low-quality aggregates

In some areas, excellent aggregates have been exploited and utilized, only low-quality aggregates left. Considering from construction economy, utilizing low-quality has been an inexorable trend .
优良的骨料在某些地区已经开发利用，剩下的是低品质骨料，从建筑经济上考虑，利用低品质骨料已成必然趋势。