Tolerance of Clay Minerals by Cement: Effect of Side-Chain Density in Polyethylene Oxide (PEO) Superplasticizer Additives

Abstract

Polycarboxylate superplasticizer (PCE) is a widely used water-reducing agent that can reduce significantly the water demand of concrete, which reduces the porosity and enhances the strength and durability of the concrete. (The PCE consists of a single backbone with many long PEO side chains.) Generally, aggregate occupies >70 wt.% of concrete; clay minerals are ubiquitous in nature and are difficult to avoid in mined aggregates. Clay minerals in aggregate often render the PCE ineffective and give rise to rapid loss of the fluidity of the concrete; this phenomenon is referred to as ‘poor clay tolerance of PCE.’ Though the poor clay tolerance of PCE is known widely, the relationship between the clay tolerance and the molecular structure of the PCE, in particular the effect of the side-chain structures, on clay tolerance is not understood completely. The objective of the present study was to determine the effect of different grafting densities of polyethylene oxide (PEO) side chains on the clay tolerance of PCE. The raw materials included mainly PCE, which was synthesized using acrylic acid and isopentenol polyoxyethylene ether, and a natural montmorillonite (Mnt), one of the most common clay minerals. The loss of fluidity of the cement paste was tested to assess the clay tolerance; total organic carbon was used to measure the amount of PCE adsorbed; X-ray diffraction, transmission electron microscopy, Fourier-transform infrared spectroscopy, and thermogravimetric analysis were used to investigate the microstructure of the intercalated Mnt. The results showed that preventing the superficially adsorbed PCE from being intercalated into Mnt was of great importance in terms of the improvement in clay tolerance of PCE, which increased with greater grafting density of PEO in the side chain of the PCE. The results also suggested the possibility that polymers which intercalate preferentially into the Mnt could improve significantly the clay tolerance of the PCE system.

Influence of experimental procedure on d-spacing measurement by XRD of montmorillonite clay pastes containing PCE-based superplasticizer

“…From Fig. 5, it can be stated that both clay d-spacing results of 18.6 Å and 19.1 Å do not differ significantly from the literature values, using the same sample preparation approaches [3][4][5]. The difference in d-spacing observed in Fig.…”

“…Thus, it can be deduced that separation method has a minor influence on d-spacing measurements on dried clay pastes. [2,3,5]. This observation indicates that not all absorbed water is lost during the drying process (even considering that the tested sample was dried for 7 days at 40 ºC).…”

“…From an initial d-spacing of 12.3-12.6 Å or 15.0-15.5 Å (depending on the degree of clay hydration) when no admixture is present [2,3], PCE intercalation produces a d-spacing expansion of up to 17-18 Å when a PCE-based superplasticizer is used. This enlargement has been reported to correspond to one single intercalated poly-glycol chain in MNT clay, coordinated with water molecules [4][5][6][7][8]. All available studies in the literature address the interaction of clays with PCE-based superplasticizers via the same experimental procedure [1,4,8].…”

“…Fig.1. a) Laboratory XRD patterns for centrifuged-dried MNT samples with different PCE based superplasticisers (adapted from[5]); b) Laboratory XRD patterns for centrifugeddried MNT samples with PCE and PEG based superplasticisers (adapted from[9])…”

Effect of the Synthesis Temperature of Polycarboxylate-Based Superplasticizer on the Performance of Concrete

“…After finished the determination of initial concrete slump, the mixtures were placed under condition of air temperature for 30 min and tested the concrete slump immediately (Figure 3) Table 3, no change was observed in slump and compressive strength of the concrete values which is used PCEs synthesized at different temperatures. In most studies, radical polymerization of PCEs was carried out under 30-80 °C [2][3][4][5][6][7][8][9]. Quantity of heat is needed during the polymerization reaction.…”