Search for: All records

Alkali-silica reaction (ASR)'s destructiveness is governed by the compositions and properties of ASR products, while methods of converting these hygroscopic-expansive gel-like products into innocuous phases remain unexploited. In this study, the influence of magnesium nitrate on the evolutions of phase, molecular structure, hydroscopic and mechanical properties of ASR gels with varying Mg/Si ratios from 0.1 to 1.1 was investigated. The results indicate that the primary phases of ASR products, tobermorite-type calcium silicate hydrate (C–S–H) and alkali kanemites, can be suppressed into brucite and eventually converted into magnesium-silicate-hydrate (M-S-H) in the presence of increasing Mg/Si ratios and the consequent decreasing pH. The Si–O–Si bridging bonds and Si–O symmetric stretching in the Q3 sites of ASR products can be suppressed. The phase and structure modifications resulted in a 93.5% reduction in hydroscopic swelling, a 94.7% decrease in strength, and a 152.3% drop in modulus of elasticity rendering the ASR products less destructive. 

The formation and swelling of alkali-silica reaction (ASR) gels, products of the reaction between amorphous silica from aggregates and alkalis from cement capable of absorbing moisture, are considered the primary mechanism of ASR-induced deteriorations in concrete. To date, the ASR mitigation approaches mainly focus on the incorporation of supplementary cementitious materials and the use of lithium-based admixtures. These traditional approaches possess limitations in the extent of ASR suppression and might compromise concrete performance. Effective ASR mitigation under carbonation has been recently documented but the underlying mechanisms still remain unclear. To fill this knowledge gap, phase evolution and property development of ASR gels under carbonation are investigated in this study A synthetic ASR gel with a calcium-to-silica ratio of 0.3 and an alkali-to-silica ratio of 1.0 was synthesized and conditioned under 20% CO2 concentration, 75% relative humidity, and 25oC. The extent of carbonation and phase evolutions were characterized and quantified through X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). The modulus of elasticity and hardness of the carbonated ASR gels were measured using nanoindentation, and the moisture uptake capacity was evaluated using dynamic vapor sorption in stepwise relative humidity levels. The results indicate complete conversion of ASR gels into stable carbonates (nahcolite, vaterite, calcite and silica gel) with increased mechanical properties and suppressed hygroscopicity. 

The formation and swelling of alkali-silica reaction (ASR) gels, products of the reaction between amorphous silica from aggregates and alkalis from cement capable of absorbing moisture, are considered the primary mechanism of ASR-induced deteriorations in concrete. To date, the ASR mitigation approaches mainly focus on the incorporation of supplementary cementitious materials and the use of lithium-based admixtures. These traditional approaches possess limitations in the extent of ASR suppression and might compromise concrete performance. Effective ASR mitigation under carbonation has been recently documented but the underlying mechanisms still remain unclear. To fill this knowledge gap, phase evolution and property development of ASR gels under carbonation are investigated in this study A synthetic ASR gel with a calcium-to-silica ratio of 0.3 and an alkali-to-silica ratio of 1.0 was synthesized and conditioned under 20% CO2 concentration, 75% relative humidity, and 25oC. The extent of carbonation and phase evolutions were characterized and quantified through X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). The modulus of elasticity and hardness of the carbonated ASR gels were measured using nanoindentation, and the moisture uptake capacity was evaluated using dynamic vapor sorption in stepwise relative humidity levels. The results indicate complete conversion of ASR gels into stable carbonates (nahcolite, vaterite, calcite and silica gel) with increased mechanical properties and suppressed hygroscopicity. 

Cracking can facilitate deteriorations of concrete structures via various mechanisms by providing ingress pathways for moisture and aggressive chemicals. In contrast to conventional maintenance methods, self-healing is a promising strategy for achieving automatic crack repair without human intervention. However, in capsule-based self-healing concrete, the dilemma between capsules’ survivability and crack healing efficiency is still an unfathomed challenge. In this study, the feasibility of a novel property-switchable capsule system based on a sustainable biomass component, polylactic acid, is investigated. Capsules with different geometries and dimensions were studied focusing on the compatibility with concrete, including survivability during concrete mixing, influence on mortar and concrete properties, and property evolution of the capsules. The results indicate that the developed elliptical capsules can survive regular concrete mixing with a survival ratio of 95%. In concrete containing 5 vol.% of gravel-level capsules, the compressive strength was decreased by 13.5% after 90 days, while the tensile strength was increased by 4.8%. The incorporation of 2 vol.% of sand-level capsules did not impact the mortar strength. Degradation and switchable properties triggered by the alkaline matrix of cement were observed, revealing the potential of this novel biomass capsule system in achieving both high survivability and self-healing efficiency in concrete.