Rice straw ash as supplementary cementitious materials for concrete: optimizing water soaking duration of rice straw to remove alkalis

In 2016, global cement consumption reached 4.65 billion tonnes, accounting for 8% of the world’s carbon dioxide emissions. Reducing the reliance on cement is an effective strategy for mitigating cement’s climate impacts. This study investigates rice straw ash (RSA) as a supplementary cementitious material (SCM) for concrete. To enable large-scale RSA production via rice straw combustion at bioenergy plants, the effect of varying water-soaking durations (3, 6, 24, and 72 h) on the leaching of salts and heavy metals from rice straw was assessed. The resulting leachate was treated by reverse osmosis, producing clean water for irrigation and a potassium-rich fertilizer concentrate. The soaked rice straw was combusted to produce RSA samples (RSA 0 for unsoaked straw, and RSA 3, RSA 6, RSA 24, and RSA 72 for soaked durations) for evaluation as SCM based on physicochemical properties, pozzolanic reactivity and strength activity index (SAI), available alkalis, and alkali-silica reactivity (ASR) mitigation.

Results revealed that a soaking duration of just 3 h was effective at removing harmful compounds detrimental to combustion systems. All RSA samples exhibited high pozzolanic reactivity (heat release > 335 J/g and calcium hydroxide consumption > 130 g/100 g RSA) and SAI exceeding 120%. Furthermore, soaking was effective in enhancing ASR mitigation, achieving an 85% expansion reduction with RSA 3. This study demonstrates the potential of rice straw as bioenergy feedstock and its ash as a viable SCM for concrete, offering a pathway to reducing cement consumption while creating value-added uses for agricultural residues.