**3. Effect of SCM on concrete properties**

The utilization of SCM from industrial by-products, such as silica fume, slag and fly ash has become more powerful in concrete industry; this is attributed to their significance in the sustainable aspect of low-carbon footprint material that enables the reduction of cement content, strength enhancement and durability. On the other hand, the research interest, and their potential commercial interest on the use of POFA, POCP, RHA and EPP as alternate non-traditional materials are gaining more momentum; numerous journals have been published and this shows the vigorous attempts on the interest of those new materials that are available in abundance in the countries. It is envisaged that the potential application of industrial and agro-based products in the concrete would enable the local industries to provide such pozzolanic materials without hindrance as these materials are regularly produced. If these products are not utilized or dumped in the vicinity of the factories, these materials will end up in the landfill, and that would cause land and air pollution; however, the utilization of such materials which have pozzolanic properties could enhance the properties of concrete. Researchers reported that the incorporation of up to 20% of RHA as SCM enhanced the mechanical behavior and developed resistance against chloride-ion penetration in comparison to the conventional OPC-based concrete [7].

It is envisioned that durability performance of SCM used in concrete is one of the prime factors that would draw attention of the concrete manufacturers in addition to their sustainable attributes. The use of SCM in concrete affects the strength development by the three known facts. The factors are filler effect, dilution and the reactivity. The type of SCM and the fineness and the replacement percentage have a significant effect on the strength and durability of concrete. It is to be taken into consideration that these factors are dependent on each other and hence the replacement level of SCM plays a major role. For instance, the replacement of SCM for the cement leads to the dilution, and this in turn could have a negative impact on hydration at the early age. On the contrary, the filler effect can have a positive impact as it decreases the voids in the concrete. As is known the presence of micro-pores creates an overall negative impact on the compressive strength and durability of the concrete. In contrast, the smaller particles of the SCM tend to have filler effect compared to that of cement.

Therefore, the addition of SCM increases the micro-filling and thus, enhances the packing capability and, this in turn effectively reduces the volume of voids.

If the SCM is used as an extra material, an enhancement in the concrete properties is expected due to the reduction in water-to-binder ratio [8]. In contrast, when SCM is used as a cement replacement material, a reduction in development of the strength occurs, more noticeable at early ages because of cement dilution as well as the slow nature of the pozzolanic reaction [9]. The replacement of cement with a SCM that has a lower reactivity will have a negative impact due to the dilution effect. This will result in a lower amount of hydration products, and as a result, it affects the pozzolanic reaction at the early ages. Also, the alkaline environment coupled with the reduction in cement could hinder the hydration of cement.

Researchers have reported that the addition of SCM might have some possible early adverse effects; however, at the later ages, the strength of concrete containing SCM could be at par with the normal concrete or even it could exceed the strength of the corresponding control concrete. Chao-Lung et al. [10] reported the quantity of RHA used in their research and expressed the opinion that incorporating up to 20% of RHA did not produce any negative impact on the strength and durability of the concrete. Further, researchers revealed that the addition of POFA reduced the early age strength, but the later strength was comparable to the control concrete, and they attributed this to the pozzolanic properties of the POFA [11]. Other group of researchers divulged that concrete containing up to 20% POFA produced comparable 28-day compressive strength to that of the control concrete; they also found out that the strength even after 28 days produced comparable strength [12]. Furthermore, researchers established the effectiveness of refinement of pore structure due to the addition of fly ash and silica fume; this in turn produced high mechanical performance by producing high compressive, tensile and bond strengths [13]. This is attributed to the presence of silica in the SCM as the pozzolanic reaction is proficient of taking part in a hydration.

It is a well-established fact that when cement is hydrated, the portlandite is produced which in turn creates an environment with a relatively high alkalinity in the cementitious paste and also acts as a reactant in the pozzolanic reaction, as shown in Eqs. (1) and (2) [2].

$$\text{2Ca}\_2\text{SiO}\_4 + 4\text{H}\_2\text{O} \rightarrow \text{3CaO} \cdot 2\text{SiO}\_2 \cdot \text{3H}\_2\text{O} + \text{Ca(OH)}\_2\tag{1}$$

$$(\text{Bellite}) + (\text{Water}) \qquad (\text{C-S-H phase}) + (\text{Portlandite})$$

$$2\text{SiO}\_2 + 3\text{Ca}(\text{OH})\_2 \rightarrow 3\text{CaO} \cdot 2\text{SiO}\_2 \cdot 4\text{H}\_2\text{O}\tag{2}$$

It is well established that the addition of pozzolanic materials utilizes the calcium hydroxide and promotes the production of additional C–S–H; the additional C–S–H contributes to strength gain. This in turn reduces the concrete pores and improves the particle packing. The presence of SCM necessitates sufficient moisture and thus, the mix design should consider adequate water to facilitate pozzolanic reaction.

Though different SCMs are available, various aspects such as particle size, reactivity index, water requirement, etc. should be kept in mind whilst incorporating the specific type of SCM in the concrete mix; further, the inclusion of SCM can be related to the morphological and physical properties as not all SCM possess similar characteristics. Researchers experimented the impact of SCM obtained from different byproducts on the mechanical properties of high-strength concrete [14]. Highly fine, ground pulverized coal combustion FA, ground RHA and ground POFA were used to

#### *Effect of Non-Traditional Supplementary Cementitious Materials in Concrete DOI: http://dx.doi.org/10.5772/intechopen.111900*

partly substitute the conventional Portland cement and they discovered that concretes containing 10–40% ground FA and 10–30% ground RHA or POFA exhibited higher compressive strengths than that of the control concrete.

Another aspect on the implication of SCM in the properties is the optimum substitute level. It can be explained by the accomplishment of an optimum particle packing ability as well as the effect of cement dilution. When the very fine SCM are added to concrete, the particle packing increases, resulting in a decrease of the overall cavities or micro-pores. On the contrary, if too much fines are added, the water demand is higher and it may have a negative impact; further, the particle packing is disrupted and due to that the packing density decreases again [14, 15]. The dilution of cement can also have a negative effect in the strength development at the early ages such that the increase of replacement level leads to reduce the cement content, thus less hydration products will be produced namely C–S–H; it should be borne in mind that the C– S–H is the foremost constituent material that is predominant for strength contributing factor in the concrete [16].
