Sterile effect in foam concrete

The paper considers the effect of a foaming agent and the steric effect initiated by it on the strength of a cement matrix and foam concrete under the classical technology of producing heat insulating cellular concrete of non-autoclaved hardening.

The presence of this effect is more convenient to experimentally reveal on the matrix, because the influence of the formed large-porous structure is excluded in this case.
Increasing mobility, reducing the water requirement of a concrete mix, or reducing the consumption of cement can be achieved using plasticizing additives. It can be assumed that the foaming agent, as the main component of the foam that forms the structure of the foam concrete at the initial stage, has a significant effect on the characteristics of the cement slurry and the rheological properties of the foam cement mixture [4,5]. Such mixtures necessarily contain an addition of a foaming agent, the effect of which on the hardening binder is not fully understood. This makes it necessary to study the interaction processes of various foaming agents with cement. The purpose of this work is to study the effect of the steric effect on the structural and mechanical properties of the foam cement mixture and the strength of the foam-concrete monolithic massif.

Materials and methods
We used the following in our research: (a) the cement PC-500-D0 being produced by the Angarskcement OJSC; (b) synthetic blowing agents, namely Penta Pav 430A and Maxpen; (c) protein foaming agents, Ompor and Bioph; hyperplasticizers Glenium Sky 591, MC Power Flow-3100, and Relamix M.
Determination of the matrix strength was carried out in accordance with GOST 310.4-81 "Cements: Methods for Determining the Ultimate Strength in Bending and Compression". For manual production of three beam samples, the required amount of binder, as well as surfactants, to which the foaming agents and plasticizers belong, are weighed, and batching is carried out. The resulting homogeneous mass is poured into a metal three-cavity mold being lubricated with oil. After 28 days of normal hardening, the halves of the beams were tested for compression. The tests consist in determining the minimum compressive loads, which destroy half of the standard beams sample of 40x40x160 mm. The compressive strength was determined by testing six halves of beams on a hydraulic press IP-1000. To transfer the load to the beams' halves, flat steel polished plates of 40x62.5 mm are used. Each half of the beam is placed between the two plates in such a way that the side coincide with the working surfaces, and the abutments of the plates fit snugly against the sample's end smooth wall. Tests on bending were not carried out, since they were not included as part of our research objectives.

Results
The nonlinear character of the strength versus porosity dependence, which was expressed in [6,7,8,9,10], was confirmed by the experiments. The graph ( Fig. 2) shows the strength of foam concrete of thermal insulation grades based on the average density of Rb on the ratio of the density of foam concrete ρb and matrix ρm. With exponential approximation with the reliability R 2 = 0.99 in the range of ρb / ρm ≤ 0.5, it corresponds to the following analytical expression: Rb=0,2Exp(7,2 ρb/ρm).
(1)  In the matrix, the steric effect is expressed in a decrease in density and strength [3,12]. Difficulty in the growth of neoplasms leads to loosening of the cement stone's structure, as well as to the formation of interconnected micropores, decreasing in density and, accordingly, in strength (Fig. 3).
When two surfactants are simultaneously present in the cement colloid, namely a foaming agent and a hyperplasticizer, the latter, as the more active surfactant, partially displaces the foaming agent from the solid-liquid interface. This allows, first, to reduce the water requirement of the mixture, and secondly, to reduce the negative effect of the steric effect [3,4].
Both these phenomena have a positive effect on the strength index, which is clearly illustrated in Figure 4.

Discussion
The sterile effect partially inhibits the course of the hydration reaction. Thus, the presence of surface-active substances close to the reacting atoms may prevent these atoms from converging and slow down the reaction, or completely make it impossible. Frothers cause a slowing down of the interaction of C3A with water and change the rate of forming hexagonal hydrate phases. Moreover, large dosages of additives exert a more significant effect on the kinetics of crystallization of hydrate neoplasms and, to a lesser extent, affect the phase composition. In solutions of foaming agents, the process of interaction of C3A with water does not take place completely, and the original alite remains in the system 28 days after hydration.
Before the solidification, the foam cement mixture is a heterogeneous free-disperse system, including solid, liquid, and gaseous phases, in which the dispersed phase is mobile.
Two dispersed mobile phases can be distinguished: the dispersed gas phase in the dispersion medium and the dispersed gel phase in the aqueous solution in the form of a dispersion medium. Particles in this system are closely approximated; therefore, this system can be conditionally referred to a free-dispersed concentrated system. It is possible that in the foam concrete mixes at the level of thin films of the interporal partitions, microscopic stratifications occur, forming zones in the solid-liquid system with different water contents, in which the separation of the solid-phase particles in size takes place. Over time, it passes into a cohesive dispersed system with a solid dispersed medium, i.e. a cement stone. This cement stone, formed during hardening of general cement, is characterized by a complex stressful state, which can lead to the emergence and development of microcracks.
The effect of hyperplasticizers is based on the joint manifestation of the electrostatic and steric effects that arise with the side hydrophobic polyester chains of the polycarboxylate ester molecule [11]. Due to this, their water-reducing effect reaches more than 50%. Thus, the mechanism of action of hyperplasticizers on cement systems is provided due to the predominant steric effect, which is also characteristic of the presence of surface active substances ( Figure 5). With this effect of hyperplasticizers, there are a great diluting ability with low dosages of the active substance, reduced sensitivity to the quality of cement, uniformity of concrete and mortar mixtures, and high performance characteristics of the finished product.
Plasticizing additives, adsorbed at the water-solid interface, affect the surface tension of the liquid phase insignificantly. Hyperplasticizers have a dispersant effect, i.e. they increase the amount of particles in the solution and reduce their size, increasing the sedimentation resistance of the cement system [12,13,14].

Conclusion
There is a nonlinear relationship between the porous structure and the strength of the foamed body ( Figure 2). This is due to the joint action of several processes and phenomena. The low strength of the expanded cement stone in comparison with the strength of the matrix and the nonlinearity of the functional dependence of the strength on the average density are due to two groups of factors: the steric repulsion effect in the matrix solution and the large cellular structure, which, for its formation, requires an increased mobility of the mixture.
Therefore, when laying a foam concrete mixture, the steric effect caused by the plasticizer and foaming agent plays a positive role, since it increases the mobility of the solution. But when grasping and typing strength, their action varies considerably. The difference lies in the fact that the plasticizers act temporarily in the solution and then enter into a physico-chemical interaction with the hydration products; and the foaming agents remain inert with respect to the solution and make it difficult to grasp and increase the strength of the cement stone.
The established mechanism of the positive effect of the investigated hyperplasticizers on the strength of the material can be used in the production of monolithic commercial foam concrete of heat-insulating grades by density.