Mixing proportion (for reference) of ordinary concrete using “Skyscraper” Cement

Note:

1. In the above table, the stone used boasts 5 to 40mm continuous gradation. Sand is medium sand (fineness modulus of about 2.6). Clay content in sand and stone is low. The slump of concrete is about 100mm. The abovementioned mixing ratio will be adjusted according to construction requirements and thickness of aggregates.
2. When the construction project requires the addition of certain concrete admixtures, the effect of water loss should be observed to reduce the amount of cement and water, while increasing an appropriate amount of sand and stone. 

Notes to the usage of cement and concrete

1. Various cement products of the Group are suitable for concrete placement for columns, beams, and foundations of buildings. According to the requirement of the relevant concrete’s strength grading, the right concrete composition is selected and applied, with the use of quality sand, stones, and strict control over cement content. After the concrete is placed, it has to be compacted using the vibration method.
2. In addition to the abovementioned, concrete slab placement requires recompaction after concrete is placed (and before it sets) as the process of recompaction further expels entrapped air (thus increasing the density of the concrete) from the placed concrete that begins to lose moisture and shrink, thereby preventing shrinkage cracks that form in the concrete when it hardens. Portland cement is not suitable for construction in hot summer as the process of cement’s strength growth and the rate at which concrete hardens are faster at high temperatures (water-reducing and retarding admixture is not commonly used in rural areas), and construction works will be behind schedule. As such, it is better to use Portland slag cement or composite Portland cement. Concrete should be kept moist and warm (in winter) after it has set. When setting the formwork for slab, it is important to prevent formwork collapses, which will cause cracks to open or form in concrete.
3. Pay attention to the rate at which the concrete develops strength when removing formwork after the concrete has hardened. Formwork for non-load-bearing structures (such as columns and walls) can be removed after one to three days of curing, and after two to three days for Portland slag cement concrete as it develops its strength at a relatively slow pace. For load-bearing slabs, removal of formwork from placement of ordinary or composite Portland cement concrete can be done after approximately 15 days of curing, and approximately 20 days for Portland slag cement concrete. The removal time of the formwork for load-bearing beams is approximately 28 days.
4. Daytime construction is recommended in the event of placing Portland slag cement concrete during winter. It is because the temperature is higher during daytime. After commencement of construction, attention should be paid to maintain warm temperatures. Given low temperature in winter, it is normal for the concrete to gain its strength slowly.
5. When placing concrete in winter, it is important to maintain favourable curing temperatures and protect concrete after the placement. Avoid construction work when outdoor temperature is below 6oC to prevent damage to the concrete from freezing which will cause the concrete to fail to set. 
6. When storing cement, it is important to prevent it from exposure to moisture and thus developing lumps, which will reduce or cause it to lose its strength as compared to fresh cement (not yet been stored). The State requires that cement more than three months old should be tested for its strength before being used. As a result of exposure to moisture, cement with lumps or hardened cement should be sieved before use. The separated lumps will be rolled or crushed for use as masonry mortar or plastering mortar for minor projects. For cement lumps that will turn into powder once they are touched or squeezed, they could be used according to their strength grading after proper downward adjustment.
7. Cement concrete or mortar should be protected from direct sunlight and excessive drying after it is placed. Should concrete and mortar be exposed to direct sunlight after it is placed, its strength will decline or will be totally lost as moisture rapidly evaporates, and cracks occur. As such, the sub-grade should be thoroughly cleaned and fully moistened before concrete or mortar is poured, and should be carefully covered after the placement, and water has to be timely added to keep it moist according to the relevant requirements.
8. After cement has been added to the concrete or mortar mix, the mix should be protected from freezing in cold weather. Should it be exposed to low temperatures, hydration in cement could not take place while water will freeze and expand. The surface of concrete or mortar will be damaged by spalling and scaling, and gradually their inner layers will be affected too. As such, strict compliance with “Requirements for Building Construction Projects in Winter (JGJ101-97)” during the construction process is a must.
9. Cement concrete or mortar should be protected against high temperatures and extremely hot weather after placement. Should the layer of mortar or components of concrete, after setting, be continuously exposed to high temperatures and extremely hot weather, the strength of mortar or concrete will decline due to dissolution of calcium hydroxide in hydrated cement under high temperature conditions. Certain aggregates will also dissolve or expand under high temperature conditions. When the area is consistently exposed to high temperatures, ordinary mortar and concrete should be lined with and protected by refractory bricks. At higher temperatures, special heat resistant concrete, or concrete mixed with a certain amount of finely ground heat resistant materials, should be used for placement.
10. Sub-grade for cement concrete or mortar should be thoroughly cleaned. Cement can firmly adhere to or bond with a compacted sub-grade with a clean surface. However, how well cement adheres to or bonds with depends on the cleanliness of the surface of the sub-grade. Prior to placing the cement, the sub-grade has to be thoroughly cleared, so that cement can be bonded firmly with the sub-grade. Dirt, grease, and acidic/alkaline substances on the surface of the sub-grade will form a barricade and thus should be cleared and cleaned. A layer of neat cement paste will then be put on the sub-grade, followed by a mortar layer or the placement of concrete. Cement will shrink during the setting process, and does not accommodate the change in volume of loose, unstable sub-grade when humidity or temperature varies, resulting in voids or cracks and weak adhesion. As such, wood, slag layer and lime soil layer cannot adhere firmly with mortar or concrete.
11. Aggregate used in cement concrete or mortar must be pure. Should sand, gravel used as aggregate of concrete or mortar be contaminated with dirt, clay or other organic materials, it will affect the rate at which cement adheres to and bonds with sand, gravel, which will eventually lead to reduced compressive strength. As such, aggregate should be cleaned before use if the amount of impurities it contains exceeds the standard requirements.
12. Avoid excess water and thick cement paste when mixing cement concrete or mortar. Often overlooked is how the strength of concrete is affected by the amount of water used. For easier placing during construction, the required water-cement ratio is sometimes not observed, resulting in a thinned concrete mix. Since hydration requires an amount of water equivalent to only approximately 20% of the cement weight, the excess water will evaporate and leave voids in the concrete, making the concrete weak. As such, water should be kept to the minimum in the mix in order to ensure the concrete is compact during placing. Many people believes that the more cement is used for plastering, the stronger the plaster membrane will be. However, on the contrary, the more cement is used, the thicker the cement mortar will be, and the greater the volume reduction of the cement paste in the plaster membrane will be, which will result in the formation of more cracks. In general, in plastering, cement sand plaster at a 1: (3-5) ratio will be applied as the first coating, followed by a second coating, which is a very thin layer of cement mortar mixed at a 1: (1.5-2.5) ratio. Remember to avoid using too much cement.
13. Prevent cement concrete or mortar from erosion by acids after it has been applied, as acids can neutralise with calcium hydroxide in cement to produce loose, expansive substances that can easily be pulverized when exposed to water, causing concrete or plaster membrane to gradually corrode and collapse. As such, cement should be prevented from exposure to acids. When exposing to acidic substances or containers with acidic properties, it is advised to use acid-resistant mortar and acid-resistant concrete. Given that slag, pozzolan and Portland fly-ash cement have good resistance to acids, these three cement products are preferred in preparation of acid-resistant mortar and concrete. Avoid using silicate or ordinary Portland cement in acid resistant projects.

