Classification of Natural Sand and Stone Aggregates
The term "sand and stone aggregates" refers to the general category of stone and aggregate materials. It includes stone products that are mechanically processed or directly extracted from natural rocks, with specific shapes and sizes. It also includes pebble and gravel aggregates formed through natural weathering of natural rocks, as well as granular crushed stone aggregates obtained through mining or rolling, and even includes metallurgical slag aggregates. Whether processed or unprocessed, sand and stone materials are the largest category of materials used in road engineering. Accurately understanding, selecting, and using these materials is of significant importance in ensuring the quality of construction projects.
Classification of natural sand and stone aggregates: The performance of sand and stone aggregates is determined not only by the mineral composition of the rocks but also by the geological formation conditions, which have a significant impact. They can be classified into three major categories based on the formation conditions of the rocks: igneous rocks, sedimentary rocks, and metamorphic rocks. They have distinct structures and textures:
(1) Igneous Rocks
Igneous rocks are formed when magma from the Earth's interior rises, cools, and solidifies. They are also known as volcanic rocks. Among them, intrusive rocks are formed deep in the Earth's surface, under the pressure of overlying layers, and through slow cooling, resulting in coarse-grained crystals and blocky structures, which are dense in structure. Extrusive rocks are formed when magma is ejected to the surface, rapidly decreasing in pressure and cooling. Therefore, most of them are not fully crystallized and often have a glassy or vesicular structure. When extrusive rocks form thick layers, their structure and texture resemble intrusive rocks. When they form thin layers, they often have a porous structure, resembling volcanic rocks. Common intrusive rocks include granite, syenite, and diorite.
Volcanic rocks that solidify after magma is ejected into the air and rapidly cools are also known as volcanic clastic rocks. They have a glassy structure and a porous texture. Examples include volcanic ash, volcanic sand, and pumice. Volcanic ash and ash sand can be used as admixtures, and pumice can be used as lightweight concrete aggregate. Volcanic ash and volcanic sand, when subjected to pressure from overlying layers, can become lithified rocks called volcanic tuff. Volcanic tuff is porous, lightweight, and easy to process, making it suitable for wall materials in thermal insulation construction. When finely ground, it can be used as a cement admixture.
(2) Sedimentary Rocks
Sedimentary rocks are formed by the long-term weathering, transportation, and deposition of exposed rocks on the Earth's surface, resulting in lithification near the surface or shallow depths. Compared to igneous rocks, the formation process of sedimentary rocks involves less pressure and lower temperatures, and they often exhibit a layered structure. Each layer has different composition, structure, color, and thickness, resulting in heterogeneity in rock properties. Compared to deep-seated igneous rocks, sedimentary rocks have lower density, higher porosity, higher water absorption, lower strength, and poorer durability. However, sedimentary rocks are widely distributed and easier to process, making them extensively used in engineering. Common sedimentary rocks include limestone, shale, sandstone, conglomerate, gypsum, kaolin, and diatomaceous earth. Loose grains such as clay, sand, and gravel are also considered sedimentary rocks.
(3) Metamorphic Rocks
Metamorphic rocks are rocks that have undergone geological metamorphism from either igneous or sedimentary rocks. Metamorphism refers to the process in which minerals in rocks recrystallize or new minerals form under high temperatures, pressures, hot gases, and infiltrated aqueous solutions in the Earth's crust. As a result, the mineral composition and structural characteristics of the original rocks change significantly, forming new rocks. Generally, sedimentary rocks that undergo metamorphism under high pressure and recrystallization become more compact. For example, limestone or dolomite metamorphoses into marble, and sandstone metamorphoses into quartzite, both of which are more durable than the original rocks. Rocks originally formed from deep-seated igneous rocks, after undergoing metamorphism, may deteriorate due to the development of foliated structures. For example, gneiss, which is a metamorphic rock derived from granite, is prone to layering and delamination, resulting in poor durability.
The distribution of rocks on the Earth's surface is approximately 75% sedimentary rocks, while igneous rocks and metamorphic rocks make up the remaining 25%.