胶结物

胶结物

胶结物指成岩期在岩石颗粒之间起粘结作用的化学沉淀物引。主要胶结物为硅质（石英、玉髓等）、碳酸盐矿物（方解石、白云石等），其次是铁质（赤铁矿、褐铁矿等），有时可见硫酸盐矿物（石膏、硬石膏等）、沸石类矿物（方沸石、浊沸石等）、粘土矿物（高岭石、水云母、绿泥石等）。

碎屑颗粒和基质之外的化学沉淀物质。在碎屑岩中含量一般不超过50%，它对碎屑颗粒起胶结作用，使其变成坚硬的岩石。粘结岩土颗粒或结构面的物质，有钙质、硅质铁质、泥质及可溶盐等。

分类：基底式胶结、孔隙式胶结、接触式胶结和镶嵌式胶结。

命名：在同一岩石中可出现二种以上的胶结物结构和胶结类型，可用复合命名法，如再生孔隙胶结、连生基底胶结等。

胶结类型指碎屑物与填隙物（包括胶结物及杂基）之间的关系。胶结类型或叫支撑性质，它首先与碎屑颗粒与杂基的相对数量比例（即基粒比）有关，另一重要因素是颗粒之间的相互关系。如当水动力强时，和碎屑同时沉积下来的杂基将被冲走，使碎屑颗粒彼此相接触，颗粒之间留有空隙，造成“颗粒支撑”的结构，成岩后形成化学胶结物的碎屑岩；如果水动力弱或介质为密度流时，大小碎屑与泥质一起沉淀，造成“杂基支撑”的结构，碎屑呈“游离状”分布于杂基之中，成岩后形成杂基填充的碎屑岩。

在成岩期的压固作用下，特别是当压溶作用明显时，砂质沉积物中的碎屑颗粒会更紧密地接触。颗粒之间由点接触发展为线接触、凹凸接触，甚至形成缝合状接触。这种颗粒直接接触构成的镶嵌式胶结，有时不能将碎屑与其硅质胶结物区分开，看起来像是没有胶结物，因此有人称之为无胶结物式胶结。

A cement is a binder, a substance used in construction that sets and hardens and can bind other materials together. The most important types of cement are used as a component in the production of mortar in masonry, and of concrete- which is a combination of cement and an aggregate to form a strong building material.

Cements used in construction can be characterized as being either hydraulic or non-hydraulic, depending upon the ability of the cement to set in the presence of water (see hydraulic and non-hydraulic lime plaster).

Non-hydraulic cement will not set in wet conditions or underwater; rather, it sets as it dries and reacts with carbon dioxide in the air. It can be attacked by some aggressive chemicals after setting.

Hydraulic cements(e.g., Portland cement) set and become adhesive due to a chemical reaction between the dry ingredients and water. The chemical reaction results in mineral hydrates that are not very water-soluble and

so are quite durable in water and safe from chemical attack. This allows setting in wet condition or underwater and further protects the hardened material from chemical attack. The chemical process for hydraulic cement found by ancient Romans used volcanic ash (activated aluminium silicates[citation needed]) with lime (calcium oxide).

The word "cement" can be traced back to the Roman term opus caementicium, used to describe masonry resembling modern concrete that was made from crushed rock with burnt lime as binder. The volcanic ash and pulverized brick supplements that were added to the burnt lime, to obtain a hydraulic binder, were later referred to as cementum, cimentum, cäment, and cement.

CaCO3→ CaO + CO2

The calcium oxide is then spent (slaked) mixing it with water to make slaked lime (calcium hydroxide):

CaO + H2O → Ca(OH)2

Once the excess water is completely evaporated (this process is technically called setting), the carbonation starts:

Ca(OH)2 + CO2→ CaCO3 + H2O

This reaction takes a significant amount of time because the partial pressure of carbon dioxide in the air is low. The carbonation reaction requires the dry cement to be exposed to air, and for this reason the slaked lime is a non-hydraulic cement and cannot be used under water. This whole process is called the lime cycle.

Conversely, hydraulic cement hardens by hydration when water is added. Hydraulic cements (such as Portland cement) are made of a mixture of silicates and oxides, the four main components being:

Belite (2CaO·SiO2);

Alite (3CaO·SiO2);

Tricalcium aluminate (3CaO·Al2O3) (historically, and still

occasionally, called 'celite');

Brownmillerite (4CaO·Al2O3·Fe2O3).