone powder, however, is not entirely an inert filler. While there is a slight interaction between tricalcium silicate (C 3 S) and CaCO 3 , there is no pozzolanic reaction and does not produce calcium silicate hydrate (CSH) gel [5]. The particle size of limestone powder in the binder phase of a mixture improves particle packing efficiency, which leads to improved blocking of capillary pores and reduced penetrability. This then results in a lower water demand due to reduced bleeding of water, thereby improving workability and durability [6] . Palm et al. [7] and Lollini et al. [8] also reported that a lower water-cement ratio is the main parameter in cement with high limestone content, which leads to higher solid volume and lower porosity in the concrete. Matschei et al. [9] found that the ettringite formation derived from the reaction of sulfoaluminate with water and calcium hydroxide increases the molar volume of paste solids and can magnify the space-filling properties of paste. This could lead to a reduction of porosity and permeability of the paste. Separate grinding of the limestone and clinker provides greater opportunity to optimize particle size distribution, and can be incorporated into concrete like other pozzolans. This process is an alternative to inter-grounding the limestone with the cement in which the limestone powder can be mixed in with the concrete while batching. Fly ash is widely used as a cement replacement because of its cementitious and pozzolanic properties. From 2009 to 2015, power generation from coal as a fuel has been reduced by 12% in the United States [10]. As the use of natural gas for power generation and green energy gains popularity, there has been a reduction in the availability of fly ash as less coal is burned at power plants. Therefore, an alternative is needed for cement replacements due to the inefficiency in the manufacture of cement. The release of CO 2 in cement production is primarily due to the calcination of the limestone. Approximately 1.6 metric tons of raw materials are essential to produce one metric ton of cement [11] . An estimated 40% of raw materials are lost in the formation, but the environmental effect can be lowered by employing limestone powder as a replacement for cement. However, this reduction effect will vary based on particle size of the limestone powder used as fine limestone powder, which requires extra milling. Limestone powder with a particle size of 8 µm produces about 24.5 kg of CO 2 per ton, whereas finer particles (4.5 µm) produce approximately 90.7 kg of CO 2 per ton [12] . This amounts to 3.4-12.5% of CO 2 emissions compared to emissions producing one ton of cement, making it sustainable as the major environmental impact of concrete comes from the CO 2 emissions during cement production. Many models describe the relationships between mix composition and property of compressive strength of the concrete. However, the mix composition of concrete does not only include cement since more materials are used to replace cement in concrete. Cement concentrations are still a major factor that determine compressive strength. There are multiple popular models, such as Féret's equation, Bolomey's formula, and Abrams' formula that focus on the relationship between water, cement and the compressive strength. These can also be used as predictive models based on water and cement content to predict compressive strength. F. de Larrard [13] documents the accuracy of these models and compares them to the same data sets. The volumetric approach by Rene Féret's model incorporates more elements of concrete that determine the strength [14] . To isolate the efficiency of limestone powder in concrete, an extension of Féret's equation can be used because it takes the air content into account, and has a mathematical form that is physically justified by the use of absolute volume. As urban expansion grows, the increasing demand for concrete may exceed cement production's capacity. The use of supplementary material in concrete to replace portions of cement is important to meet demand and reduce the environmental impact of cement production. With fly ash supplies decreasing, an alternate cement replacement is needed. While most research focuses on inter-ground limestone, this study focuses on the efficiency and feasibility that CaCO 3 , or limestone powder, can be used as a cement replacement while batching concrete. The use of limestone powder during the batching process will be investigated for the mechanical and environmental effects the particle size and replacement level have on concrete efficiency for optimal performance.