At present, not many studies have considered methods to quantitatively evaluate the reaction efficiency of granulated blast furnace slag (GBFS) at different curing temperatures. For high volume slag concrete, when the replacement ratio exceeds a certain ’threshold’ value, the superfluous and ineffective slag will no longer react in concrete but simply behave as a fine aggregate, which may cause the decrement of strength. The ’threshold’ value depends on the reaction efficiency of slag. In this study, experiments on mortars with different replacement ratios by slag were conducted at different curing temperatures (20, 30, and 50 °C, respectively), the threshold values of effective replacement ratio by slag were comprehensively analyzed through the reaction efficiency of slag mortar. The results showed that the turning point of the strength curve with replacement ratio can be considered as the threshold value of the effective replacement ratio by slag in mortar. Along with the curing temperature enhancement, the threshold value of the effective replacement ratio by slag in concrete decreased, whereas the reaction efficiency of slag increased. Meanwhile, the analysis of cement effective coefficient (k value) and basicity was also calculated. Based on the obtained threshold values of effective replacement ratio at different curing temperatures, the formula for the determination of reaction efficiency coefficient of slag in the mortar can be established. Therefore, the reaction efficiency coefficient and upper limit of the effective replacement ratio of slag at different temperatures can be calculated more intuitively and quantitatively, providing a theoretical basis and reference for practical engineering applications.

Recycled aggregate can be used as a substitute for natural aggregate to produce good regenerated concrete, so the text use recycled coarse aggregate and recycled fine aggregate to substitute for natural aggregate with different replacement rates preparing double recycled aggregate concrete, and researching the effect of the quality and replacement rate of recycled aggregate on the mechanical properties of recycled aggregate systematically. The results show that the compressive strength of double recycled aggregate concrete is increased gradually with the improvement of the quality of recycled aggregate, and the effect from weak to strong is class I, class II and class III. On the other hand, the compressive strength of double recycled aggregate concrete is decreased gradually with the increase of the replacement rate of recycled aggregate, and the maximum reduction is up to 57.2%. Therefore, the simultaneous use of recycled coarse aggregate and recycled fine aggregate have an effect of “double weakening” on the compressive strength of recycled concrete, and it can be applied to the concrete products of low strength grade.

The effective mineral admixture in mortar can consume the calcium hydroxide produced by hydration reaction because of pozzolanic effect. For high volume mineral admixture mortar, when the replacement ratio exceeds a certain ‘threshold’ value, the supply of Ca(OH)₂ amount is insufficient, the superfluous and ineffective mineral admixture will no longer react in mortar but as fine aggregate. This study presents an experiment study on the threshold value of effective replacement ratio of fly ash mortar by the comprehensive analysis of Ca(OH)₂ amount and consumption curves. Under the conditions of different curing temperature, the effective replacement ratios of fly ash mortar have been determined ultimately. The results showed that Ca(OH)₂ amount and consumption in mortar both decreased with replacement ratio. For fly ash mortar, at 20°C curing temperature, when the replacement ratio was less than 40%, the Ca(OH)₂ amount decreased obviously with replacement ratio. However, when replacement ratio was more than 40%, Ca(OH)₂ amount at 91d changed slightly with replacement ratio, the Ca(OH)₂ amount was consumed to a limit value at this time. Under the condition of 30°C curing temperature, when the replacement ratio exceeded 30%, the change of Ca(OH)₂ amount with replacement ratio was close to a straight line, the superfluous fly ash no longer consumed Ca(OH)₂ basically. Moreover, the consumption of Ca(OH)₂ was not much difference to the Ca(OH)₂ amount at the same replacement ratio under the condition of 20°C curing temperature. This result has a great significance on effective utilization of mineral admixtures in engineering application.

In this study, a new similar simulation material for surrounding rock of roadway engineering under deep high-stress environment had been developed. This simulation material was used in the ratio of conventional Portland cement, sand, and water, adding iron powder, barite powder, talcum powder, gypsum, and glycerin. Through the high-precision LEICA EM ACE200 carbon sprayer equipment, scanning electron microscope and energy spectrum analysis equipment (SEM-EDS), the mineral composition, morphology, micro pore structure morphology and phase composition of the similar simulation materials were revealed. The microscopic observation results showed that the hydration reaction produced a large number of 3CaO·2SiO ₂ ·3H ₂ O gel and calcium hydroxide. The amorphous hydration product C-S-H had a large surface area, which accelerated the cementation between aggregates and made the internal structure of the simulation materials more dense at different scales. The similar materials had obvious hard and brittle characteristics based on SEM/EDS results, and can be used to predict the strength development characteristics of surrounding rocks in actual engineering through high-precision microscopy instrument.