Effect of Lakhra Fly Ash as Partial Replacement of Cement in Traditional Concrete

This study was aimed to determine the effect of LFA (Lakhra Fly Ash) as partial replacement of cement in concrete. To conduct this study, a total of 72 standard concrete cylinders were cast with 1:2:4 proportions at 0.5 water-binder ratios. Cement was partially replaced by weight with LFA and the percentage replacements of FA (Fly Ash) were kept as 0, 5, 15, 25, 40 and 50%. In addition to that, workability of each mix was determined by Slump Cone test and Compaction Factor test. Out of 72 cylinders, 36 cylinders were tested to determine the compressive strength while the remaining 36 cylinders were tested to determine the tensile strength of concrete, at 7 days curing period, by using UTM (Universal Testing Machine). The results revealed that 25% replacement of binder with LFA, determined an increase in compressive strength of concrete up to 15% as compared to normal cement concrete. Moreover, the tensile strength of concrete also increased up to 16% for the same percentage replacement. Furthermore, it also increased the workability, cohesiveness and surface finishes of concrete. This finding can help to reduce the admixture dosage or the water content of the mix. On the contrary, the compressive strength as well as the tensile strength of concrete decreased in case of 40 and 50% replacements of the cement with FA.


INTRODUCTION
C oncrete is a mixture of binding material, fine aggregates, coarse aggregates, and water. It has become an important material for the society's infrastructure. Its versatility, durability, sustainability, and economy have made it the world's most widely used construction material. One of the major advantages of concrete is that it can be molded into any required shape. That is why it is frequently used for the construction of buildings, bridges and other civil engineering structures.
Concrete plays a vital role in the construction industry; therefore, improvements are continuously being made in properties of this material. The decreasing quantity of raw materials used for manufacturing of cement is a big challenge for the construction industry. Hence, there is dire need to replace cement with alternative materials to overcome the problem of decreasing resources of this component.
FA is a fine residual material obtained from the ignition of coal in power generation plants. It can be used as a partial substitute for cement in concrete. In addition to that, this valorization option can provide an environmental friendly way of its disposal and reuse [1]. It is the most common pozzolana in the world.
Furthermore, particles of FA are almost spherical in shape, ranging in diameter varying from 0.5-150 microns.
As a result, FA freely blends in the mixture, which makes it a desirable admixture for concrete. It is generally heterogeneous, consisting of a mixture of glassy particles such as quartz, mullet and iron oxide. On the other hand, FA reduces the density of concrete; therefore, it can be used in producing lightweight concrete which may reduce the production cost of concrete. Thus, the use of such by-products as the partial substitution of cement, make the concrete economical. Additionally, it has also environmental and technical benefits. Hence, FA is recommended to be used in the manufacturing of cement and concrete rather than disposed off to spoil the environment [2].
Only a few years ago, FA generated by the burning of coal was considered as a waste material. Later on, its use as a pozzolanic additive to cement has changed the way this residual material is considered.
Apart from that, the industrial wastes are a big problem for the environment. So, it is necessary to make an attempt to use these waste materials in the construction industry to overcome the environmental problems.

LPS (Lakhra Power Station), Sindh, Pakistan, is
Pakistan's second-biggest power station based on lignite coal. It has been producing a huge quantity of FA as residual material from its chimney. Lakhra coal reserves are estimated to about 1328 million tons [3].
Sil and Roy [4] conducted a study and concluded that by partial replacement of cement as well as sand with FA, the properties of concrete can be improved. In addition, Sabet,et. al. [5] also reported that mechanical and durability characteristics of the mixes can be improved by using mineral admixtures. Moreover, FA reduces the environmental pollution, so it may be used as a partial replacement of cement in the lightweight concrete mixture. It is possible to produce a lightweight concrete cylinder with a compressive strength of 25 Mpa by the use of FA [6]. It is also observed that with an increase in age, the compressive strength of concrete also increases for FA replaced concrete [7].
The concrete made by using FA had good strength and durability as compared to conventional aggregate in the severe environment [8]. In addition to that, Reddy et. al. [9] reported that replacement of cement with FA up to 20% significantly improved the compressive strength of concrete mix. The application of FA as partial replacement of cement also improves the durability of concrete [10]. Shahab et. al. [11] Table 1.
Workability of plastic concrete used for the present study was determined using Slump cone as well as by Compaction Factor test and it was concluded that use of FA can increase the workability of concrete.

RESULTS AND DISCUSSION
In this section, the results obtained from the experiments are analyzed in order to assess the influence of LFA  Fig. 9 shows the loss of slump at different percentage replacements of cement with LFA. Fig. 9 clearly depict that the slump loss of normal cement concrete is less than that of concrete in which cement is partially replaced with LFA. The results also show that as and 5% LFA increased up to 29, 20, and 14% respectively as compared to normal cement concrete.

Compacting Factor Test
Compacting factor indicates the degree of compactness.
As the percentage of LFA increases, the consistency of the mix also increases.
The compacting factor values for different percentages of LFA are given in Fig. 10.

Compressive Strength
The compressive strength test was performed and the compressive strength of normal concrete and the concrete made with different percentage replacements of cement were compared. Table 2  and 15% respectively as compared to normal concrete.

Tensile Strength
The tensile strength test on concrete cylinders was performed. The tensile strength of normal concrete and the concrete made with different percentages of LFA were obtained (Table 3 and Fig. 12). Concrete cylinders were placed in the UTM and the load at failure was observed and recorded. The tensile strength was determined by using the Equation (1).
The tensile strength of concrete after 7 days curing with 5, 15 and 25% replacement of cement with LFA increased up to 7, 11 and 16% respectively as compared to normal concrete. However, the tensile strength of concrete with 40 and 50% replacements decreased by 11 and 20% respectively as compared to normal concrete.

CONCLUSION
Partial replacement of cement with LFA in concrete increased consistency (workability), cohesiveness and surface finishes of concrete. The test results indicated that there is a direct relationship between the percentage of FA and workability. So, the workability of concrete can be improved by using FA. This can help to reduce the admixture dosage or the unit water content of the mix.
Moreover, the results showed that the addition of 25% of LFA significantly increased the compressive strength of concrete up to 15% at 7 days curing period at 0.5 waterbinder ratios, as compared to normal cement concrete. However, the compressive strength of concrete with 40 and 50% replacement decreased by 9 and 15% respectively as compared to normal concrete.
Likewise, the tensile strength of concrete after 7 days curing with 5, 15 and 25% replacement of cement with LFA increased up to 7, 11, and 16% respectively as compared to normal concrete. However, the tensile strength of concrete with 40 and 50% replacement decreased by 11 and 20% respectively as compared to normal concrete.

SUGGESTIONS
It is suggested that for higher percentage replacements of cement with LFA as in concrete (for instance 40 and 50%), the curing period of concrete can further be extended to evaluate the properties of concrete in the hardened state.  On the basis of this study, it can be suggested that the durability of composite materials made by the application of FA as partial replacement of cement may be further increased by adding the necessary admixtures in higher percentages at higher curing periods. The use of FA in concrete by replacing cement to some extent will possibly reduce the cost of concrete.
FA can be used in making concrete economical, strong, durable, and environment friendly. Additionally, it also reduces the heat of hydration, therefore, it is also suggested that higher percentages of FA can be used in mass concreting.