Cement Definition
Cement is a soft, dark-colored, hydraulic bonding and binding agent which is obtained by heating, then grinding, a mixture of limestone and clay. By such properties, it is able to bind concrete with steel bars used for reinforcement. Most cements are made of three basic raw materials: calcium carbonate found in limestone; silica found in clay and sand; and alumina (i.e. aluminum oxide).
Types of Cement Products
There are several types of cements; the purpose of and for their application determine the cement naming. Yet, cement basic constituents are one despite that the ratios of these components differ from one type of cement to another. Most include the followings:
Ordinary Portland Cement
There are different types of ordinary Portland cement. These kinds include white cement products with a lower percent of ferric oxide; and oil well cement which is used to cover oil wells from their interior; and the other type is the speedy setting time cement. Add to that several other kinds for special and different uses.Uses of Ordinary Portland Cement
Ordinary Portland cement is used in the making of all types of concrete structures and constructions not directly exposed to sulfate attacks or in a sulfate-fed environment Examples include reinforced concrete for structure foundations :- all sorts of piles; constructional works done in water tanks and sewages.
- all sorts of piles; constructional works done in water tanks and sewages.
- road furniture and bridges.
- It is also used in the manufacturing of concrete pipes and in the interior covering of metal pipes .
Basic Components of ordinary Portland Cement
- Tri-calcium silicate whose ratio ranges between 45-55%; this ingredient is the substance providing a concrete mixture the strength within the first twenty-eight days from application.
- Di-calcium silicate whose ratio ranges between 15-25%; this substance is used to do self-curing to the mortar and concrete by mending the extremely tiny cracks and to provide concrete with tensile bond strength.
- Tri-calcium aluminate whose ratio ranges between 12-15%; it reacts promptly at the time of mixing the ingredients and produces a high level of temperature. By doing so, the concrete is given the strength on the first day but does not impact the final strength of the concrete.
- Tetra-calcium aluminoferrite with a ratio ranging between 7- 12%. This component has the feature of promptly reacting with other compositions on the first couple of days and produces high temperature faster that than that produced by tri-calcium aluminate.
- In addition to the abovementioned ingredients, cement has other secondary components as oxides; these include potassium, sodium, magnesium, titanium oxides, and sulfur dioxide. But these compounds constitute a very small proportion of cement weight.
Chemical analysis of Portland cement and its components statement :
| المكون | المدى المتوسط |
|---|---|
| أوكسيد الكالسيوم (Ca o) | 60-65 ═ 62.5 |
| السيلكا (Sio2) | 25-20 ═ 22.5 |
| الألومنيا (Al2o3) | 9-5 ═ 7 |
| أوكسيد الحديديك (Fe2o3) | 5-1 ═ 3 |
| الماغنيسيا (Mgo) | 4 ═ 2 |
| ثالث اوكسيد الكبريت (So3) | 1.6-0.4 ═ 1 |
Rapid Hardening Cement Products
Rapid hardening cements differ from ordinary cement product types in many respects. One is that the ratio of limestone to silicates and tri-calcium silicate a rapid hardening cement product is higher than in other ordinary cement products. This type of cement is characterized by being smoother than other ordinary cement products, which leads to rapid hardening and rapid temperature generation. Rapid hardening cement is also used in the construction of roads.Low Heat Portland Cement
