Elsevier

Cement and Concrete Research

Volume 35, Issue 12, December 2005, Pages 2278-2289
Cement and Concrete Research

Forced and natural carbonation of lime-based mortars with and without additives: Mineralogical and textural changes

https://doi.org/10.1016/j.cemconres.2004.12.012Get rights and content

Abstract

We have studied the carbonation process in different types of mortars, with and without pozzolana or air-entraining additives, subject to a CO2-rich atmosphere and compared the results with those of similar naturally carbonated mortars. We used X-ray diffraction technique to demonstrate that high CO2 concentrations favour a faster, more complete carbonation process with 8 days being sufficient to convert portlandite into 90 wt.% calcite. Full carbonation, however, is not reached during the life-span of the tests, not even in forced carbonation experiments. This could be due to at least one of the following phenomena: a premature drying of samples during carbonation reaction, the temperature at which the carbonation process was carried out or the reduction of pore volume occupied by newly formed calcite crystals. This last option seems to be the least probable. We observed a more prolific development of calcite crystals in the pores and fissures through which the carbonic anhydride flows. Under natural conditions, carbonation is much slower and similar levels are not reached for 6 months. These differences suggest that the carbonation process is influenced by the amount of CO2 used.

Both the mineralogy and texture of mortars vary depending on the type of additive used but the speed of the portlandite–calcite transformation does not change significantly. Pozzolana produces hydraulic mortars although the quantity of calcium aluminosilicate crystals is low. The air-entraining agent significantly alters the texture of the mortars creating rounded pores and eliminating or reducing the drying cracks.

Section snippets

Introduction and objectives

Lime mortars have been used as building materials since ancient times [1], [2], [3]. In the 19th century, the appearance of Portland cement led to a considerable fall in their use [4] because cement offered certain advantages such as fast setting and high mechanical resistance [5], [6]. Lime mortars are now beginning to be used again in the restoration of historic buildings because they are compatible with traditional building materials [7], [8], [9], with which Portland cement shows low

Materials and methods

Four types of lime mortars (non-hydraulic and hydraulic) were tested against forced and natural carbonation by CO2 (the abbreviation for each group of mortars is defined in brackets):

  • 1)

    pure lime (L);

  • 2)

    lime+air-entraining agent (LA);

  • 3)

    lime+pozzolana (LP);

  • 4)

    lime+pozzolana+air-entraining agent (LPA).

The weight percentage of additives was: 0.1% for the air-entraining agent (according to the recommendations of the manufacturer) and 20% for the pozzolana in accordance with the UNE 80-301-87 standard for

Weight increase

After 30 days all the samples subjected to forced carbonation registered weight increases of approximately 6% (at which time it was decided to stop the test as no further weight changes were discernible in the mortars). The highest values were for the L group (6.6%) and the lowest for the LP group (5.7%). The lower values registered by LA and LPA mortars if compared with the L group can be explained by the fact that the air-entraining agent generates highly porous mortars with a low degree of

Conclusions

The following conclusions can be reached:

  • 1)

    Different techniques, such as the determination of the weight of the mortars or the quantification by XRD of the different mineral phases that constitute them, provide very similar results. They enable the speed of mortar carbonation to be calculated and ensure that the process is completed quickly and reliably.

  • 2)

    In the mortars studied in this work, a ≥90 wt.% portlandite–calcite transformation was achieved by XRD in just over 1 week by subjecting the

Acknowledgements

This research has been supported by a Marie Curie Fellowship of the European Community Programme “Energy, Environment and Sustainable Development” under contract number EVK4-CT-2002-50006, by the Research Group RNM179 of the Junta de Andalucía and by the Research Project DGI-MAT-2000-1457 from the Spanish government. We thank the Centro de Instrumentación Científica of the Universidad de Granada for technical assistance during SEM, thermogravimetry and granulometry analyses and Nigel Walkington

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