Fillers such as fly ash, slag, and biochar offer potential solutions for addressing carbon emissions from cement manufacturing and improving waste management. However, concrete with fillers experiences severe thermal damage at elevated temperatures due to issues like thermal incompatibility, pore pressure build-up, thermal stress, and phase transformation. This paper offers a comprehensive review of how fly ash, slag, and biochar impact concrete when subjected to high temperatures. It reviews phase stability, alterations in microstructure, thermal damage, and mechanical behaviour, as well as approaches to improve concrete's fire resistance. Fly ash and slag reduce microcracks in concrete during heat exposures by consuming free portlandite (Ca (OH)2) during cement hydration, while biochar mitigates pore pressure in the matrix. However, fillers lower concrete's thermal conductivity, increasing temperature gradients and reducing fire resistance. A mix of steel and polypropylene fibers enhances fire resistance more effectively than using a single fiber type.

Porous supplementary cementitious materials (SCMs) are known to adversely affect the rheology of fresh concrete. When cement is replaced on a weight basis, these effects are often intensified due to the high number of porous particles introduced relative to the volume of cement displaced. In this study, biochar was selected as a representative porous SCM owing to its high porosity. To address this, a volumetric water-to-cementitious materials ratio (w/c) approach was employed, in which biochar replaced cement by volume at levels of 5 %, 10 %, and 20 % and were compared with a conventional 5 wt% replacement to assess their impact on workability, hydration and mechanical properties. The results demonstrated that weight-based replacement introduces a large number of porous SCM, leading to severe slump loss (69 % reduction at the 5 wt% level), shortened setting times, decreased moisture retention, and limited internal curing capacity. In contrast, the volumetric replacement method provided a more balanced substitution by incorporating a volume of porous SCM equivalent to that of the cement replaced. It ensured workability of concrete, as indicated by only a 12 % reduction in slump at the 20 vol% replacement level. Remarkably, due to improved moisture retention, although the 10 vol% biochar mixture contains 5 % less cement than the 5 wt% mixture, it still achieved 1 %, 0 %, 3 %, and 6 % higher compressive strength at 3, 7, 28 and 56 days, respectively. This underscores the effectiveness of volumetric replacement for porous SCMs in concrete.

Using biochar as a replacement for sand in concrete provides a sustainable solution to the ecological degradation and environmental pollution. This study investigates the use of biochar for internal curing when substituting sand in concrete. Various properties, including slump, mechanical strength, and hydration products, were assessed. The results show that prewetted biochar effectively preserves the slump of fresh concrete. The prewetted biochar provides internal curing by extending the hydration process, resulting in a 56 % increase in the duration between the first and third hydration peaks at the 100 % replacement level. This extended hydration process reduced the amount of unhydrated cement clinkers and enhanced the development of Calcium Silicate Hydrate (C-S-H) and CaCO₃, indicating a more complete degree of hydration of cement. Furthermore, the addition of biochar effectively reduces the total shrinkage of concrete by 65 % at a 100 vol% replacement level at 28 days and guarantees later strength development of concrete.

This study investigates the high-temperature performance of concrete (representing fire exposure) where cement and sand were partially replaced with biochar powder and biochar aggregate, respectively. The sand replacement levels were 30 vol.% and 60 vol.%, and the cement replacement ratios were 10 vol.% and 20 vol.%. The target temperatures were 400 °C and 800 °C. The results indicate that biochar powder as the cement replacement had a limited effect on the temperature gradients in concrete. In contrast, biochar aggregate significantly increased the temperature gradients in concrete due to its substantial volume, which leads to destructive thermal damage. Biochar increases the mass loss of concrete after exposure to high temperatures, and its use as an aggregate further enhances this effect. Similar to the control samples, concrete with biochar powder exhibited a compressive strength increase after 400 °C exposure due to accelerated hydration. However, concrete with biochar aggregate showed no such strength gain, as the beneficial effect was negated by destructive thermal damage.

Biochar holds significant potential for reducing carbon emissions in the concrete industry. However, despite increasing interest in biochar-concrete, there is a lack of Life Cycle Assessment (LCA) studies to elucidate the mechanisms by which biochar reduces carbon emissions of concrete across various stages, such as raw material acquisition, transportation, manufacturing, etc. This study compares the environmental impacts of using biochar derived from wood and fruit waste as partial replacements for cement and sand in concrete. The research applies LCA methodology, in line with ISO 14040 and 14044, to evaluate the environmental impacts and resource use of biochar-concrete across its cradle-to-gate life cycle. Cement exhibits high carbon emissions of 0.63 kg CO₂-eq per kg, of which 80 % originates from the calcination whereas crushed sand demonstrates very low carbon emissions. On the other hand, both wood and fruit biochar exhibit negative carbon emissions due to the combined effects of co-product energy recovery and carbon sequestration. Using wood biochar to replace 20 vol.% of cement reduces carbon emissions of concrete by 42 %. Replacing 60 vol.% of sand with fruit biochar reduces concrete carbon emissions by 167 %, indicating a net-negative carbon footprint and demonstrating its effectiveness as a carbon sink. Moreover, fruit biochar shows a greater potential for reducing the carbon footprint of concrete and exhibits higher sensitivity to intensity analysis due to the larger volume replacement it offers for sand. The findings indicate that key parameters, such as biomass moisture content and biochar yield rate significantly influence the decarbonization potential, whereas applied average transportation distance contributes minimally to overall emissions.