Replacement of Portland cement (PC) by fly ash (FA) is currently limited to 15–30% by mass, mainly due to low early age strength development of concrete. This research uses calcium formate (Ca(HCO2)2; CF) as an admixture to high-volume FA (HVFA) composites to improve its strength properties. HVFA represents 60–70% of cement replaced by FA and dosage of CF varies from 0.5% up to high dosage of 9% of cement content. Compressive strength, isothermal calorimetry, thermogravimetric analysis, X-ray diffraction and scanning electron microscopy were conducted to investigate the effects of CF on hydration and microstructural aspects. The results show that both HVFA pastes with 60% and 70% FA achieved the highest strength at the CF dosage of 3%. At the age of 28 days, adding 3% CF to HVFA mixes led to higher consumption of FA as well as higher formation of calcium hydroxide (Ca(OH)2), calcium silicate hydrates, calcium carbonate (CaCO3) and ettringite, which contribute to the increase of strength. The addition of very high dosages of CF at 9% increased the hydration of tricalcium aluminate but could hinder the hydration of tricalcium silicate in both PC and HVFA pastes with 60% and 70% FA.

This review presents the research conducted to date in the field of cement-based composites reinforced with waste paper-based cellulose fibres, focusing on their composition, mechanical properties, and durability characteristics. The literature demonstrates that the properties of raw material (depending on their own chemical composition) significantly influence the formation of the cement composite binders. When considering fresh properties, the presence of silica and magnesium compounds generally lead to favourable effects on the setting of the cement composite when combined with waste paper cellulose fibre. Reduction in density values, i.e., approximately 25%, was observed with the inclusion of waste paper fibres from 20 to 80% in cement composites. The homogeneous dispersion of fibres in the matrix is one of the crucial factors to achieve in order to develop composites with well-balanced mechanical properties incorporating waste paper cellulose fibres. Hence, dispersion of fibres can be improved by increasing water quantity corresponding to the optimal value, which was a water/cement ratio of 0.64 leading to optimum strength properties of the composite. Even though the effect of fibre dispersion in the matrix improves with the addition of water, higher porosity and voids govern the strength properties beyond an optimum water-to-cement ratio. Higher porosity leads to an increase in the water absorption and a lowering of the thermal conductivity properties with the addition of paper fibre in cement binders. Paper fibre absorbs a high amount of water leading to higher water absorption. This phenomenon is related to the hydrophilic nature of cellulosic fibres absorbing some volume of water due to their microporous structure.