Effect of Admixtures in Concrete Containing Manufactured Sand

The construction industry is currently facing a significant challenge: the depletion of natural river sand resources. River sand has long been the primary fine aggregate used in concrete production. However, environmental restrictions and the sheer volume of demand have necessitated the search for sustainable alternatives. Manufactured sand (M-sand), produced by crushing hard granite rocks, has emerged as a viable substitute. While M-sand offers ecological benefits and consistent gradation, its angular shape and higher fines content can negatively impact the workability and strength characteristics of concrete. To mitigate these issues and enhance performance, the use of admixtures has become essential in modern concrete technology.

Understanding Manufactured Sand

Unlike natural river sand, which is rounded and smooth due to the erosive action of water, manufactured sand is characterized by its angular and rough texture. This geometry is a double-edged sword. On one hand, the angular particles provide better mechanical bonding with the cement paste, which can potentially increase strength. On the other hand, the rough texture and flaky nature of some M-sand particles increase internal friction, leading to reduced workability. Furthermore, the crushing process often generates a significant amount of micro-fines (particles smaller than 75 microns). While some fines are necessary for cohesion, an excess can increase water demand and negatively affect the durability of the concrete if not properly managed.

The Role of Admixtures

Admixtures are materials other than cement, water, and aggregates that are added to the concrete mix immediately before or during mixing. In the context of M-sand concrete, admixtures serve two primary functions: modifying the rheology (workability) of the fresh concrete and enhancing the durability and strength of the hardened concrete. These are generally categorized into chemical admixtures (such as superplasticizers) and mineral admixtures (such as fly ash, silica fume, and ground granulated blast furnace slag).

Effect on Workability

The most immediate challenge when using M-sand is the decrease in workability. The angular particles interlock tightly, requiring more lubrication to flow. If one simply adds more water to compensate, the water-cement ratio increases, leading to a loss in strength and durability. This is where chemical admixtures, specifically Superplasticizers (SP), play a critical role.

Superplasticizers, particularly polycarboxylate ether (PCE) based admixtures, work by dispersing the cement particles. They induce a negative charge on the cement grains, causing them to repel one another. This dispersion breaks up flocculations and releases trapped water, making the mix fluid without additional water. In concrete containing M-sand, the use of SP allows for high workability (high slump) even with the sharp, angular aggregate. This ensures that the concrete can be placed and compacted efficiently, filling formworks and enveloping reinforcement without honeycombing.

Influence on Compressive Strength

The impact of admixtures on the compressive strength of M-sand concrete is profound. By enabling a reduction in the water-cement ratio while maintaining workability, superplasticizers directly contribute to higher strength. Lower water content results in a denser microstructure with fewer capillary pores. When combined with M-sand, which offers superior interlocking due to its angularity, the result is a concrete matrix that often exhibits higher compressive strength than equivalent river sand concrete, particularly at later ages.

Mineral Admixtures and Microstructure

Mineral admixtures are often used in conjunction with M-sand to refine the microstructure. Fly ash, a pozzolanic material, improves workability due to its spherical particle shape, which acts as ball bearings within the mix. However, its primary benefit is long-term durability. When fly ash reacts with the calcium hydroxide produced during cement hydration, it forms additional calcium silicate hydrate (C-S-H) gel. This reaction fills the voids in the concrete, making it impermeable.

Silica fume is another highly effective admixture for M-sand concrete. Being extremely fine, it fills the spaces between cement particles. M-sand often lacks the very fine particles found in river sand; silica fume compensates for this deficiency. The combination results in a very dense, high-strength concrete with low permeability. This is particularly beneficial for aggressive environments where chloride ingress or sulphate attack is a concern.

Shrinkage and Cracking

Drying shrinkage is a common concern in concrete, leading to cracking and structural integrity issues. M-sand concrete can exhibit different shrinkage characteristics compared to river sand concrete. The higher water absorption of M-sand can initially lead to higher shrinkage if not cured properly. However, the use of shrinkage-reducing admixtures (SRA) can mitigate this effect. SRAs reduce the surface tension of pore water, decreasing the capillary stresses that cause shrinkage during drying. When used with M-sand, these admixtures ensure volumetric stability, reducing the risk of early-age cracking.

Conclusion

The transition from river sand to manufactured sand is inevitable for sustainable construction. While M-sand presents challenges regarding particle shape and workability, these are not insurmountable. The strategic use of chemical and mineral admixtures allows engineers to tailor the properties of M-sand concrete to meet specific requirements. Superplasticizers restore workability without compromising strength, while mineral admixtures like fly ash and silica fume enhance the microstructure, durability, and long-term performance. Ultimately, the synergy between manufactured sand and modern admixture technology produces a high-performance concrete that is both environmentally responsible and structurally superior.