NEW-AGE CEMENT

The cement industry is facing numerous challenges in the 21st century due to depleting natural resources, shortage of raw materials, exponentially increasing cement demand and climate linked environmental concerns. Needless to say, these novel binders will emanate less CO2 and utilize less energy without compromising their quality. Also, the requirement of construction industry towards speedy construction with varied requirements and attributes have pressed for the need to explore new binders which are waiting for commercial production and usage.  Some interesting formulations are detailed below:

Alkali Activated Slag Cement

Alkali-activated cements belong to family of hydraulic cements that are characterized by a high content of aluminosilicates bonding phase. Aluminosilicates are not reactive with water, or their reaction is too slow. Alkali-activated cements are in competition to Portland cement in cost, performance, and less CO2 emissions. In addition to this, they proved to have more durability and ability to recycle the millions of tonnes of industrial by-products and waste. The prime materials used are blast furnace slag, steel slag, metakaolin, fly ash, kaolinitic clays, and red mud.

Among alkali-activated pozzolan cement, alkali-activated fly ash cement and alkali-activated metakaolin cement are of great interest. To produce geopolymer, fly ash or metakaolin and an alkali activator, such as sodium hydroxide, calcium hydroxide, or potassium hydroxide, are used. The curing temperature of geopolymers usually ranges between 40⁰C and 95⁰C, which is higher than that of Portland cement, which requires room temperature for curing. Higher curing temperature promotes the reactivity of fly ash and results in higher early strength. For metakaolin-based geopolymers, curing temperature is also crucial on setting and hardening. Elevated curing temperatures leads to higher early-age compressive and flexural strengths. Geopolymers are found to be very resistive against acid and alkali-silica reaction and exhibit higher strengths when cured at higher temperature

Belite Cement

The clinker mineralogy for belite-rich Portland cement is same as OPC. They are also known as high belite cements. The key difference between Portland cement and the type of cement manufactured a century or more ago is the alite/belite ratio in clinker composition. Belite content is more than 50% in belite-rich Portland cement, while alite constitutes about 35%, which makes belite the abundant phase, compared to the conventional OPC where alite is the most abundant phase that constitutes around 50-55% and belite is only 18-26%.

It was found that high belite cement not only lowers the heat evolution but also shows excellent workability and durability. It also exhibited better sulphate and chloride resistance. Belite-rich cement are manufactured with same process as conventional OPC but with less amount of limestone in clinker raw mix. The commercial production of belite-rich Portland cement is carried out in dry process kilns with preheaters. It uses the same types of materials but mix design is different. Clinkering temperature inside the kiln is approximately 1350⁰C, which is 100⁰C less than is needed for traditional OPC.

Calcium Sulphoaluminate Cement

Calcium sulfoaluminate (CSA) cements are types of cements that contain high alumina content. To produce CSA clinker, bauxite, limestone, and gypsum are mixed together in a rotary kiln. CSA cements were developed in China and came to prominence since the late 1970s. The main constituents of the cement powder contain belite phase (C2S), ye'elimite (C4A3S), and gypsum (CSH2) [90-92]. Upon hydration, CSA cements form ettringite according to the following reactions.

The classical calcium sulfoaluminate clinkers are predominately based on 35-70% ye'elimite (C4A3S), 30% belite (β−C2S), with lesser percentages 10-30% of phases like, C12A7, C4AF, and CaO, but C2AS and CS are not desirable due to their deleterious nature.  Raw mix design of CSA compositions, needs less limestone that not only benefits in reduced thermal energy (up to 25%) but also decreased CO2 emissions (up to 20%) compared to the Portland cement. Industrial waste materials can also be used as raw materials for manufacturing CSA cements and therefore, calcium sulphoaluminate cements have significant environmental advantages.

Magnesia-based Cements

Magnesia cements are based on magnesium oxide (MgO) as main ingredient. It is developed by Sorel in 1867 and is known as “magnesite” or magnesium oxychloride cements. At early stages, this type of cements was produced by using magnesium oxide and aqueous magnesium chloride. The resulting hardened product consists of four major bonding phases as: 2Mg(OH)2 • MgCl2 • 4H20, 3Mg(OH)2 • MgCl2 • 8H2O, 5Mg(OH)2 • MgCl2 • 5H2O, and 9Mg(OH)2 • MgCl2 • H2O. However, it was soon recorded that magnesium oxychloride phase is not stable after an exposure to water over long time as it results into leaching out in form of magnesium chloride and magnesium oxide. This limits the practical application of the cement to certain properties in construction even though it showed high strength properties, high fire resistance, high abrasion, and exemption of wet curing compared to traditional OPC. In the recent decade, after Harrison patented reactive MgO cements the production has been significantly increased to 14 Mt per year. Magnesium oxysulphate cements, based on magnesium sulphate solution and magnesium oxide, has similar properties to Sorel cements but poor weathering resistance has confined its utilization on mass scale.

Conclusions

Advance technological and experimental methods are needed to establish for these novel cement systems. These binders are supposed to provide a simple yet promising solution for the needs of construction industry as well as for the abatement of CO2 emission at commercial scale. This potential of novel binder can be fully realized only through detailed investigation and characterization of binder with help of cutting-edge technologies. Establishing codes, standards, and setting guidelines with trainings will play a key role in developing commercial manufacturing and application concept.

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