A recent study has presented the potential reusing of calcium wastes to improve the strength of low-carbon fly ash.

Three calcium wastes, namely calcium carbide residue (CCR), limestone waste, and waste cement (WC) slurry in powder form were used as additives and compared with the use of ordinary Portland cement (OPC).

Low-carbon fly ash was replaced with the calcium additives at 20%. However, 20% CCR resulted in flash setting, hence 5% CCR was used instead. A durability test using 3% HCl solution was also performed.

Recently, green cements or low-carbon cements have been developed to reduce CO2 emissions and energy consumption in cement production.

These cements include “pozzolan cement,” in which pozzolanic material is used to partially replace Portland cement, and “geopolymers,” which utilize silica- and alumina-rich materials to produce alkali-activated composites.

Coal fly ash is widely used as a source material for geopolymer synthesis since it contains enough reactive alumina and silica for geopolymerization.

Geopolymer products are known to have beneficial fire resistance and mechanical properties.

However, further development of the composite properties is required to achieve environmentally friendly, strong, and durable products with an increased service life.

The results showed that the reactivity of calcium additives played an important role in strength development.

In the calcium–aluminosilicate–alkali system, calcium silicate hydrate (CSH) and calcium aluminosilicate hydrate (CASH) were formed.

The maximum strength of 21.9 MPa was obtained from the OPC/LCFA geopolymer, and 3% HCl solution had a harmful effect on the strength.

OPC and CCR were favourable reactive sources of calcium compounds to blend with LCFA.

Thermogravimetric results showed that lower thermal weight changes with higher strength gains were achieved.