A study of the mechanical properties and appearance of efflorescence in paving bricks under different curing environments

Taiwan has witnessed rapid economic development in recent years, and with urban development, the planning and design of compressed paving units and pervious ground tile structures are focused on the demand for esthetic appearance in addition to practical functioning and legal requirements for strength and water permeability. In this study, the efflorescence appearance of structures was effectively reduced using calcium carbonate crystallization from the water purification and softening process to fill the pores in generic high-pressure bricks. The same water-to-binder ratio of 0.35 was used for three different mix proportions, and 20 cm × 20 cm × 6 cm general high-pressure bricks, calcium carbonate high-pressure bricks, and pervious bricks were made under a 50 kg/cm² molding pressure at a concrete-brick-making plant. The samples were cured in five different environments with curing times of 2, 7, 28, 56 and 91 days. The effect of the curing age, the curing environment and the brick type on the hardness, solidity and appearance of efflorescence of the bricks were analyzed in terms of the compressive strength, the ultrasonic pulse velocity, and the coefficient of permeability, using Paint.net image analysis, scraping rate tests, SEM analysis and EDS analysis. The test results showed that the compressive strengths of the general high-pressure bricks, the calcium carbonate high-pressure bricks and the pervious bricks were 46.78 MPa–84.97 MPa, 56.89 MPa–100.23 MPa and 35.38 MPa–59.20 MPa, respectively. The ultrasonic pulse velocities were 3296 m/s–4524 m/s, 3567 m/s–5703 m/s and 2623 m/s–4319 m/s, respectively. The coefficient of permeability of the pervious bricks was larger than 1 × 10–2 cm/sec. Quantitative analysis of efflorescence using Painet.net image analysis and scraping rate tests showed that the ratio of the efflorescence to the total area at the surface or bottom of the bricks and the scraping rate were larger for curing on hardened concrete than under other curing conditions. The efflorescence that was generated on the bottom layer of the bricks was higher than that on the surface layer by 5.31%–19.82%. Microscopic analysis of the specimens (SEM) showed the existence of many crystalloid Ca (OH)₂ colloids that accounted for 20–25% of the cement volume. EDS analysis showed that Ca was the main element in efflorescence, at 6.64%, which was much higher than the percentage of other elements. To summarize the results, curing in a cling film seal can inhibit efflorescence crystallization.