With the depletion of natural resources and the growing awareness of environmental protection, it is increasingly important to apply more renewable resources in construction material. Considering the low-cost and abundance of plant fibers, this paper proposes a novel cementitious material called plant fibers reinforced alkali-activated slag cementitious material (PF-AASCM). Specifically, the author investigated the types and features of fibers and how they affect the AASCM properties. On this basis, the influencing factors of the mechanical properties and microstructure of PF-AASCM were determined, and the measures to enhance the compressive and flexural strengths of the PF-AASCM were evaluated one by one. Next, the potential of applying plant fibers as internal enhancements of the AASCM was thoroughly explored. The results show that the PF-AASCM is economically and technically viable for many construction applications. The plant fiber reinforcement can enhance the ultimate strength of the AASCM rapidly (70 % growth in 7d and 120 MPa/17,000psi in 28d), resulting in excellent acid resistance and freeze-thaw durability. In addition, the optimal mix ratio is 1:4 wt.% between sodium hydroxide and potassium silicate. Under this ratio, the mechanical properties and microstructural features of the sample will be comparable to those of Portland cement. The compressive strength of single-row-hole sample of wheat straw reinforced AASCM was 10.75MPa at room temperature, equivalent to that of standard concrete sample MU7.5. The compressive strength of such a PF-AASCM decreased linearly with the growth in temperature. After the temperature reached 600 ℃, the plant fiber still exerted a certain tensile force on the matrix. By contrast, the polypropylene fiber started to melt down after the temperature increased to 200 ~ 400 ℃, causing brittle failure of the matrix. Under the high temperature of 200 ~ 400 ℃, the fine steel fiber enhanced the compressive properties of samples, which showed clear plastic deformation. The research results shed new light on reducing pollution and enhancing AASCM ductility.

Recycled concrete is a kind of green and new material. With the application of construction, more research should be carried on its carbonization performance, not only focusing on its mechanical properties. Recycled concretes with different W/C (0.45、0.55 and 0.65) and recycled aggregate mix proportions (0、30%、50%、70%and 100%) are made to test their carbonation depth. The results show that water-cement ratio and recycled aggregate mix proportions have interactive influence on carbonization performance of recycled concrete. Influence of recycled aggregate on carbonation depth depends on two conflicting aspects: one is beneficial effect due to high water absorption; another is harmful effect caused by damage structure. A new model of RAC carbonation depth is suggested based on the study.

This paper studied the carbonation resistance of recycled aggregate concrete (RAC) designed by two mix proportion design methods through accelerated carbonation test. The investigated variables include water-cement ratio, replacement ratio of recycled coarse aggregate (RCA), and carbonation time. Based on the test results, the effects of water-cement ratio, replacement ratio of RCA, mix proportion design method, and carbonation time on the carbonation resistance of RAC were analyzed and discussed. The results showed that the mix proportion design method has a significant effect on the carbonation resistance of RAC. The replacement ratio of RCA and the water-cement ratio alternately affected the carbonation resistance of RAC. The replacement ratio of RCA is the major factor affecting the carbonation resistance of RAC. In addition, a carbonation model for RAC considering the effect of the mix proportion design method was proposed. The comparison of the test values and calculated values indicates that the proposed model can properly predict the carbonation depth of RAC.

The underground environment is much more complicated than the overground environment. Concrete in underground engineering bears complex environmental effects such as soil stress, water penetration stress, and soil salt erosion. The durability of concrete directly determines the service life of underground engineering. In this paper, considering the soil stress, the chloride ion, sulfate attack and permeability resistance of concrete were tested in the laboratory. The results show that fly ash has excellent resistance to sulfate attack, and when fly ash content is 20% or 30%, the concrete specimens have the best chloride ion penetration resistance and water penetration resistance. Under the effect of preload stress, when the preload stress level is less than 20%, the capacity of concrete stress cannot be reduced greatly, but when the preload stress level is greater than 20%, the concrete durability performance decreases rapidly.