--By Rajendra Prakash Manandhar

 

In previous articles, we discussed some historical facts of the Portland Pozzolana Cement (PPC) - its short chemistry, production, applications and advantages. Now, we must move on to try and understand the major factors that make concrete durable. The PPC consists of some protective qualities with benefits other than a 28-day compressive strength which give the concrete a long life. Being aware of these qualities is important for a consumer to select the best cement that fulfils their dream of making a durable house. 

 

The 28-day compressive strength is not only a determinant of the quality of cement - there are many other factors that make concrete capable of withstanding acid rain, hazard and a polluted environment. PPC offers several unique advantages to the concrete - its production growth and popularity has shown its universal acceptance in different types of construction work. But in Nepal, consumers are still confused as they are not familiar with the terms PPC and ‘Ordinary Portland Cement’ (OPC). Blended PPC is a safe and durable option today though the parent material is the OPC clinker.

 

Factors of durability in concrete

 

Physical and chemical parameters

Primarily, the quality of clinker used in PPC production should be the best. Loss on ignition (LoI), insoluble matter, FCaO, MgO, chloride, alkalis, and C3A should ideally be low whereas that in C3S, C2S should be optimum. Again, the quality of fly ash (pozzolana) is also important. Its LOI and MgO content should be low but the specific area (blaine value) should be high, because fine particles give better reaction. The produced PPC should have its parameters within the range stated by NS 385, the Nepali Standard code for PPC. The quality of fly ash, on the other hand, should be tested by performing the lime reactivity test. 

 

Optimum fineness:

The produced PPC should have optimum fineness as the cement paste, which may get coated on the surface, would be bound together better with fine particles. Another reason is that the finer the particles of cement, the better the reaction as setting and hardening take place easily.

 

High strength:

PPC gives high strength in comparison to OPC even after five years due to the primary reaction with the formation of C-S-H (I) gel which liberates the calcium hydroxide [Ca(OH)2]. During the secondary reaction, the pozzolana reacts with the liberated Ca(OH)2 forming the C-S-H (II) gel – the more of which makes  the concrete stronger. But in case of OPC, the secondary reaction is not possible. The coating of the liberated Ca(OH)2 in the OPC causes the concrete  to grow weak and come out of its surface. For durability of concrete, the secondary reaction of the C-S-H(II) gel is important where Ca(OH)2 is absorbed during C-S-H (II) formation, for high strength  reduces cement consumption.

 

Soundness: 

Soundness of cement requires true PPC cement mixed with nothing other than clinker, fly ash and gypsum. It is measured by performing the Le-chatelier expansion, autoclave expansion and a simple hot pat test.   

 

Resistance against chemical attacks:

Higher density of gel structure reduces the permeability to protect the concrete structure against external attacks. Certain aggregates react with the alkalis available in the cement to produce an expansive gel and subsequently cause disintegration of concrete. Sulphates of magnesium, sodium and calcium react with the calcium hydroxide and C3A to form expansive gels which disintegrate the concrete structure. While using PPC, the availability of both Ca(OH)2 and C3A are less, which gives better resistance against sulphate attacks. The effect of pozzolana “fly ash” on the binary blends (OPC + slag) has been studied and this ternary blend shows better sulphate resistance compared to binary blends.

 

A well-made dense concrete may, under many conditions, suffer deterioration very slowly. Conversely, a poor quality, porous concrete even though made with a cement of high chemical resistance, may deteriorate rapidly; hence, low permeability is most important. An example that would demonstrate the significance of low permeability would be the case of using such cement in coastal areas. Seawater contains so many hazardous chemicals that come into contact with the concrete used in the construction of harbours and docks that it may suffer attack owing to the chemical action of dissolved salt. This may lead to crystallization of salts within the concrete under the condition of alternate wetting and drying, to frost action, to mechanical attrition and impact by waves, and to corrosion of the reinforcement embedded in it. The chemical action of seawater on concrete, mainly due to the presence of magnesium sulphate, was first studied by Vicat in 1812. Blended cement PPC can be used in construction of silos, tanks and underground construction where water resistance is a must. 

