Mortar preparation and compressive strength measurement

Mortar specimens were prepared based on ASTM C349, a standard test method for the compressive strength of hydraulic-cement mortars. Type I Portland cement and ISO standard sand (ISO 679:2009) were mixed to manufacture four types of mortar (Table ​(Table1):1): reference mortar (Ref), oyster shell-derived calcium ion-inclusive mortar (Oys), mortar supplemented with AK13 spores (Spo), and mortar supplemented with both calcium ions and spores (Mix). All mortar mixtures were cast in a 40 mm × 40 mm × 160 mm mold for compressive strength measurements, a 40 mm × 40 mm × 10 mm mold for crack-sealing experiments, and then cured at room temperature (23 °C) for 24 h before demolding. Small-sized mortar for crack-sealing experiments were prepared to facilitate microscopic observation of the specimens. Mortar specimens were submerged in tab water at room temperature until testing. Compressive strength was measured on 40 × 40 × 160 mm prismatic specimens according to the ASTM C349 after 3, 7, and 28 days of water curing. Six samples were created for the measurement of each mortar type, and the average compressive strength and standard deviation was calculated.

The mortar specimen mixtures

^a2.23 × 10¹⁰ CFU/g