Evaluation of mechanical and microstructural properties of eggshell lime/rice husk ash alkali-activated cement

Searching for innovative solutions to produce alternative construction materials using residues is a need to reduce the exploitation of natural resources. Alkali-activated cements (AACs) have been studied in recent years and pointed out as a possible alternative to Portland cement (PC). AACs can still be produced from waste and/or by-products from different sectors of the economy. In this sense, this research proposes the development of an alkali-activated binder produced from rice husk ash (RHA) and eggshell lime (ESL), activated with NaOH or KOH. A dosage study using a second-order composite design sought to evaluate the influence of parameters such as ESL/RHA ratio, alkali content (%Alk), water-binder ratio (w/B), curing temperature (TEMP) and type of alkaline activator (TYPE). Specimens were produced, thermally cured for 24 h and tested at 3 days of age. Unconfined compression strengths (UCS) and ultrasonic pulse velocities (UPV) were determined. Results indicated the ESL/RHA 30/70 ratio, 3 % alkali content, and curing temperature of 60 °C as optimal parameters, with the mixture reaching UCS of 18 MPa at 3 days of age. XRD and FTIR results show that CSH gel formation increases when increasing ESL/RHA ratio up to 30 %, alkaline content up to 3 %, and curing temperature up to 60 °C. Above these values, the formation of CSH tends to decrease. Binders activated by KOH obtained UCS results on average 10 % higher than those activated by NaOH.