Increasing flexural strength of CO₂ cured cement paste by CaCO₃ polymorph control

This study investigates the impact of Mg(OH)₂ on the in-situ formation of fibre-like aragonite and the mechanical properties of the cement mortar by accelerated carbonation. The results showed that the addition of Mg(OH)₂ showed a weak effect on the compressive strength of carbonated samples, but it significantly improved the flexural strength of the carbonated samples. The flexural strength of carbonated sample increased with the addition of Mg(OH)₂ content and the carbonated sample with 10% Mg(OH)₂ showed approximately twice times than that of the hydrated sample at 28 d. The FTIR and XRD analysis also confirmed that aragonite became the dominate CaCO₃ polymorph instead of calcite in carbonated sample with beyond 5% Mg(OH)₂ content. The in-situ formed fiber/needle-like aragonite increased the total porosity of carbonated sample when adding Mg(OH)₂ compared to the formed calcite in control sample due to the lower carbonation degree, while it refined the pore sizes of carbonated samples. Rhombohedral/cubic calcite and small amounts of long rod-like aragonite were present in the control sample. However, the dominant CaCO₃ polymorph of aragonite with a length of 10–20 μm and an ammeter of 1–2 μm were generated in the samples incorporation of Mg(OH)₂ and the length/diameter ratio of aragonite increased with the increased Mg(OH)₂ dosages. This paper confirmed that reinforced cement composites can be prepared by controlled in-situ formation of fiber-like aragonite under carbonation curing and that an appreciable amount of CO₂ can be sequestered.