The precast products are custom-ordered concrete structures that are prepared, cast, and cured using molds. The concrete structures are built at a manufacturing facility and then moved to the construction site for easy installation. The precast concrete may be used as beams, girders, doors, windows, etc.
The major advantage of precast structures is that they make the construction of modern structures quick and save on costs. Since the concrete components are produced in bulk at a manufacturing site, it helps achieve economies of scale. On the other hand, specialists teams can produce high-quality concrete components with specifications received from the building project manager. In a case the components are needed for identical structures like Hospitals, dorms, etc., they can be built with great speed.
Industry Trends
The ethos of the concrete precast industry has been progressive urbanization. The massive scales of ongoing manufacturing expansion and building commercial spaces and residencies have fueled demand for precast products. Precast concrete reduces overall building costs, improves the construction speed, and reduces wastage of building resources. The precast products find their popular use in the construction of commercial office spaces, bridges, stadiums, etc. And materials used in such structures can be reused to avoid wastage. The developing and underdeveloped nations have emphasized infrastructure projects that help in the development of local industry and upgrade the living standards of their population. The precast industry foresees tremendous growth thanks to its products being deployed in major infrastructure projects worldwide.
The precast industry offers products for different end uses like residential and non-residential categories. The products can also be classified as per type, i.e., walls, floors, roofs, stairs, beams, etc.
Growth in the precast industry
During the year 2020, the overall sale of precast products was valued at approx. USD 92.14 Bn. However, the pandemic brought about a momentary standstill in orders, as most construction projects were halted. The industry’s compounded annual growth rate is projected at 5.3% from the years 2021 up to 2028. As discussed earlier, the majority of future growth is expected in growing economies in the Asia Pacific region. Construction projects have mostly resumed in all places. Infrastructure continues to be a major growth driver for the industry.
Carbon Dioxide emissions
When a conventional method of cement is deployed, it gives out about 0.9 lbs of carbon dioxide per pound of cement produced. Concrete thus used for making precast products involves several other ingredients apart from cement. It is noted that the production of one cubic yard of concrete, i.e., 3,900 lbs, emits approximately 400 lbs of carbon dioxide. Such a huge release of CO2 gas into the atmosphere greatly contributes to the greenhouse effect. Also, the amount of carbon dioxide gas produced by one cubic yard of concrete is equivalent to burning 16 gallons of gasoline.
In a recent report published by IDTechEx, the commercial and technical side of Carbon Capture Utilization is discussed. The report named “Carbon Capture, Utilisation and Storage 2021-2040” also discusses the potential of mitigating carbon dioxide emissions.
Significance of CO2 in Concrete mix
Carbon dioxide, by nature, is a tasteless and colorless gas that makes up about 0.04% of the atmosphere. On average, humans exhale out approx 0.0043 oz of carbon dioxide every minute. The gas is in turn utilized by green plants and is converted into breathable oxygen gas. The natural breathing cycles of humans and other animals are stabilized by the natural photosynthesis of plants. However, rapid industrialization has affected the natural balance. The unnatural carbon dioxide has fossil fuel burning as its primary source. The excessive presence of carbon dioxide gas in the atmosphere leads to trapping heat and raising temperatures. Thereby leading to the “greenhouse effect.” Global warming is believed to be a side effect of the greenhouse effect.
Carbon dioxide reacts with calcium compounds present in the concrete mix. It produces solid calcium carbonate material as part of the binding matrix. Steam cured concrete blocks and fiber-cement panels are produced in large quantities under the typical manufacturing process.
In many cases, carbon dioxide gas is effectively used to replace steam in the curing process for early strength. Such a step helps with long-term durability and reduction in emissions and energy utilization. The 24-hour process windows require the concrete to have a maximum carbon absorption of 29%.
To achieve maximum absorption, carbon dioxide curing is performed to promote reaction efficiency of 60-80%. Such a process assists in resistance improvement to freeze-thaw cycling and sulfate ion attack.