 

Lower heat of hydration:

The reaction of Portland cement with water is exothermic and generates a lot of heat. This can be a disadvantage in the construction of large size structures. Heat produced during hydration of clinker minerals C3S, C2S, C3A, C4AF is known as heat of hydration. Heat of hydration in the order of C3A>C3S>C4AF>C2S which depends on clinker minerals composition, fineness of cement, surrounding temperature and water cement ratio, is observed. Low heat cement is used in mass concrete construction and manufactured by balancing clinker minerals such as optimising C3S, C2S and minimising C3A content in cement. The heat of hydration in OPC is high in comparison to PPC, because 95 per cent of the clinker is used in OPC, 50 per cent in PSC and the amount of C3S, C3A are low, with its heat of hydration obviously lower to prevent the mass of concrete from cracking, thus enhancing durability.

 

Low impurities:

Impurities are unwanted materials in cement and come from different raw materials such as lime stone, clay, coke/coal, gypsum and fly ash. Primarily, the used clinker should contain low impurities. Secondly, select fly ash with minimum lime reactivity greater than 4 MPa - the permissible per cent of insoluble residue in PPC is given by the empirical relation:

X + 4 (100 – X)/ 100                                

where X is  the declared per cent of fly ash in PPC as stated by NS 385. Lower impurities mean better bond formations between cement particles for high strength and durability.

 

Lower permeability:

Permeability measures how porous a sample of concrete can get. Its compactness relates directly with its strength and durability. Some hazardous materials penetrate the concrete through the pore and attack the reinforcement inside the concrete causing the structure to weaken very soon. Low permeability protects the concrete from carbonation and frost action which contribute to making the concrete structure weak. The specific surface of OPC, which indicates the fineness of cement particles, is much lower than PPC whereas specific surface of fly ash is around 5,000-6,000 cm2/gm. Such micro particles of fly ash in PPC cement fill the pore in the concrete, making compact and dense concrete mass, so that the structure becomes strong externally as well as internally. This also does away with the need to add plasticizers like silica cement.

 

There are more than 50 cement industries operating in Nepal including limestone based and clinker based factories. Most of them produce PPC, whereas some produce OPC, and very few produce Portland Slag Cement (PSC). On the other hand, a lot of cement is being imported. Import substitution is possible only when Nepali cements are able to compete with the imported quality. For this, a solid commitment from cement producers is required. Control and monitoring from the Nepal Bureau of Standard and Metrology is also most important. But primary importance should be given to creating awareness and faith upon Nepali PPC to consumers.

 

 

Truly speaking, the days of Pure OPC are gone and it is now the time of PPC/PSC blended cements. Associations with the government, builders, contractors, designers, construction engineers and consumers would play a significant role in promoting PPC and to highlight its usefulness. This will also help dispel apprehensions with regard to using PPC for construction work. 

 

At the national level, the production of PPC/PSC conserves raw materials by 30-50 per cent. A true and healthy PPC/PSC can play an important role in import substitution while consumers get cheaper PPC compared to OPC cement.

 

As far as I remember, a meeting in 2005 with some members of the cement manufacturing association of Nepal had decided to set up a common independent quality control laboratory to control and monitor the quality of cement produced by private cement plants in Nepal. The next agenda was to produce good quality cement to compete with and substitute imports. Some video clips were also prepared for publicity to take effective measures to highlight the usefulness of PPC, but all these concepts could not come to life, the reasons being unknown. Now the time has come to take such good and bold steps again in order to substitute imports by producing quality cement, highlighting the usefulness of PPC. It is only dependable cement that gives safety and durability to concrete which looks smooth after hardening.

 

Cement has been and continues to be the prime building material in construction activities all over the world. Corrosion of reinforcement, frost action, carbonation, alkali aggregate reaction and sulphate attacks are major causes which adversely influence the durability of concrete structures. Blended cements have helped to considerably enhance the durability of concrete. Blended cement PPC/PSC which has taken upon a pivotal place as more energy economical and cost effective cement shall continue to occupy a prominent place in future.

 

(The writer is QC/Production Manager (SR) at Rolpa Cement (P) Ltd. E-mail: [email protected])