Sodium sulfate boosts strength of lime binders made from rice husk and corn cob ash

Researchers replaced half of the hydrated lime in mortar mixes with either rice husk ash or corn cob ash, then mixed the blends with a dilute sodium sulfate solution instead of plain water. After 28 days of curing, the activated lime–corn cob ash mortar reached 6.75 megapascals of compressive strength, up from 2.91 megapascals in its water-mixed counterpart, while the activated lime–rice husk ash mortar climbed from 3.70 to 7.91 megapascals. Porosity fell in parallel, from 12.47 to 9.23 percent for the corn cob system and from 12.32 to 7.95 percent for the rice husk system. The work, published in Discover Concrete and Cement, was led by Odiwuor Vincent Onyango of the reporting institution with co-author Onesmus Mulwa Munyao.

The motivation behind the study lies in the environmental footprint of ordinary Portland cement, which the authors note contributes nearly 8 percent of global anthropogenic carbon dioxide emissions through energy-intensive clinker production and limestone calcination. Conventional supplementary cementitious materials such as fly ash, silica fume, and slag have long been used to dilute clinker content, but their availability is gradually declining as coal combustion falls and industrial systems change. Agricultural residues offer an alternative supply of reactive silica. Rice husk ash is widely recognized as a highly reactive pozzolan because of its abundant amorphous silica, while properly processed corn cob ash contains reactive silica and alumina phases that can participate in pozzolanic reactions and, under suitable activation, form calcium aluminosilicate hydrate phases.

The raw materials were sourced locally. Rice husks came from a processing plant in Ahero, Kisumu County, and corncobs from farmers in Uasin Gishu County. Both residues were washed, dried, and calcined in a muffle furnace at 700 degrees Celsius, the corn cobs for 2 hours and the rice husks for 3 hours, before being ball-milled and sieved through a 75-micrometer sieve. X-ray fluorescence showed that rice husk ash was dominated by silica at 88.94 percent, with a low loss on ignition of 2.93 percent, and it satisfied the ASTM C618 requirement that combined silica, alumina, and ferric oxide exceed 70 percent. Corn cob ash, by contrast, contained 46.47 percent silica, a loss on ignition of 17.72 percent, and a combined alkali oxide content of 12.96 percent, so it fell short of the ASTM C618 oxide-sum criterion even though it retained reactive aluminosilicate phases.

To compare the intrinsic reactivity of the two ashes, the team used a rapid electrical conductivity test in which each ash was added to a saturated calcium hydroxide solution and conductivity was tracked for four hours. Rice husk ash caused a rapid and pronounced drop, reaching about 37 percent within the first 30 minutes and roughly 63 percent after 240 minutes, while corn cob ash lost only about 15 percent at 30 minutes and about 28 percent at 240 minutes. The authors interpret this as evidence that rice husk ash has higher apparent pozzolanic reactivity, consistent with its higher silica content, amorphous character, and lower residual carbon. They caution, however, that the conductivity method measures net changes in solution chemistry and can be influenced by soluble alkali salts, temperature, and carbonation, so the values serve as a comparative screening indicator rather than a direct quantitative measure of reaction extent.

The mortar formulations used a constant binder-to-sand ratio of 1:3 and a liquid-to-binder ratio of 0.55, with the pozzolan replacing 50 percent of the hydrated lime by mass. Five mixtures were produced: a pure lime control, water-mixed and sodium sulfate-activated versions of each blend, designated LC50 and LR50 respectively. The 0.5 molar sodium sulfate concentration was adopted from earlier studies that identified it as an optimum for enhancing reactivity in calcined-clay systems. Specimens were cast as 40 by 40 by 160 millimeter prisms, compacted on a vibrating table, and air-cured at 22 degrees Celsius and 60 percent relative humidity to promote atmospheric carbonation, which is an essential hardening mechanism for lime-based binders.

