Is Pure Cement Stronger Than Concrete? A Materials Science Perspective

Concrete Components: It‘s What‘s Inside That Counts

Cement and concrete – two terms often used interchangeably by novice DIYers that actually refer to remarkably different materials. Cement is simply one ingredient that makes up concrete, albeit a crucial one. And on its own, pure cement powder is extremely feeble compared to expertly mixed, properly cured concrete.

But why? What exactly occurs during concrete curing that bestows much greater strength capacity? And what about the other components – the water, gravel, sand, and any special admixtures?

This guide will provide deep-dive analysis into cement, concrete, and the underlying chemistry to settle the debate on strengths once and for all.

Cement Composition

Cement production begins with precision mixing of special limestone, aluminosilicate clay, iron ore, silica sand, and other raw quarry materials in exact chemical formulations. This raw mix enters a rotating kiln heated to over 1400°C to initiate extreme pyroprocessing reactions and fuse the mixture into hard nodules called clinker.

The clinker then undergoes fine grinding with a small amount of gypsum to regulate setting time, resulting in the final cement powder. This ultrafine gray powder consists primarily of the following compounds:

• Tricalcium silicate – Makes up 50-70% of cement; largely responsible for initial strength development
• Dicalcium silicate – 15-30% of cement; contributes strength during later stages
• Tricalcium aluminate – 5-10% of cement; important for early reaction kinetics
• Tetracalcium aluminoferrite – 5-15% of cement; impacts coloration

Various minor elements like MgO, K2O, Na2O, TiO2, Mn2O3 also appear at concentrations under 5%, influencing cement properties.

Table 1: Major cement powder compounds

Now that‘s a whole lot of calcium, silica, and aluminate compounds! But what ultimately gives cement its binding properties is what happens upon mixing with water…

Cement Hydration Reactions

Pure cement powder seems rather unremarkable on its own. However, the magic occurs when water enters the picture.

Upon combining cement powder with water, complex exothermic hydration reactions are activated between the chemical compounds listed above and water molecules. These reactions produce submicroscopic gel-like hydration products that first coagulate, then harden over time, literally interlocking with the aggregate fillers discussed later to provide the underlying basis for concrete strength.

The key hydration products include:

Calcium silicate hydrate (CSH) – Provides strength and binds aggregates
Calcium hydroxide [Ca(OH)2] – Formed as byproduct; less useful

Here is a simplified cement hydration equation showing the formation of CSH gel as bonds form between the calcium, silica, and water:

2Ca3SiO5 + 7H2O → 3CaO·2SiO2·4H2O (CSH gel) + 3Ca(OH)2

As you can see, the cement reacts with water to produce lime and CSH gel which harden over time. But cement on its own, without aggregates, still does not have the structural integrity to support load-bearing structures. Pure cement also exhibits substantial shrinkage as water evaporates during curing, developing internal stresses and cracks.

This is where the "concrete" portion – the gravel, sand, crushed rock, etc – comes into play…

Concrete Aggregates

While cement forms the binder component, aggregates constitute the remainder 60-80% of a concrete mix by volume, providing enhanced strength and durability. These aggregates intersperse throughout the curing cement paste and resist shrinkage as water evaporates. Well-graded aggregates are designed to optimize particle packing density as the cement hydration products form in surrounding spaces.

Typical aggregates used in concrete include:

• Coarse aggregates – gravels, crushed stone, blast furnace slag, etc > 4mm
• Fine aggregates: sand, crushed fines, etc. < 4mm

Aggregates must be clean, hard, strong, and durable with no surface coatings or soft/structurally defective particles. Normal weight aggregates common in general use have typical densities between 95-100 lbs/ft3 compared to pure cement at 94 lbs/ft3.

But when properly mixed, the combined cement gel and aggregates together develop strong interfacial transition zones (ITZ) that vastly improve strength beyond what cement could ever achieve on its own.

Figure 1: Illustration of the interfacial transition zone (ITZ) between cement paste and aggregates

The Importance of Concrete Curing

Adequate curing is absolutely vital for concrete to properly set, hydrate, and achieve maximum intended strength. Best practices generally call for water-curing concrete for 3-7 days or using specialty curing compounds. During this window, the cement hydration reactions discussed above progressively interconnect through the concrete to bolster structural bonds.

Concrete left to dry too rapidly exhibits pronounced shrinkage cracking that severely undermines strength. Flawed curing also contributes to a porous microstructure, reducing durability. Overall curing impacts include:

• Compressive strength – Poor curing causes up to 30% strength loss
• Surface durability – Proper curing enhances abrasion/erosion resistance
• Impermeability – Minimizes water/chemical intrusion through density
• Dimensional stability – Reduces cracking/stress from shrinkage

Here is some actual test data quantifying the immense strength benefits of proper wet curing. Reported values represent 28-day compressive strength in psi:

Table 2: Strength data demonstrating importance of curing

This table demonstrates how the same concrete can exhibit nearly double the strength with ideal curing methods!

Now back to the central question – how does pure cement stack up?

Pure Cement Strength Properties

Earlier we touched on some core deficiencies of pure cement:

• Severe cracking and shrinkage
• Brittle with low tensile strength
• Powdery weakness without aggregates

But quantitatively, pure cement powder has rather pitiful structural properties on its own, even with perfect curing!

The average compressive strength of pure cement is only about 8 MPa, or roughly 1160 psi. That‘s nearly 6X less than properly cured concrete! This holds true across compositions and brands.

Without aggregates to provide reinforcement, dimensional stability, and shrinkage resistance, cement alone simply lacks the composition necessary to excel as a building material beyond functioning as a binder. Concrete judiciously applies just enough cement to fully bind the much stronger aggregates together into a composite material that far outperforms its constituents individually.

Concrete Strength Enhancements

While conventional concrete with ordinary Portland cement makes an exceptionally versatile, strong building material suitable for most applications, specialized needs in infrastructure construction have spurred innovations enhancing strength even further:

• Steel reinforcement – Rebar, fibers, structural skeletons
• Enhanced cement types – Calcium aluminate, calcium sulfoaluminate
• Crystalline admixtures – Nanomaterials promote better crystallization
• Polymer admixtures – Fortifiers that bolster the cementitious matrix
• Ultra-high-performance concrete (UHPC) – Advanced mixes cured under heat/pressure

By incorporating these leading-edge technologies, today‘s strongest concrete can compress beyond 22,000 psi – over 30X stronger than pure cement! And this number continues rising through ongoing materials science advancements.

Applications Demanding Concrete Strength

Before designing any concrete mix, the appropriate strength capacity should be carefully evaluated against the demands of the intended application:

Table 3: Concrete strength guidelines across common applications

Skyscrapers, for example, place especially rigorous strength demands on load-bearing concrete structural elements. Laboriously hand-mixing pure cement alone into formwork simply wouldn‘t cut it!

Consult the Concrete Experts

Decades of materials science and structural engineering advances have only bolstered concrete‘s standing as civilization‘s most prolific composite construction material, valued worldwide for its unmatched balance of economy, customizability across applications, and structural integrity.

Yet realizing concrete‘s maximum potential requires meticulous precision – in mixing procedures, consistency, placement/consolidation, and curing conditions. And balancing cost considerations is paramount across this highly compositional material.

Simply put – pure cement powder alone does NOT make very good concrete!

So be sure to enlist trusted ready mix suppliers, contractors, technicians, and engineers when undertaking any structural project. Their collective wisdom across centuries of refined best practices will prove indispensable towards engineering concrete designed to safely withstand the unique demands of any application.