The quick answer

Researchers at Kaunas University of Technology in Lithuania have developed alkali-activated mortar in which the powder used to make the binder came entirely from two waste streams: crushed ceramic-brick waste and metakaolin waste. When the most promising binder was combined with heat-stable fine aggregate, it retained useful compressive strength after exposure to temperatures as high as 800°C.

The headline result was obtained with a binder containing 75% ceramic-brick waste and 25% metakaolin waste. After specimens were heated to 800°C, held for two hours, cooled and then tested, mortar made with ceramic-waste aggregate reached 34.7 MPa, or 78.7% of its original room-temperature strength. Mortar made with corundum aggregate reached 53.0 MPa, equivalent to 108.6% of its initial strength.

The research also produced an important warning. Ordinary river sand, granite and basalt did not perform as well at the highest temperature. The binder chemistry mattered, but the type of fine aggregate and its thermal compatibility with the surrounding matrix strongly influenced cracking, mass loss and residual strength.

What did the 2026 research report?

The study, published in Scientific Reports on 25 May 2026, is titled “Influence of fine aggregate type on geopolymer mortar performance in an elevated temperature environment.” It was carried out by Martynas Statkauskas, Danutė Vaičiukynienė and Audrius Grinys of Kaunas University of Technology. A university news report, later covered by Tech Xplore on 28 July 2026, presented the material as a greener, fire-resistant alternative to conventional cement mortar.

The scientific question was precise: if the binder is made from ceramic-brick waste and metakaolin waste, which fine aggregate enables the mortar to retain the most strength after high-temperature exposure?

The work is valuable because many studies optimise geopolymer chemistry at ordinary temperature but pay less attention to the aggregate. In a fire, the paste and aggregate expand, shrink, dehydrate and transform differently. A high-strength binder can still crack badly if its aggregate undergoes a sudden mineral change or expands at a different rate.

What is geopolymer mortar?

Conventional mortar normally uses Portland cement as the binder. When water is added, the cement hydrates and produces calcium-rich compounds that glue the sand particles together. A geopolymer or alkali-activated mortar takes a different chemical route. It uses a powder rich in reactive silica and alumina, together with an alkaline activating solution, to build a hardened three-dimensional mineral network.

The term “geopolymer” covers several chemistries, so it should not be treated as one universal product. In this study, the reactive powders were ceramic-brick waste and metakaolin waste. The activating solution contained 8 molar sodium hydroxide and sodium silicate, with a sodium-silicate-to-sodium-hydroxide ratio of 1.5.

Why ceramic brick waste can react

Clay bricks are fired during manufacture. The heat changes the clay minerals and can leave aluminosilicate phases that are capable of dissolving in a sufficiently alkaline solution, particularly after the brick is finely ground. Silicon- and aluminium-bearing species are released, reorganise and form binding gels. The exact reaction depends on the original clay, firing history, particle size, activator and curing conditions.

Why add metakaolin waste?

Metakaolin is produced by heating kaolin clay so that it becomes highly reactive. Metakaolin waste can therefore supply readily available alumina and silica, helping strength development. Yet “more reactive” is not automatically “better at every temperature.” In the study, mixtures rich in metakaolin waste often developed high initial strength but needed more activator and water and experienced greater mass and strength loss at 800°C.

The binder and aggregate have different jobs

This distinction explains why two mortars made with the same binder can behave very differently in a furnace. The aggregate is not an inert filler at high temperature; its mineralogy and thermal expansion become part of the performance system.

How the experiment was conducted

The researchers produced five binder compositions, labelled F1 to F5, by gradually replacing ceramic-brick waste with metakaolin waste. F1 contained ceramic-brick waste only; F5 contained metakaolin waste only. Between them were blends, including the 75:25 ceramic-brick-waste-to-metakaolin-waste mixture identified as F2.

Each binder was mixed separately with five fine aggregates: ordinary river sand, crushed granite, basalt, ceramic waste and corundum. The fine aggregate was 0–3 mm and the aggregate-to-precursor mass ratio was 1.5. This produced a matrix of mixes that allowed the researchers to separate the influence of binder composition, aggregate mineralogy and exposure temperature.

Heating at 5°C per minute to a uniform target, holding for two hours and cooling in the furnace is a sensible laboratory comparison method. However, a real fire can heat much faster, create steep temperature gradients and cool suddenly through firefighting water. Structural members also carry load, are restrained by adjacent members and may contain reinforcement. These differences are central when moving from material research to building design.

What were the key results?

