Concrete Alternatives

Portland cement concrete is the most widely used building material on earth — roughly 30 billion tonnes placed annually. The material itself is straightforward: cement powder, water, sand, and aggregate, mixed and allowed to hydrate into a stone-like mass. The cement is the active ingredient, and its manufacture is energy-intensive — limestone is heated in a kiln to approximately 1,450 degrees Celsius, driving off carbon dioxide and producing clinker, which is then ground to a fine powder. Each tonne of Portland cement releases approximately 600 to 900 kilograms of carbon dioxide, roughly half from the chemical decomposition of limestone and half from the fuel burned to reach kiln temperature.

This is not an argument against concrete. Concrete provides compressive strength, fire resistance, thermal mass, and durability in a form that can be poured into virtually any shape and cured on site. No other material matches its combination of versatility, availability, and performance. The question is not whether to use concrete but whether the binder — Portland cement — can be partially or wholly replaced by materials that perform the same function with less energy and fewer emissions. Several alternatives exist, each with distinct chemistry, performance characteristics, and limitations.

Supplementary Cementitious Materials

The simplest approach is partial replacement of Portland cement with supplementary cementitious materials — pozzolans and latent hydraulic binders that react with water or with the byproducts of cement hydration to form additional binding compounds. The most widely used are fly ash and ground granulated blast-furnace slag.

Fly ash is a fine powder collected from the exhaust gases of coal-fired power plants. It is composed primarily of silica and alumina, and when mixed with the calcium hydroxide produced during Portland cement hydration, it undergoes a pozzolanic reaction — forming calcium silicate hydrate, the same compound that gives Portland cement concrete its strength. Fly ash can replace 15 to 35 percent of the Portland cement in a mix without significant change in performance, and at higher replacement rates — up to 50 or 60 percent — with adjustments to mix design and curing time. The resulting concrete is typically slower to gain early strength but achieves equal or greater strength at 56 to 90 days compared to a pure Portland cement mix. It is also denser and less permeable, which improves resistance to chloride penetration and sulfate attack.

Ground granulated blast-furnace slag — the glass-like granular material produced by quenching molten slag from iron smelting — is a latent hydraulic binder. Unlike fly ash, which requires the calcium hydroxide from cement hydration to react, slag can hydrate on its own when activated by an alkaline environment. In concrete, the alkalinity of Portland cement activates the slag, and replacement rates of 30 to 70 percent are common. Slag cement concrete develops strength more slowly than Portland cement concrete in the first days, but continues gaining strength over months and years. It is particularly resistant to chemical attack and is widely specified for infrastructure exposed to marine environments, de-icing salts, or aggressive soils.

Geopolymer Binders

Geopolymer concrete replaces Portland cement entirely. Instead of calcium silicate hydration, geopolymer chemistry relies on the dissolution and polycondensation of aluminosilicate materials — fly ash, slag, or calcined clay — activated by an alkaline solution, typically sodium hydroxide or sodium silicate. The resulting binder is an amorphous aluminosilicate polymer — chemically distinct from Portland cement but performing the same structural role: binding aggregate into a monolithic mass.

The performance characteristics of geopolymer concrete differ from Portland cement concrete in ways that matter for long-term maintenance. Geopolymer binders develop strength rapidly — often reaching 70 to 80 percent of their final strength within the first 24 hours when heat-cured. They are highly resistant to fire, retaining structural integrity at temperatures where Portland cement concrete spalls and loses strength. They resist acid attack more effectively than Portland cement, which is susceptible to dissolution in acidic environments. And because geopolymer chemistry does not require the calcination of limestone, the binder can be produced at significantly lower temperatures, with correspondingly lower carbon dioxide emissions.

The limitations are practical rather than chemical. Geopolymer concrete requires precise control of the alkaline activator solution — its concentration, temperature, and ratio to the aluminosilicate source affect setting time, workability, and final strength. The activator chemicals — sodium hydroxide and sodium silicate — are caustic and require handling precautions beyond those for Portland cement. And the properties of the concrete depend heavily on the specific source material: fly ash from different power plants, or slag from different furnaces, produces different results. This variability makes standardization more difficult than for Portland cement, where the clinker composition is controlled within narrow tolerances.

Recycled Aggregate

The aggregate in concrete — the sand and gravel or crushed stone that makes up 60 to 75 percent of the total volume — can also be reconsidered. Recycled concrete aggregate is produced by crushing demolished concrete structures, removing reinforcing steel, and screening the resulting material to appropriate size fractions. The crushed particles retain a coating of old cement paste, which affects the properties of the new concrete: recycled aggregate is more porous and absorbs more water than virgin aggregate, and concrete made with recycled aggregate typically has slightly lower compressive strength and higher shrinkage than concrete made with natural aggregate of the same grade.

These differences are manageable. Blending recycled aggregate with natural aggregate — typically at 20 to 50 percent replacement — produces concrete with acceptable structural performance for most applications. Higher replacement rates are used in non-structural applications: road sub-bases, fill, drainage layers, and mass concrete where high strength is not required. The material is not inferior. It is different, and the difference can be accounted for in the mix design.

Lime and History

Before Portland cement existed, concrete was made with lime. Roman concrete — opus caementicium — combined a lime binder with volcanic ash to produce a material that has survived two thousand years of exposure to seawater, ground movement, and atmospheric weathering. The volcanic ash, rich in reactive alumina and silica, served the same pozzolanic role that fly ash serves today, reacting with the lime to form stable binding compounds. The Pantheon's unreinforced concrete dome, spanning 43 meters, has stood since the second century without structural reinforcement.

Roman concrete is not a curiosity. It is evidence that durable concrete does not require Portland cement. The binding chemistry is slower, the early strength is lower, and the production demands specific pozzolanic materials — volcanic ash, calcined clay, or finely ground brick — that are not universally available. But the resulting material is resistant to the mechanisms that degrade Portland cement concrete over time: carbonation, chloride penetration, alkali-silica reaction. Roman marine concrete, in particular, actually gains strength over centuries as seawater reacts with the volcanic ash to form aluminium tobermorite, a mineral that reinforces the concrete matrix. The material improves because its chemistry continues, driven by the very environment that would attack a Portland cement structure.

What Holds

Each of these alternatives addresses the same fundamental question differently: what happens when the binder is changed? Fly ash and slag modify the Portland cement system incrementally, reducing the proportion of clinker while retaining the basic hydration chemistry. Geopolymer binders replace the chemistry entirely, producing a different binding compound with different long-term behavior. Recycled aggregate changes the inert fraction rather than the binder, reclaiming material that would otherwise be waste. Lime concrete returns to pre-industrial chemistry, trading speed for durability.

None is universally superior. Each imposes its own constraints — on supply chains, on quality control, on construction schedules, on the skill of the workforce. The choice depends on what the concrete must do, for how long, and in what environment. What is consistent across all of them is a recognition that the composition of concrete is not fixed. It is a designed material, and its design can be changed to suit the conditions of its use and the resources available at the place of its making. The concrete of the next century need not be made the same way as the concrete of the last.