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Geopolymers vs. Cement Matrix Materials
Geopolymers are spreading more and more in the cementitious materials field, exhibiting technological properties that are highly competitive to conventional Portland concrete mixes.
Raw materials preparation. After quarrying, principal raw materials (such as limestone and clay) are crushed to a maximum of about 7.5 cm in diameter by a two-step grinding treatment. In a “dry” production method, crushed aggregates are fed directly into the kiln. Conversely, the “wet” method consists of the mixing of ground material with water to form a slurry, a suspension of creamy consistency composed of water (35–50%) and fine particles. Next, the slurry is inserted into the kiln. “Wet” preparation ensures an easier control of the chemistry process and is suitable in the presence of moist raw aggregates. However, it has higher energy requirements due to the need for slurry water evaporation. On the other hand, the “dry” process is faster and less energy expensive .
Clinker production. Ground ingredients are burned in the kiln at a temperature of about 1350 °C to 1500 °C. Various materials can be used as kiln fuels. Traditional sub-stances are fossil fuels, oil, coal, and gas. Secondary raw materials, including waste oils, plastics, waste tires, and sewage sludge, are considered alternative fuels for the cement industry . Thermal treatment involves the partial fusion of raw materials, the breaking of their chemical bonds, and the recombination into new compounds. The result is a nodular-shaped clinker product.
Final grinding process. After cooling, clinker nodules are crushed into a superfine powder by steel ball milling treatment. During this process, the clinker is mixed with a small amount of gypsum (3–5%) to produce PC. Gypsum prevents the flash setting of the cement and regulates its hardening.
Use of alternative fuels or raw materials to mitigate CO2 emission for Portland cement manufacturing.
Adoption of CO2-capturing technologies in the cement plants.
Development of Portland-free alternative binders.
2. CO2 Emission Mitigation Strategies in the Cement Industry: An Overview
2.1. Alternative Fuels in PC Manufacturing
2.2. CO2-Capturing Technologies
2.3. Alternative Cementitious Binders
3. Geopolymer Cements: Basic Features and Novel Advances
Preparation of the activator solution. The alkali solution is prepared at least one day prior to its use, employing distilled water to avoid contamination by unknown substances. Common molarity values investigated are in the range of 8 to 16 M.
Raw material mixing. One or more precursor types are mixed with the mineral aggregates (fine sand or coarse fraction), in dry conditions, for 2–3 min. The aggregate part occupies about 75–80% by mass of geopolymer concrete/mortar.
Activator addition. The alkaline solution is added to the solid fractions and mixed for 3–5 min. To improve the rheology of the compound, in terms of workability, a part of water-reducing admixture could be added to the mixture. Firstly, the admixture is mixed with the alkali solution and then added to the solid dry material.
Casting. The fresh material is cast in specific molds (cylinder, cubes, or beams) kept under vibration for 10 s.
Curing. The specimen curing can be performed under various temperature and time regimes. The material hardening can be completed entirely at room temperature or provide oven treatments between 30 °C to 90 °C, accelerating the curing. Typical treatment times are selected in the range from 6 h to 96 h.
The entry is from 10.3390/nano11082007
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