Concrete that needs nothing more than water to harden has long been one of the more tantalizing promises of sustainable construction. Geopolymer binders, which rely on aluminosilicate industrial by-products rather than Portland cement, could in optimized formulations cut carbon dioxide emissions by up to 80 percent and energy consumption by roughly 60 percent compared with conventional cement production, according to figures cited in a new experimental study. Yet the conventional two-part version of this technology demands corrosive liquid alkaline activators, a handling and transport burden that has slowed its adoption on real building sites. The one-part, or just-add-water, alternative sidesteps that problem by blending a solid activator into the dry binder, so geopolymerization begins only when water is added at the point of use. What that water does once it arrives, however, turns out to matter enormously.

A study published in the journal Discover Concrete and Cement by Tuğba Özdemir Mazlum and Nihat Atmaca has now mapped, in fine detail, how the water-to-binder ratio shapes nearly every property of one-part geopolymer concrete made from two underused industrial wastes. The researchers prepared five mixtures with water-to-binder ratios ranging from 0.42 to 0.60, using a fixed binder blend of 30 percent magnesium slag and 70 percent ground granulated blast furnace slag, activated by solid sodium metasilicate powder. Magnesium slag is generated during the thermal reduction of magnesite or dolomite in magnesium production, while blast furnace slag is a well-established by-product of iron and steel making. The combination had received little previous attention in one-part systems, which the authors identify as the novelty of their work.

The mix design was deliberately held constant except for water. The total precursor content stood at 398.70 kilograms per cubic meter, split into 119.61 kilograms of magnesium slag and 279.09 kilograms of ground granulated blast furnace slag, with the solid activator dosed at 63.79 kilograms per cubic meter, or 16 percent of the precursor mass. Crushed coarse aggregate with a maximum size of 16 millimeters and natural river sand were combined at a 58:42 ratio by weight, contributing 961 and 693 kilograms per cubic meter respectively. Aggregates were oven-dried before mixing to keep initial moisture from distorting the effective water content, so that any change in performance could be attributed to the water-to-binder ratio alone. A total of fifty 100-millimeter cube specimens were cast, and after mixing, the fresh concrete was heat cured at 60 degrees Celsius for 24 hours before demolding and sealed storage.

The fresh-state results were unambiguous. As the water-to-binder ratio rose from 0.42 to 0.60, slump values climbed from 50 millimeters to 150 millimeters, a workability improvement the authors quantify at 200 percent. More water simply means more free liquid to reduce internal friction between particles and let the mixture flow. But flow came at a price. Hardened density peaked at a water-to-binder ratio of 0.47 and then fell steadily, dropping from 2.18 grams per cubic centimeter to 2.04 grams per cubic centimeter at the highest water content, a reduction of about 6.4 percent. Ultrasonic pulse velocity, a non-destructive probe of internal integrity, followed the same pattern, reaching its maximum at 0.47 and declining progressively as excess water left behind capillary pores and micro voids that impede wave propagation.

Compressive strength told the same story with particular clarity. At 28 days, strength increased as the ratio moved from 0.42 to 0.47, peaking at 54 megapascals, then fell to roughly 50 megapascals at 0.50, 43 megapascals at 0.55 and 40 megapascals at 0.60. That final figure represents a 26 percent loss relative to the optimum. The 7-day results mirrored the trend, with a maximum of about 42 megapascals also at 0.47, before declining to 27 megapascals at 0.60. Interestingly, the mixtures with higher water content gained more strength between 7 and 28 days, which the authors attribute to the extra water sustaining activator dissolution, ion mobility and continued gel formation at later ages. Too little water, by contrast, can restrict dissolution of the solid activator and limit matrix development, which helps explain why the driest mixture did not win.

Durability testing reinforced the picture. Following the ASTM C642 procedure, 24-hour water absorption was lowest at 2.89 percent for the 0.42 mixture and highest at 5.88 percent for the 0.60 mixture, an increase of roughly 103 percent. Absorption stayed low within the 0.42 to 0.47 range and then rose sharply beyond 0.50, reflecting a more interconnected capillary pore network left by unbound water. The optimum mixture at 0.47 maintained relatively low absorption while delivering the highest strength, confirming in the authors’ analysis that matrix densification governs both mechanical and durability performance. Scanning electron microscopy of fractured specimen interiors provided qualitative support: the optimum mixture showed a continuous geopolymeric gel matrix with few visible discontinuities, while higher water contents brought local pores, minor microcracks and reduced matrix continuity. The authors caution that SEM images capture only selected local regions and served as supporting evidence rather than a quantitative pore measurement.

The most striking result emerged from the thermal experiments. Cube specimens cured for 28 days were heated in a fire furnace to 600 and then 900 degrees Celsius at 3 degrees Celsius per minute, held for two hours, and cooled naturally before residual strength testing, following the general recommendations of RILEM TC 129-MHT. Every mixture lost strength, but the ranking reversed. At 900 degrees Celsius, the 0.42 mixture retained only about 40 percent of its original strength, while the 0.60 mixture retained nearly 70 percent; residual retention at 600 degrees climbed from 51.7 percent to 77.0 percent across the same range. The authors explain this counterintuitive outcome through pore structure: the more open network of high-water mixtures gives physically bound water and vapor an escape route during heating, lowering internal vapor pressure and limiting thermally induced microcracking, whereas denser low-water mixtures trap vapor and crack more severely.

The authors are careful about the limits of their work. Shrinkage and expansion under ambient conditions were not measured, a notable gap because magnesium slag contains calcium oxide and magnesium oxide, phases that can influence volumetric stability through hydration and carbonation, although no visible cracking or dimensional instability appeared during specimen storage. Microstructural characterization was restricted to ambient-cured specimens, so the mechanisms of thermal degradation, including gel dehydration, pore coarsening and possible phase transformations at 900 degrees Celsius, could not be directly observed and await future SEM, X-ray diffraction and thermogravimetric study. Extended durability testing under aggressive environments, cyclic thermal exposure, and full life cycle and techno-economic assessments are also flagged as necessary before large-scale implementation.

Even with those caveats, the study offers a practical design reference. A water-to-binder ratio of 0.47 balanced workability, density, strength, absorption and microstructural compactness better than any alternative tested, and the authors suggest potential applications in precast products, non-structural and semi-structural elements, paving blocks and partition units where industrial by-product valorization and reduced Portland cement consumption are priorities. The dry solid activator, they note, simplifies handling and transportation compared with the alkaline solutions of two-part systems. The broader lesson is a trade-off rather than a single winner: less water buys ambient strength and durability, more water buys flowability and thermal resilience, and engineers working with magnesium slag and blast furnace slag one-part geopolymer concrete must now weigh which of those matters more for the structure in front of them.

Subject of Research: Effect of water-to-binder ratio on the fresh, mechanical, durability, thermal and microstructural performance of magnesium slag–GGBS one-part geopolymer concrete

Article Title: Impact of water-to-binder ratio on the physical, mechanical and durability-related performance of industrial waste-based one-part geopolymer concrete

Article References: Özdemir Mazlum, T., & Atmaca, N. (2026). Impact of water-to-binder ratio on the physical, mechanical and durability-related performance of industrial waste-based one-part geopolymer concrete. Discover Concrete and Cement, 2(1), Article 91. https://doi.org/10.1007/s44416-026-00124-x

Image Credits: AI Generated

DOI: 10.1007/s44416-026-00124-x

Keywords: one-part geopolymer concrete, water-to-binder ratio, magnesium slag, ground granulated blast furnace slag, sodium metasilicate activator, compressive strength, water absorption, high-temperature exposure, workability, durability, microstructure, sustainable construction