Polypropylene fibre (PPF) is a kind of polymer material with light weight, high strength, and corrosion resistance. The crack resistance of concrete can be improved by adding PPFs. PPF can optimize the pore size distribution of concrete. As a result, the durability of concrete is significantly enhanced since PPF can block the penetration of water or harmful ions in concrete. This paper summarizes the influence of polypropylene fibre on the durability of concrete, including drying shrinkage, creep, water absorption, permeability resistance, chloride ion penetration resistance, sulfate corrosion resistance, freeze-thaw cycle resistance, carbonation resistance, and fire resistance. The authors analysed the effects of fibre content, fibre diameter, and fibre hybrid ratio on these durability indexes. The durability property of concrete can be further improved by combining PPFs and steel fibres. The drawbacks of PPF in application in concrete are the imperfect dispersion in concrete and weak bonding with cement matrix. The methods to overcome these drawbacks are to use fibre modified with nanoactive powder or chemical treatment. At last, the authors give the future research prospects of concrete made with PPFs.

Alkali activated slag (AAS) mortar is becoming an increasingly popular green building material because of its excellent engineering properties and low CO2 emissions, promising to replace ordinary Portland cement (OPC) mortar. However, AAS’s high shrinkage and short setting time are the important reasons to limit its wide application in engineering. This paper was conducted to investigate the effect of internal curing(IC) by super absorbent polymer (SAP) on the autogenous shrinkage of AAS mortars. For this, an experimental study was carried out to evaluate the effect of SAP dosage on the setting time, autogenous shrinkage, compressive strength, microstructure, and pore structure. The SAP were incorporated at different dosage of 0, 0.05, 0.1, 0.2, 0.3, 0.4, and 0.5 percent by weight of slag. The workability, physical (porosity), mechanical, and shrinkage properties of the mortars were evaluated, and a complementary study on microstructure was made. The results indicated that the setting time increased with an increase of SAP dosage due to the additional activator released by SAP. Autogenous shrinkage decreased with an increase of SAP dosage, and was mitigated completely when the dosage of SAP ≥ 0.2% wt of slag. Although IC by means of SAP reduced the compressive strength, this reduction (23% at 56 days for 0.2% SAP) was acceptable given the important role that it played on mitigating autogenous shrinkage. In the research, the 0.2% SAP dosage was the optimal content. The results can provide data and basis for practical application of AAS mortar.

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Calcium acetate is the major component of the waste solution produced in treating recycled concrete aggregate (RCA) with acetic acid. Thus, the current work aims to explore the influence of calcium acetate solution on the performance of ordinary Portland cement. For this purpose, cement pastes and mortars were prepared using different concentrations, namely, 1, 2, and 3%, of calcium acetate solution. The heat evolution results confirmed that the calcium acetate solution at a concentration of 2% could accelerate the hydration of the cement. The XRD/TGA results and SEM images also indicated that more ettringite and calcium hydroxide (CH) are produced in the mortars prepared by the 2% calcium acetate solution. Moreover, the mortar containing the 2% calcium acetate solution has a denser microstructure than the control group according to the MIP tests results, and mortars cured for 3 and 28 days have a respective compressive strength, 23.34 and 15.43%, higher than the control group. The finding of this research could also contribute to studying the effect of adding metal ions to weakly acidic solutions as mixing liquids on the performance of cementitious materials.

A safe and zero hazardous method is proposed to recycle and reuse fresh mortar waste. In this method, a dormancy agent and an awakening agent were used together in this study to reuse waste cement mortars. Citric acid (CA) was used as the dormancy agent to retard the hydration of cement, and an alkaline accelerator (AA) was used as the awakening agent to accelerate the hydration of cement retarded by CA at an early age. Autogenous shrinkage, dry shrinkage and quality loss, compressive strength, capillary water absorption, and rapid chloride penetration testing were performed for the mortars. A setting time test, X-ray diffraction, and thermogravimetric analysis (TGA) were performed for the pastes (without sand). The results showed that the addition of 0.3% of CA solution increased the setting time significantly. This was because the citrate ions were absorbed onto the surface of the cement particles and a protective film was formed around the cement particles to retard dissolution. However, the retarding effect was eliminated by the AA, which was mainly composed of sodium silicate due to the contribution to initial calcium consumption. With the increasing dosage of the AA, the autogenous shrinkage, dry shrinkage, water absorption, and total charges passed were increased continuously compared to the reference. However, the influence was insignificant at a low dosage of the AA (1% and 1.5%). Obviously, the compressive strengths of the mortars with the AA were increased compared with the reference at the curing times of 3 d and 7 d. On the basis of not reducing the compressive strength of mortar, the reutilization of fresh mortar waste is realized. From this work, it was found that the reuse of waste cement mortars was feasible with the combined utilization of CA and the AA, which could provide some theoretical basis and experimental data for engineering applications.