Portland cement can be replaced or supplemented with finely divided materials known as “alternative cementitious materials.” Concrete’s technical properties and/or cost are improved and decreased due to their utilization. Fly ash, ground granulated blast furnace slag, condensed silica fume, limestone dust, cement kiln dust, and natural or manufactured pozzolans are a few examples of these materials. The cement industry is boosting the manufacturing of blended portland cement using industrial by-products, including blast-furnace slags and coal combustion fly ashes, to reduce CO2 emissions. Even though burning fossil fuels is responsible for most CO2 emissions, this only accounts for 50% of the cement industry. The other 50% cannot be eliminated by improving efficiency or switching to alternative energy sources because it is inherent in the chemical reaction that yields calcium carbonate (CaCO3), the main ingredient in cement. Because of this, no cement can be made without releasing carbon dioxide into the atmosphere. As a result, efforts to decarbonize cement manufacturing focus on capturing CO2 rather than reducing the quantity of CO2 in concrete mixtures. However, the high-quality byproducts are few, which calls for an alternative.
Fig 1: Ground Granulated Blast Furnace Slag (GGBS)
Courtesy: Tata Steel
This article talks about whether or not it is possible to replace portland cement with other hydraulic cement, which could lead to lower total CO2 emissions per unit volume of concrete that works the same.
It is essential to collect data on the impact of unconventional cementitious materials. Beyond a 20–30% replacement of OPC by POFA, POCA, and BA, the compressive strength deteriorates. These unconventional cementitious materials’ replacement levels depend heavily on their particle size, shape, and LOI. Thus it’s critical to check their physical qualities as well as their chemical generation.
The data analysis methodology consists of the estimation of CO2 with respect to heat generation, raw materials CO2 emission as well as fuel-derived CO2, and finally on natural sources suitable for producing hydraulic cement.
Regarding heat generation, from data analysis, it was observed that there was electrical consumption in large quantities along with heat generation. It varied country-wise depending on raw materials used and the type of production process implemented during the manufacture. Further, analysis of by-products from CaCO3 proved beneficial compared to CaSO4 due to CO2 emission in the former in contrast to H2SO4 in the latter. Some limitations came to light regarding the RMCO2 and FDCO2, with FDCO2 less practically feasible since a detailed knowledge of processing equipment is required in addition to identified temperature and heat restrictions under which it has to be modified. Since limestone is found abundantly on the earth, the RMCO2 of different pure cement compounds was observed, leading to pozzolans, calcium (sulfo) aluminate-based cement, and calcium sulfate-based cement as answers to the analysis. Also, the volume of hydrates was estimated that was found to be related to RMCO2.
Benefits and Drawbacks
The following are the benefits and drawbacks of cement substitute products:
Inadequate initial power (except CSF).
Lower raw material costs (assuming CSF is used to save cement).
Increase production costs and the risk of mix proportions being incorrect.
Improved durability is a possibility.
Improved curing is required, which raises the cost of placement. However, PFA generally increases cohesion over GGBS.
The combination of PFA and CSF yields a deeper shade. It produces a nearly white color when mixed with GGBS (it may be a bit blue or green initially, but this soon fades).
Industrial by-products, including GGBS, PFA, and CSF, can be hazardous to the environment if they aren’t properly disposed of
Several knowledge gaps are identified in this sector, such as
Lack of comparison of equivalent structures made from alternative sets of materials
Lack of estimate of the lifetime of the structure emitting CO2
Lack of statistical analysis in emission due to the differences in the manufacturing process, methods of calculations, etc.
Lack of sources other than limestone that has comparatively better performance
Lack of practicality in reducing FDCO2 emissions
No reliable technology that will allow rapid but thermally efficient quenching
Low durability of C-S-H in pozzolans
Difficulty in using fly ash due to high curing temperature
Lack of substitutes usability from MgO since its diffusion-controlled
Limitation of usage of calcium sulfoaluminate-based cement in China
Unclear results on TCS
Lack of performance and durability characteristics of hydraulic cement
Problems with Fly Ash
Cement replacement research has been going on for decades, but until now, fly ash research has not matured. It is still happening because of the difficulties of employing fine aggregate in concrete. There is a major problem with employing fly ash in cement since the qualities of fly ash can’t be universally applied. This problem arises due to the large range of chemical compositions found in fly ash. It is impossible to reap the benefits of worldwide research due to the wide variation in fine aggregate characteristics between countries and even within a single country. Even within a single coal power plant, it is well-known that fly ash characteristics are influenced by the coal source, the kind of processing, and many other variables. The fly ash properties change over time as a result of this.
Conclusion and Suggestion
Despite all possibilities in temperature variation, heat generation, source dependability, RMCO2, and FDCO2 emissions, it is clear that more research has to be done that has been identified in the knowledge gap. Researchers can propose researching pozzolans, calcium (sulfo) aluminate-based cement, and calcium sulfate-based cement as time-based performance analyses related to CO2 emissions. Some authors have mentioned that CO2 emissions in cement are less than in steel, but durability is one criterion that makes CO2 emissions in cement more impactful. So if structures are erected using such alkali-activated cement, then CO2 emissions can be well monitored, which impacts durability as well as performance in structures. Further, a comparison can be drawn among structures constructed with such types of cement to see the effect of variation in raw materials by conducting durability tests.
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