The strongest high-temperature result came from the F2 binder containing 75% ceramic-brick waste and 25% metakaolin waste. With corundum fine aggregate, the residual compressive strength after 800°C was 53.0 MPa—108.6% of the room-temperature value. With ceramic-waste aggregate, it was 34.7 MPa—78.7% of the original value.

These averages combine several binder compositions, so they should not be used as design values. Their purpose is to show the trend: corundum was the most robust aggregate, ceramic waste offered a strong circular alternative, and ordinary quartz-rich sand was the least favourable for severe temperature exposure.

Why could strength increase after 800°C?

At first this seems impossible. Heat normally damages cementitious material by evaporating water, dehydrating binding phases and creating cracks. In a well-matched geopolymer system, however, high temperature can also promote additional condensation and sintering. Particles and partially reacted phases fuse more tightly, pores close and the matrix becomes denser. If thermal expansion is compatible and cracking remains limited, this densification can outweigh the damage in a small specimen.

The F2-corundum mortar appears to have benefited from that balance. The paper reports a density of about 2,164 kg/m³ after 800°C and mass loss of about 4.0%, alongside the 53.0 MPa residual strength. By contrast, metakaolin-rich F5 mixtures generally experienced larger mass losses—approximately 7.9% to 11.4%—and much lower strengths at the highest temperature.

Why the aggregate controls high-temperature behaviour

At normal temperature, engineers often focus on binder strength and treat fine aggregate mainly as a clean, well-graded filler. At elevated temperature, aggregate mineralogy becomes critical. Each mineral expands at its own rate, may change crystal form and may lose or retain stiffness differently from the binder around it.

Quartz-rich river sand

Many natural sands contain substantial quartz. At approximately 573°C, quartz undergoes a reversible crystal transformation from alpha quartz to beta quartz. The transformation creates a sudden volume change. In a mortar specimen, the surrounding matrix may not expand in the same way, so stress develops around the aggregate-paste interface. Microcracks connect, stiffness decreases and compressive strength falls.

Granite and basalt

Granite contains several minerals, commonly including quartz and feldspar, each with different thermal properties. Basalt is generally stronger and may be more stable than quartz-rich sand at moderate temperature, but it is still not immune to differential expansion, oxidation of some constituents or thermal microcracking. The study found reasonable performance up to intermediate temperatures, followed by substantial loss at 800°C.

Ceramic-waste aggregate

Crushed ceramic material has already experienced high temperature during brick manufacture. It may therefore offer better dimensional compatibility with a ceramic-derived geopolymer matrix. Its porous surface can also create mechanical interlock. In this study it produced good strength retention and limited surface cracking, while adding a second route for demolition waste to re-enter construction.

Corundum

Corundum is crystalline aluminium oxide. It is a hard refractory mineral with a melting point and service-temperature capability far beyond the temperatures of ordinary building fires. It remained stable while the binder densified, which helped the mortar retain or gain strength. Its disadvantages are practical: refractory-grade aggregate may be more expensive, heavier to process and more energy intensive than locally recovered ceramic waste.

A fire-resistant binder and an ordinary aggregate do not automatically make a fire-resistant mortar. The two materials must move, transform and transfer heat compatibly.Core engineering lesson

Residual strength is not a structural fire rating

This is the most important distinction in the entire story. The study measured compressive strength after small specimens had been heated, held, cooled and stored for 24 hours. That is called residual strength. It is useful for screening materials and understanding damage, but it is not the same as the fire resistance of a wall, slab, beam, column or tunnel segment.

A structural fire-resistance test evaluates a full assembly or representative member while it is exposed to a prescribed time-temperature curve. Depending on the requirement, the test examines load-bearing capacity, integrity against flames and hot gases, and insulation against temperature rise. The member may be loaded and restrained; deformation and spalling are monitored over time.

Concrete and mortar do not need to ignite to fail. Heat can reduce stiffness and strength, drive vapour pressure, cause explosive spalling and expose reinforcement. Steel loses strength rapidly as its temperature rises. A material with excellent post-heating cube strength could still perform poorly as a thin, restrained or heavily loaded member if it transfers heat quickly or spalls.

Is the mortar truly greener?

The material has a strong circular-economy logic. Demolished bricks and a metakaolin-rich industrial residue are used as binder precursors rather than landfilled. Alkali activation can avoid Portland-clinker production, including the limestone calcination and very high kiln temperatures that make conventional cement carbon intensive.

But the research paper was a performance study, not a full Life Cycle Assessment. It did not publish an Environmental Product Declaration or a verified carbon footprint for one tonne of the mortar. “Greener” is therefore a credible direction, not a complete quantified conclusion.

