Concrete brick is a type of brick-shaped building element made from Portland cement, water and aggregate. Along with the times, the need for concrete bricks is increasingly varied, such as the need for lightweight concrete bricks. To produce lightweight concrete in the mix, substitution materials can be used as developers or materials that have a light specific gravity. One of the materials is Silica. Geothermal Sludge has pozzolanic properties because it contains silica minerals, so it can be used as an aggregate in brick mixtures because it has a silica content of 97.3% which is said to be quartz sand. Aggregate greatly affects the properties of bricks, so the selection of aggregates is an important part in the growth of concrete bricks. The method used in this study was mixing Silica, Cement, Pasir, water, and Foam Agent then put into the mold then dry. After the concrete brick is finished, the compressive strength and unit weight tests are carried out. It was found that in one of the samples the compressive strength was 69.54 kg/cm2 and the unit weight value 1.7838grams/cm³. It can be concluded that the effect of adding Silica Geothermal Sludge to concrete bricks is that it can reduce the unit weight of a concrete brick but can also increase the compressive strength of a concrete brick.