In the United States, the following operations are considered major sources of carbon dioxide emissions (data source Environmental Protection Agency):
- Electricity generation – 40%
- Transportation (local and long-distance) – 31%
- Industrial operations – 14%
Carbon Dioxide and the Cement Industry
A typical cement manufacture plant produces carbon dioxide during two operations, i.e., Calcination and Combustion. Calcination is the larger CO2 contributor at about 60%. And combustion, which includes burning fossil fuels, leads to 40% of carbon dioxide production. The calcination process uses carbon dioxide emitting compounds like clay and limestone. As they are heated to high temperatures, they produce a lot of CO2. Cement manufacturing accounts for roughly 5% of carbon dioxide emissions across the world.
Technologies have been developed at a fast pace in order to curtail or capture carbon dioxide emissions. For example, now the carbon dioxide produced during calcination reaction can be captured and stored safely. It can also be used for a wide variety of industrial applications, such as concrete manufacturing. The UN estimates that Carbon Capture Utilization and Storage can help with mitigation in the range of 1.5 and 6.3 gigatons of carbon dioxide by 2050. Such CCUS technologies need to be adopted at a large scale to amplify their impact on environment preservation.
A new paradigm in the precast industry
Evolution has been the name of the game in the precast industry. Kyoto protocol in 1990 set in motion a lot of progressive developments aimed at reducing carbon dioxide emissions. The technological advancements in precast development have gradually brought down the clinker content used in cement manufacture. One ton of clinker produced an equivalent one ton of carbon dioxide.
The precast industry is concerned that clinker substitutes have led to a reduction in reactivity at early ages. Mechanical performances are a must for materials to be used in the precast process, especially at the day 28 ages.
Hence the fact that CEMI-52.5R cement, compliant with European Standard of NF EN 197-1, is used. It combines high clinker content (around 95% by weight) and gives out a greater reactivity early age (28-day). It also helps achieve a compressive strength of 52.5MPa. Curing, thus introduced at a high temperature, facilitates the development of mechanical properties at an early age.
Metakaolin and its prowess
Metakaolin (mineral addition) can be added in combination with the CEMII-52.5N type. Metakaolin as a product is given out due to the calcination of kaolinite clay at a temperature range of 600-700°C. It is a notable fact that the global dehydroxylation reaction of kaolin does not produce carbon dioxide. No more issue of storing or utilizing the carbon dioxide produced with the conventional process. Also, the minimal carbon dioxide emissions witnessed during metakaolin production essentially come from the process itself (i.e. extraction of raw materials, kiln, etc).
The mineral metakaolin causes the pozzolanic effect on hydration. During the hydration of cement, the siliceous and aluminous components thus issued by the dissolution of metakaolin react with calcium hydroxide to give out a mixture of C-S-H, C4AH13, C3AH6, C2ASH8, etc. Such an addition produces the most favorable durability and mechanical improvements and avoids bad effects on the environment.
Hence the conventional addition of CEMI-52.5R can be substituted by low clinker content such as CEMII/A-S-52.5N.
Mechanical testing performed after the addition of CEMII-A/S-52.5N along with the metakaolin binder has enhanced concrete mix designs. Hence it proves to be a boon for the precast industry. The microstructural strength of the metakaolin binder is worth noting as it has a favorable effect on the early age of the concrete. An increase in the substitution rate of metakaolin proportionately increases the compressive strength of mortar.
Other effects include:
- The mechanical performances of CEMI-52.5R and CEMII-A/S-52.5N along with metakaolin lead to the development of hydrated silica calcium aluminates, which is caused by an increase in the amount of C-S-H.
- Greatly reduced carbon dioxide emissions – CEMII and metakaolin binder reduce clinker by about 30%, directly reducing CO2 gas released to the atmosphere.
- Precast industry can swiftly replace CEMI cement with low carbon dioxide producing binders like CEMII and metakaolin blend.
As the industry moved towards developing sustainable manufacturing processes, the CEMII and metakaolin blend presents itself as a giant leap towards curtailing the release of carbon dioxide into the atmosphere.