This type contains a low percentage of the Tri-calcium sulfate and tri-calcium aluminate, leading to the low temperature generated. Ferric oxides are used to reduce the ratio of tri-calcium aluminate and thus, the ratio of tetra-calcium aluminoferrite is increased in this type of cement product.Sulfate Resisting Cement
This Portland cement is a resistant chemical effects and attacks, in particular, sulfate. It is called Sulfate Resisting Cement to distinguish it from other types of Portland cements. It has a low ratio of Tri-calcium aluminate and is characterized by a high level of resistance to sulfate substances. This could be attributed to the components of this cement or to the processes followed in its production. Thus, it is used in conditions where high resistance to sulfates is required. Also, it is worth noting that the higher the alumina in Portland cement, the higher the danger of the effects of sulfate-based solvents on Portland cement. Thus, it is inescapable not to substitute alumina with iron to produce the type of sulfate resisting cement. Experiments have shown that the adding a small quantity of barium carbonate to the rawmix would make the cement resistant to sulfate ( (because barium carbonate reacts with sulfur water to produce insoluble barium sulfate).Portland Pozzolana Cement
It is produced by adding Pozzolana to the ordinary Portland cement clinker. Portland Pozzolana cement is featured by having different specifications and ingredients from other types of Portland cements. It is composed of (70% -80%) of Portland cement, added to that a ratio of (15-20%) of Pozzolana. ( Pozzolana is a naturally siliceous, aluminous and volcanic material extracted directly from volcanic mountains. It is not subject to any reactions neither does it need any chemical additives. Therefore, it is recommended to use it as the different uses of this material brings no no damages or negative effects.الاسمنت البوزلاني
انواع ومصادر البوزلانا
- بوزلانا طبيعية متجددة ناتجة عن رماد وحمم بركانية
- بوزلانا صناعية ناتجة عن تدوير مخلفات المصانع
- رماد الفحم الناتج عن حرق الفحم لمحطات توليد الكهرباء .
- خبث الحديد الناتج عن مخلفات حرق الحديد في الفرن المصانع الحديد والصلب .
الاسمنت البوزلاني
أنواع الاسمنت البوزلاني
- PULVERISED FUEL ASH AFA يتكون من خلط الاسمنت البوزلاني العادي مع OPC مع نسبة 20-30 % من رماد الفحم .
- GGBFS ويتكون من خلط الاسمنت البورتلاندي العادي OPC مع نسبة 70 % من خبث الحديد .
- POZZLANA CEMENT PPC PORTLAND يتكون من خلط الاسمنت البورتلاندي العادي OPC مع نسبة 20-30% من البوزلانا الطبيعية .
خواص وفحوصات الأسمنت:
يجرى على الاسمنت العديد من الفحوصات لتحديد صفاته وللتأكد من جودته ومطابقته للمواصفات، ومن أهم هذه الفحوصات:- نعومة الأسمنت Fineness of Cement
- فحص القوام القياسي للعجينة الأسمنتية.
- زمن الشك الابتدائي والنهائي Initial & Final setting time
- التحليل الكيماوي للاسمنت.
- ثبات الأسمنت .
- مقاومة الأسمنت للضغط المباشر.
- مقاومة الاسمنت للشد المباشر.
- فحص الانثناء
المواد الخام الأولية في صناعة الأسمنت :
- الحجر الجيري
- الطفلة
- رمل السيليكات
- طفلة وأتربة البوكسيت (جسيمات متوسطة الحجم من الطبقات الرسوبية مثل الطفلة، ولها محتوى منخفض من الماء( .
- أكاسيد الحديد : وتستخدم كعوامل مذيبة أو صهارة (Fluxing Agent) لتقليل درجة الحرارة الناتجة عن التفاعلات الكيميائية .
- الجبس: ويضاف في المرحلة النهائية بمطاحن الأسمنت
خصائص المواد الأولية اللازمة في صناعة الاسمنت :
- المواد الكلسية : يعتبر الكلنكر الخليط الأولي اللازم لإنتاج الاسمنت البورتلاندي ويتكون الكلنكر من :
- الشق الكلسي : ( الحجر الجيري ) الذي يعتبر مصدرا أساسيا لكربون الكالسيوم CaO3
- الشق الطيني : الذي يعتبر مصدرا أساسيا لأكسيدات السليكون – الألمنيوم – الحديد (SiO2 - Al2O3 - Fe2O3 )
- المواد المضافة : والتي تعتبر تصحيحية لتغطية النقص في إحدى المكونات الكيميائية للخليط الخام في حال إنتاج الاسمنت العادي او لتحضير أنواع أخرى من الاسمنت .