Recycled brick waste can do more than fill dumpsters. Researchers reported that the particle size of silica sand powder added to recycled fired clay brick geopolymers has a major impact on strength, and the best results came from sand sieved to 125 micrometers.

In the study, published in Discover Civil Engineering, the team made alkali-activated pastes using recycled fired clay brick fragments and river sand sourced in Cameroon. They compared “raw” brick powder to brick powder that was thermally activated by calcining it at 700 degrees Celsius for four hours. The mixes used a sodium waterglass activator with a silicon dioxide to sodium oxide molar ratio of 1.6, were cast as 40-millimeter cubes, and were cured for 28 days under controlled temperature and humidity.

The key variable was adding 4% crystalline silica sand in four size fractions: 45, 63, 125, and 200 micrometers. Strength did not simply improve as the particles got finer. The 125-micrometer fraction delivered the top compressive strength in both systems, reaching 50.88 megapascals in uncalcined brick mixes and 64.89 megapascals in calcined mixes after 28 days. Very fine 45-micrometer powder reduced strength, and 200-micrometer powder also pulled performance down, including a steep drop to 29.54 megapascals in the calcined series.

Microscopy helped explain why. At 125 micrometers, quartz grains were well-embedded in a denser gel matrix with stronger-looking interfaces. With very fine sand, the study linked lower strength to higher water demand and reduced availability of activator for dissolving the brick precursor. With coarse sand, the grains acted more like passive inclusions, creating weaker interfaces and stress concentrations that can crack early under load.

For mason contractors watching low-carbon binder research, the takeaway is straightforward: if a spec, supplier, or pilot project involves recycled brick-based geopolymers, don’t treat “sand powder” as a generic add-in. Ask what particle size range is being used, how workability is maintained when fines are added, and what 28-day compressive results were achieved under the stated curing conditions.

Read the full, original article from Bioengineer.org here.