The effect of temperature and humidity treatment on the strength set of fine-grained concrete with the addition of saponite-containing material

Introduction. To restore and develop the Solovetsky Islands road network concrete slabs could be used. Currently, compositions with additives of industrial waste, including micro- and nanoscale, are being actively introduced to improve the performance characteristics of the road slabs. For the Arkhangelsk region, saponite-containing waste from the mining industry could be used as a material. At the same time, one of the ways to accelerate the process of gaining strength of composites is steaming. However, studies related to the effect of temperature and humidity treatment on the hardening process of a composite with a highly dispersed saponite-containing material (SCM) have not been previously studied. It is known that the process of moisture sorption with a finely dispersed additive allows controlling structure formation during concrete hardening, improving its operational characteristics. However, under conditions of high humidity, the quantitative content of tempering water, calculated based on the value of water absorption of the SСM, can significantly change and affect the process of gaining the strength of the composite. Therefore, the purpose of this research is to study the effect of temperature and humidity treatment on the strength gain of fine-grained concrete with the addition of SCM. It has been established that the accelerated method of concrete hardening has only a positive effect on the formation of a strong and dense structure. However, in the case of using highly dispersed saponite-containing material as an additive, the opposite effect (decrease in strength) associated with sorption properties and features of its structure can be observed. Methods and Materials. The SCM recovered from the recycled water was dried to constant weight and dispersed on a planetary ball mill. Particle size was determined by dynamic and electrophoretic light scattering, and specific surface area was determined by nitrogen sorption (BET theory). The strength of samples of fine-grained concrete of the control and experimental (with the addition of SCM) compositions was gained in two ways: under normal conditions and by an accelerated method using steaming. Strength tests of cube samples with dimensions of 70×70×70 mm were carried out on an automatic test press according to GOST 10180. The microstructure of the samples was examined by scanning electron microscopy. Results and Discussion. The separated, dried and ground saponite-containing material (powder) had an average particle size of 445±40 nm and a specific surface area of 50 670±10 m2/kg. In continuation of the studies, control (FGC) and test samples of fine-grained concrete (FGCscm) were made. The amount of a highly dispersed additive was introduced into the concrete mixture based on previously obtained results of kinetic studies of the water absorption process of saponite-containing material. The determination of the strength characteristics of 1-FGC and 1-FGCscm, hardening under normal conditions, was carried out on day 28. Samples of 2-FGC and 2-FGCscm, a day after sealing with water, were placed in a steaming chamber. After the expiration of the holding time, they were gradually cooled and their strength characteristics were determined. It has been established that the accelerated method of gaining strength of finegrained concrete, by steaming, has a positive effect only on control samples. For a composite with the addition of SCM, temperature and humidity treatment has the opposite effect. Thus, the dynamics of strength gain of 1-FGC and 1-FGCscm are of the same type. At the initial moment of time, an active site of strength gain is observed, but on day 7, the compressive strength of the samples of the experimental composition is 40% higher than that of the control one. At the accelerated curing method in the first two hours (isothermal heating stage), an active strength increase was observed in 2-FGC and 2-FGCscm. The subsequent curing of the control specimens has a linear dependence with a gradual increase in strength to the design strength in six hours. For 2-FGCscm, after two hours of steaming, the strength sharply begins to decrease, and after six hours – visible destruction of the concrete structure occurs. Consequently, prolonged temperature and humidity exposure of concrete with highly dispersed SSM admixture leads to a decrease in the strength characteristics of the specimens. Most likely, it is connected with moisture oversaturation of the composite structure. Therefore, in continuation of the research, electronic photographs were taken of the microstructure of concrete after three hours of holding in the steaming chamber. Thus, the microstructure of 2-FGCscm is mainly represented by spongy particles, and the number of formed needle-like (tobermorite crystals) – decreased significantly, compared to the control.