Due to increasingly stringent environmental authority requirements for the extraction of the slowly diminishing resource of humic substance-free quartz sands (raw material safety reports, LAWA, Substitute Building Materials Ordinance, Groundwater Ordinance, Federal Soil Protection Ordinance), sand-lime brick producers will in future be forced to increasingly consider the use of sands containing humic substances. The negative influence of sands containing humic substances on quality-determining properties of sand-lime bricks (e.g. stone compressive strength, discoloration) is commonly known in the sand-lime brick industry. However, the prevention of production damage caused by humic substances is not yet possible with certainty. Therefore, a practical test method for the evaluation of humic substances in sands for sand-lime brick production is to be developed. The exact mechanisms of action that lead to the negative influence on the properties of sand-lime bricks are still largely unknown due to the non-uniform and very complex composition of humic substances. It is currently assumed that both hydrophobic, hydrophilic or gelatinous humic substance coatings originally present in the sand on the sand particles, as well as humic substances dissolved during production and then adsorbed on sand and hydrated lime particles, hinder the formation of the CSH phase seams, which are important for the lithographic strength, during the autoclaving process. The NaOH test (rapid test according to POST, DIN EN 1744-1) used for decades both in the sand-lime brick industry and in concrete technology for risk assessment of aggregates contaminated with humic substances, in which the sand sample to be tested is treated with 3% NaOH solution to obtain an extract containing humic substances, only gives the sum parameter "NaOH-extractable humic substances". The results of this test are not meaningful, as they are based only on the assessment of the color gradations of the NaOH extract (the darker = higher loaded the less suitable). However, it has been shown that a darker colored extract does not necessarily have a negative influence on the quality of the sand-lime brick (and, on the other hand, a lighter colored extract can also lead to significant quality losses). In this context, there is often talk of "harmful" and "harmless" humic substances. Consequently, the harmful mechanisms of action of humic substances will be investigated more closely in this research project so that they can be differentiated into critical and non-critical ones. This will be followed by the development of a practical rapid test that eliminates the weaknesses of the NaOH test described above. This will lead to a reliable identification of uncritically contaminated sands and thus to a more comprehensive use of the increasingly scarce resource of sands suitable for sand-lime brick production.