“Made from waste” needs a boundary

The binder powders in the study were waste-derived. The complete mortar was not necessarily 100% waste. The activator was manufactured chemical material, and corundum aggregate is generally not a demolition waste. A transparent product claim should state whether it refers to the binder precursor, total binder, total dry solids or complete mortar.

What a proper environmental comparison should include

• collection and sorting of demolished ceramic material;
• transport distances and moisture content;
• energy required for crushing and fine grinding;
• production of sodium hydroxide and sodium silicate;
• aggregate extraction or recycling, especially the corundum-versus-ceramic choice;
• curing energy, yield and production waste;
• equivalent strength, durability, thickness and service life;
• repair frequency and end-of-life recovery.

The fairest comparison is not one kilogram of geopolymer against one kilogram of cement. It is the quantity of each material required to deliver the same structural or protective function for the same service life.

Where could this material be useful?

The researchers suggest applications in environments where heat resistance and resource efficiency are both valuable. The most realistic early uses may be non-primary or protection-focused applications, because they can be qualified without immediately redesigning an entire structural system.

Direct use in load-bearing reinforced concrete is a much larger step. The reported material contained no coarse aggregate and was not tested as a reinforced member. A concrete version would need fresh-property control, pumpability, bond to reinforcing steel, shrinkage and creep data, chloride and carbonation resistance, member-scale fire performance and reliable production standards.

Engineering pathway from laboratory result to construction product

A promising journal paper begins the qualification process; it does not finish it. A responsible development programme should progressively increase scale and realism while maintaining traceability of raw materials and batches.

What should a project specification ask for?

Indian adoption perspective

For an Indian project, compliance must be demonstrated against the applicable current building, fire, material, testing and structural requirements accepted by the client, designer and approving authority. Conventional concrete provisions cannot simply be assumed to cover a novel alkali-activated mortar in every respect. A project-specific performance specification, independent testing and formal engineering approval may be required.

Start with a controlled non-structural or protective pilot where failure risk is manageable and measurements can be collected. Record raw-material batches, ambient conditions, mixing time, workability, curing, dimensional change, adhesion and field defects. Only expand the application after the evidence supports it.

Common misunderstandings

What the construction industry should take from this study

The research shows that demolition waste can be more than low-grade fill. With appropriate separation, grinding, chemical activation and aggregate selection, ceramic waste can become part of a high-performance binder and fine-aggregate system. That creates an opportunity to connect two difficult challenges: construction-and-demolition waste and demand for heat-resistant materials.

It also demonstrates why material development must be system-based. The “best” precursor did not perform independently of aggregate. Corundum delivered exceptional thermal stability, while recycled ceramic aggregate offered a more circular balance. Ordinary river sand—perfectly reasonable in many conventional mortars—was not the best companion for severe heat.

The next research stage should include:

• repeatability using waste from different demolition sources;
• full chemical and mineralogical control of the waste powders;
• thermal cycling and rapid cooling, including water exposure after heating;
• hot-state strength, not only strength after cooling;
• spalling, thermal conductivity and temperature transmission;
• bond to concrete, masonry and reinforcing steel;
• larger elements under restraint and load;
• long-term durability in moisture, chlorides, sulphates and carbonation;
• product-specific lifecycle assessment and cost analysis;
• safe, automated handling of alkaline activators.

If those steps confirm performance, waste-based geopolymer mortars could become a valuable specialised product class. They need not replace every Portland-cement mortar to make an impact. Durable linings, industrial repairs and fire-protection components are substantial markets where performance can justify controlled material processing.

The breakthrough is not simply that waste became strong. It is that waste chemistry and aggregate mineralogy were engineered together for a demanding exposure.Testoraa engineering perspective

How PM Testoraa Labs can support material evaluation

PM TESTORAA LABS (OPC) Private Limited supports construction-material testing, concrete assessment, structural investigation and non-destructive testing. For a new mortar, the useful test programme begins with the proposed function and the consequence of failure—not with a generic list of tests.

Our laboratory and engineering support can include representative sampling, dimensional checks, compressive-strength testing, density and water-absorption assessment, concrete core testing, rebound-hammer and ultrasonic pulse-velocity testing, pull-off or bond-test coordination, crack mapping, reinforcement scanning, condition surveys and documentation of controlled field trials within the scope of applicable methods.

Specialised chemical, mineralogical, refractory and standard fire testing may require coordination with appropriately equipped facilities. Testoraa Labs can help owners and engineers define the evidence plan, maintain traceability and interpret results in relation to the actual structure and exposure.