Basic magnesium sulfate (BMS) cement was prepared by magnesia with different reactivities derived from calcination of basic magnesium carbonate at 600 to 1000°C. The effects of reactivity of magnesia on the setting time and compressive strength of BMS cement was investigated. Furthermore, XRD, SEM, MIP, hydration heat evolution and the pore solution composition were used to analyze the action mechanism of magnesia reactivity. The results show that the main strength phase of 5Mg(OH)2•MgSO4•7H2O (5•1•7 phase) was formed in BMS cement prepared by magnesia with calcination temperature higher than 600°C. The magnesia with a high BET surface area and low crystallite dimension showed a high adsorption capacity for citrate ions in magnesium sulfate solution, decreasing their concentration in the pore solution. By acting as a soft template for 5•1•7 phase formation, the decrease in citrate ions resulted in a low amount of 5•1•7 phase, forming Mg(OH)2 instead, leading to a low-strength BMS cement.
In this paper, the specimens of phosphor-gypsum (PG) based binder and the specimens of its lightweight mortar (LM), further the specimens of its lightweight wall materials (LWM) were prepared using original PG, straw powder (SP), expanded perlite (EP) and expanded polystyrene particles (EPP) as raw materials. Moreover the properties of the specimens were investigated systematically. In the preparation and characterization of the specimens of PG based binder with fixed PG content being 50 wt.%, and the remains of it being Portland cement, blast furnace slag, and fly ash, the specimen with 15 wt.% fly ash has the largest values of flexural and compressive strengths among the binder specimens prepared, and all the binder specimens have the promising softening coefficient more than 0.88. For the LM specimens consisted of PG based binder and SP, their softening coefficient are ≥ 0.89 as the adding content of SP is ≤ 30 vol.%. For the LWM specimens consisted of PG based binder, SP, EP and EPP, the bulk density and thermal conducting coefficient are slightly decreased with the increase of EP and EPP contents, respectively. The LWM specimen with 50 vol.% EPP has the promising comprehensive properties.
The bond behaviour of plain rebars embedded in ambient cured geopolymer concrete (GPC) prepared with a mix of fly ash and ground granulated blast furnace slag (4:1 by mass) was studied to evaluate the adhesive bond between these rebars and concrete. As GPC is an inorganic polymer concrete, the adhesive bond strength of concrete is evaluated for its suitability to reinforced concrete applications. Pull-out tests were conducted to measure the slip of the applied load. Diameters of the embedded bars were varied as 12, 16 and 20 mm, providing compressive strengths of 39 to 68 MPa. Chemical adhesion occurs between the bar and concrete as the bar surface is polished. We observed a gain in adhesive bond stress as bar diameter is increased. The bond stress-slip curves of GPC and ordinary Portland cement concrete were different and the peak bond stress was about 70% higher in GPC. Regression analysis showed that the adhesive bond strength in GPC is about one-fourth of the bond strength of the deformed rebars. Higher bond strength in GPC is due to chemical bonding and formation of both sodium aluminosilicate hydrate and calcium aluminosilicate hydrate gels besides the denser interface as seen from scanning electron microscopy and the ordering of silicon and aluminium as identified in MAS-NMR.
Rebar corrosion is a main cause affecting the durability and safety of reinforced concrete structures. Many mathematical models (empirical and analytical models) have been proposed to predict the corrosion-induced cracking of concrete cover over the recent decades. These models have differences in many aspects, such as constitutive laws for cracked concrete. However, each of them claimed that it could well agree with the test data. This may be due to the fact that a small amount of test data were used to verify their models. Besides, there are many uncertain factors such as the rust volumetric expansion factor and elastic modulus relating to these predictive models, which seriously weaken their effectiveness and accuracy. Thus, in this paper, the predictive performances of various mathematical models are tested by collecting an amount of test data. After that, the uncertain factors are summarized, and the reasons are analyzed though reviewing relevant knowledge in various literature. It is found that different performances are obtained for different mathematical models and test data. These uncertain factors involve many fields, such as electro-chemical factors (corrosion current density, corrosion current efficiency, corrosion rate, loading effect of corrosion current), rust properties (rust volumetric expansion factor, rust elastic modulus), long-term behavior of concrete (creep coefficient), rust penetration into concrete (amount of rust penetrated into concrete pores and cracks) and corrosion morphology. These uncertain factors are raised due to either the limit of experimental techniques, the discrepancy of parameter values, time-dependent properties, or the lack of detailed environmental information. In addition, other factors are too complex to be delicately taken into account, which have to be ignored in the predictive models. Through parameter analysis, it is found that these factors have large effects on cracking time of concrete cover. Prospective modifications of these factors are suggested, and more detailed and systematic tests should be carried out by using novel and sophisticated apparatuses in future.