Characteristics of Skyscraper Brand clinker

By using internationally advanced short, rotary kiln with precalciner and preheater, the Company ensures the Portland cement clinker it produces are of consistent high quality. It is a hydraulic substance made up of molten minerals consisting essentially of calcium silicate, which is produced as a result of heating a mixture of ground limestone (consisting of CaO, SiO₂, Al₂O₃, and Fe₂O₃), sand rocks, and corrective materials such as iron and alumina-silicam, in the correct composition. The mineral components of the clinker, now in their crystalline forms, are very small (10-60μm). They are premium “synthetic crystals”, made of different types of minerals that crystallised. The four main types of minerals are C₃S, C₂S, C₃A, and C₄AF that determine the principal properties of Portland cement. Over 95% of the Portland cement clinker is made up of the four minerals, with C₃S and C₂S accounting for approximately 80%, which together are known as silicate. C₃A and C₄AF account for approximately 15% of the clinker content, and will melt at 1250-1280℃ providing a liquid phase, thereby facilitating the formation of C₃S, which is a solvent mineral. Characteristics of the clinker minerals are as follows:

Characteristics of slag powder

Slag micropowder, when used to substitute an equal amount of cement in concrete and concrete products, can significantly improve the overall properties of the concrete and concrete products. Slag micropowder is a new mineral admixture for high performance concrete. The benefits of using slag micropowder to improve various properties of concrete are as follows:

1. can significantly increase the strength of cement concrete at later stage, and can be used to prepare ultra high strength cement concrete;
2. can effectively suppress alkali aggregate reaction in cement concrete, and significantly improve cement concrete’s resistance to alkali aggregate reaction, as well as enhance durability of cement concrete;
3. can effectively improve cement concrete’s resistance to sea water corrosion, which is particularly suitable for marine construction projects;
4. can significantly reduce bleeding in cement concrete, and improve its workability;
5. can significantly enhance compactness of cement concrete and improve its impermeability;
6. can significantly reduce heat of hydration in cement concrete, suitable for use in major concrete projects.