Mineralogical analysis by X-ray diffraction revealed that calcite was the dominant crystalline phase in all mortars, reflecting carbonation of calcium hydroxide, with portlandite also detected, indicating that some lime remained unreacted after 28 days. The activated corn cob ash mortar showed distinct reflections attributable to ettringite and gypsum, likely linked to the higher alumina content of that ash, whereas no sulfate-bearing phases were detected within the analytical limits in the activated rice husk ash system. Fourier transform infrared spectroscopy showed carbonate bands, hydroxyl stretching, and a key region between 1024 and 1059 wavenumbers associated with silicon-oxygen and silicon-oxygen-aluminum stretching, consistent with the incorporation of reactive silica into reaction products. The authors emphasize that these qualitative techniques cannot conclusively identify calcium silicate hydrate gels, so such phases should be regarded as proposed mechanisms rather than directly confirmed features.

The mechanical results placed the binders firmly in the territory of non-structural construction materials. The pure lime control reached only 0.5 megapascals, since hydrated lime hardens mainly through slow carbonation. Adding either ash raised strength dramatically, with the water-mixed corn cob blend increasing 482 percent and the rice husk blend 640 percent relative to the control, although the water-mixed corn cob blend remained below the 3.5 megapascal characteristic value specified in EN 459-1. Against the BS EN 998-1 classification for rendering and plastering mortars, the developed binders fall within strength classes CS II to CS IV depending on composition. The authors note these levels are comparable to conventional air lime and natural hydraulic lime mortars used in masonry, rendering, and conservation of historic buildings, but remain substantially below ordinary Portland cement mortars, which commonly exceed 10 megapascals.

Durability was tested by fully immersing 28-day-cured prisms in a 3 percent sulfuric acid solution for a further 28 days, with the acid replaced weekly. The pure lime mortar completely disintegrated. Both activated mortars deteriorated, but less than their non-activated counterparts, showing lower mass losses and higher residual strengths. The authors attribute this partly to the higher initial strength and lower porosity of the activated systems, which may have limited acid penetration. Interestingly, the rice husk ash system outperformed the corn cob ash system despite the latter containing detectable ettringite and gypsum before exposure, suggesting that accelerated pozzolanic reactions and a denser matrix mattered more than sulfate-phase formation. Because post-exposure X-ray diffraction was not performed, the formation of additional gypsum or ettringite during acid attack could not be directly confirmed.

The study carries clear limitations that the authors state plainly. Performance was evaluated only at 28 days, so long-term strength development and durability remain unknown, and the air-curing regime at 60 percent relative humidity may not be optimal for pozzolanic reactions, meaning the reported values may be conservative. The electrical conductivity results reflect ash reactivity in a simplified solution and should not be read as direct evidence of strength-development kinetics in the mortars. Only two binary systems were tested at a single replacement level and a single activator concentration, and the sand was not characterized for alkali–silica reactivity. The authors recommend future work with scanning electron microscopy, thermogravimetric analysis, quantitative pore-structure characterization, post-exposure mineralogical analysis, extended curing ages, and life-cycle assessment before these low-carbon, breathable binders are deployed in masonry, rendering, or conservation applications.

Subject of Research: Sodium sulfate activation of lime binders incorporating rice husk ash or corn cob ash as partial lime replacements

Article Title: Comparative properties of sodium sulfate-activated lime–pozzolan cements incorporating rice husk ash or corn cob ash

Article References: Onyango, O. V., & Munyao, O. M. (2026). Comparative properties of sodium sulfate-activated lime–pozzolan cements incorporating rice husk ash or corn cob ash. Discover Concrete and Cement, 2(1), Article 85. https://doi.org/10.1007/s44416-026-00129-6

Image Credits: AI Generated

DOI: 10.1007/s44416-026-00129-6

Keywords: lime-pozzolan cement, rice husk ash, corn cob ash, sodium sulfate activation, compressive strength, porosity, sulfuric acid resistance, pozzolanic reactivity, agricultural waste, sustainable construction, X-ray diffraction, mortar durability