In concrete technology, the strength of hardened concrete generally increases with which of the following changes in the properties of cement and mix design?

Introduction / Context:
Concrete strength depends on many factors, including the properties of cement, aggregates, water–cement ratio and curing conditions. Engineers must understand how each factor affects compressive strength to design durable structures. This question focuses on a specific factor: fineness of cement and other mix parameters. It asks which change generally leads to an increase in the strength of concrete. Knowing the correct relationship helps in selecting materials and mix proportions in practice.

Given Data / Assumptions:

• We assume ordinary Portland cement concrete.
• Options mention fineness of cement, aggregate size, water–cement ratio, curing time and aggregate grading.
• Other conditions such as proper compaction and curing are assumed unless stated.
• We are looking for a change that generally improves strength.

Concept / Approach:
Concrete strength increases when the cement hydration is more complete and when the microstructure is denser. Finer cement has a larger surface area per unit mass, which allows faster and more extensive hydration, leading to better bonding and higher strength. Extremely small aggregates or excessive water can weaken concrete by increasing paste volume and porosity. Shorter curing times reduce strength because hydration is incomplete. Poorly graded aggregates create voids and require more paste, which can reduce strength. The approach is to select the factor that clearly improves microstructure and hydration without introducing new problems.

Step-by-Step Solution:
Step 1: Consider fineness of cement. Increasing fineness increases surface area, promoting more complete and faster hydration, which tends to raise strength, especially at early ages. Step 2: Decreasing aggregate size beyond an optimum may lead to higher water demand and increased paste volume, which can increase shrinkage and may not improve strength. Step 3: Increasing the water–cement ratio generally increases porosity in the hardened paste, reducing strength. Step 4: Decreasing curing time reduces the period during which hydration can occur, leading to lower ultimate strength. Step 5: Poorly graded aggregates create more voids and require extra cement paste to fill gaps, which does not generally increase strength and may reduce it. Step 6: Therefore, among the given options, increasing the fineness of cement is the change most directly and consistently associated with increased concrete strength.

Verification / Alternative check:
Concrete technology texts and standards specify fineness requirements for cement, noting that finer cements typically produce higher early strength. Laboratory tests show that, within limits, cement with higher specific surface area leads to greater compressive strength at given ages, provided curing and water–cement ratio are adequate. In contrast, higher water–cement ratios are universally recognised to reduce strength, and poor curing conditions harm strength development. This evidence supports the choice of increased cement fineness as the correct factor for strength improvement.

Why Other Options Are Wrong:
Option B suggests decreasing aggregate size beyond optimum, which can raise water demand and lead to higher shrinkage and potential cracking, not necessarily greater strength. Option C, increasing water–cement ratio, definitely reduces strength because it leaves more capillary pores in the hardened paste. Option D, decreasing curing time, prevents full hydration and lowers both early and ultimate strength. Option E, using poorly graded aggregates, causes non uniform packing and more voids, which weakens the concrete. Only option A clearly corresponds to a practice that generally increases strength: using finer cement.

Common Pitfalls:
Learners sometimes think that simply using smaller aggregates or more water will make concrete stronger or easier to place, but this can backfire due to increased porosity and shrinkage. Another pitfall is to underestimate the importance of curing time; concrete continues to gain strength over many days if kept moist. Remember the key relationships: lower water–cement ratio and adequate curing improve strength, and reasonable fineness of cement supports good hydration. However, excessively fine cement can create other practical issues, so in design practice an optimum must be chosen.

Final Answer:
The concrete strength generally increases with Increase in the fineness of cement particles, which promotes more complete and effective hydration and a denser microstructure.