Cold-bonded biochar-cement lightweight aggregates for evaporation-enhanced permeable bricks

Abstract

Permeable concrete pavement is considered an effective means to alleviate the increasingly severe urban heat island effect. Traditional design emphasizes rapid water discharge but does not retain water to regulate the localized high temperature. To design high-evaporation permeable bricks, biochar is mixed as a moisture-absorbing filler in the artificial aggregates (AA). The compressive strength, bulk density, pore range, and crystal phase distribution of AA with 0–30% biochar content were tested. Then, the permeable bricks prepared by replacing natural aggregates (NA) by 0–50% AA were evaluated in terms of compressive strength, porosity, pore structure distribution, water absorption, permeability, and evaporation rate. Our results showed that biochar had a relatively high specific surface area and effectively improved the thermal performance of permeable bricks. The AA produced with 30% biochar met the standard for lightweight aggregates (LA). Replacing NA by AA would unavoidably affect the compressive strength, permeability, pore structure, and evaporation rate of the permeable bricks. Nevertheless, 50% AA substitution displayed better elongation and evaporation ability while complying with the strength requirements. The capillary water absorption rate of permeable bricks increased from 0.364 to 1.108 kg/m²h^0.5, and the surface temperature was ∼14 °C lower than that of traditional design under wet condition. Moreover, the cooling process was extended by 21 h and AA improved the connected pore structure of permeable bricks. The life cycle assessment proved that our novel design could be carbon-negative, with a carbon sequestration of 110 kg/t of permeable bricks, thus contributing to the reduction of carbon footprints and promoting the development of green construction.