Influence of Waste Latex and Acrylic Paint on Concrete Masonry Blockfill

A significant volume of waste latex and acrylic paint exists in New Zealand, with the rate of supply growing at a substantial rate. The paper specifically investigated the use of waste paint as a polymer admixture in concrete masonry blockfill. The objective of this study was to produce a blockfill mix capable of maintaining or improving the properties of the hardened material whilst increasing the efficiency of the construction process. Study into the fresh properties of blockfill focused upon workability, with specific attention given to investigating the change in rheological properties. Compressive strength, tensile strength, and seismic performance were studied to assess the hardened properties of the blockfill. It was established that waste latex and acrylic based paint was a suitable additive to concrete masonry blockfill, resulting in maintained strength and improved workability, providing a viable substitute to standard chemical admixtures currently used to achieve comparable results. INTRODUCTION An advanced cementitious material that is environmentally sustainable has been developed as a flowable blockfill for concrete masonry. Waste paint is a valuable resource, which is currently being disposed of in landfills at a substantial economic and environmental cost. This waste paint, which includes all water based acrylic and latex varieties available in New Zealand, exhibits many properties that are similar to polymer admixtures used in concrete production. Polymers make up the majority of the solid mass in paint, which for this investigation was convenient as polymer admixtures are used in concrete production to increase the matrix bond between cement and aggregate and to increase the workability and flow of cementitious materials, but they are often too expensive for many applications. Consequently, by adding waste paint to blockfill the addition of polymer assists the blockfill’s ability to flow without negatively influencing strength, as would be the case if water were added for the same purpose. This increase in workability, which is attributable to the waste paint additive, results in a less labour intensive construction process. The improved workability properties of the blockfill enable it to settle around congested reinforcement and small void areas and reduce the need for compaction and vibration, resulting in faster, cheaper and safer construction. PAINT AND POLYMER CONCRETE A polymeric admixture is described as an admixture which consists of a polymeric compound as a main ingredient effective at modifying or improving properties such as strength, deformability, adhesion, waterproofness and durability of cement, blockfill and concrete. The properties of polymer-modified concrete depend specifically on the polymer content or mass based polymer-cement (p/c) ratio, rather than the water-cement (w/c) ratio used when assessing ordinary cement concrete (Ohama 1998). Although polymer-based admixtures in many forms are used in cementitious composites such as blockfill and concrete, it is important to ensure that both cement hydration and polymer film formation proceeds in order to yield a monolithic matrix phase with a network structure in which the cement hydrate phase and polymer phase interpenetrate (Van Gemert et al. 2005). It is the formation of such a co-matrix phase that results in superior properties compared with conventional cementitious materials. Recycling waste latex paint (WLP) was used in urban concrete sidewalks in Ontario, Canada. It was determined that WLP contributed in a similar form to virgin latex by exhibiting the same advantages in cementitious materials, such as increasing flexural strength and decreasing chloride ion penetrability. A field demonstration sidewalk modified with WLP exhibited enhanced workability and finishing, and better durability to surface scaling and aggregate pop out (Nehdi and Sumner 2003). Paint is made up of numerous fine and ultra fine particles, in the range of 0.1 μm to 10 μm. The addition of fine particles and the application of particle packing theory allows concrete to be manufactured using poorly shaped or poorly graded sand and aggregates while still producing workable concrete. It had been determined that by incorporating large amounts of ultrafine fillers with reduced cement content, it is possible to produce a high strength concrete which implied a great potential for saving cement and resultant decrease in the production of carbon dioxide (Lagerblad and Vogt 2003). The waste paint used in this study was sourced directly from a paint collection program and stored in large tanks, which results in a consistent end product with little variation in properties. This ensures a stable national average in material quality, given the variety of paints collected. The paint was sampled and tested at the paint collection facility to determine the variability of water content, pigment content and polymer content. The data indicated that paint is made up of approximately 50% water, indicating that when water is replaced by waste paint, 50% of that water content remains present in the mix. The remaining solid content of the paint, made up of polymers and pigments, is the source of possible variability in air content, workability, setting times, and concrete density. Across the several New Zealand paint producers and their paint varieties a large number of unique constituents are included. The primary constituents expected to occur in high volume were; Polymers The continuous phase of latex and acrylic water based paint is primarily made of polymer. This phase carries and then binds the other components of the paint such as the pigments and extenders, and then provides the continuous film forming component of the coating. (Steward et al. 2000). Surfactants A surfactant or surface active agent is a substance that reduces the surface tension of a liquid. Surfactants are chemicals whose molecules have two parts of widely differing polarity and solubility (Porterm 1994). Foam Controllers Antifoams or defoamers are active in paint to control foaming, which is an unwanted product of surfactants. These are present in waste paint, as foaming of the paint is not desirable for its application. Titanium dioxide Titanium dioxide is the primary white pigment used by the paint industry as it is the only pigment (other than zinc oxide) that is non toxic and easily obtainable. (Marrion 2004). Titanium dioxide was not expected to have any substantial chemical effect on cement. Thickeners Thickening agents are active in paint to control consistency and ensure that workability is maintained during storage and application. THE VIABILITY OF PAINT REPLACEMENT OF CONVENTIONAL CHEMICAL ADMIXTURES A standard chemical admixture as used by a New Zealand ready mix concrete supplier for the production of standard 17.5 MPa blockfill is a nonretarding, non-chloride, water reducing, strength enhancing admixture made up of two key ingredients; calcium lignosulfonate and triethanolamine (BASF 2006b). Calcium lignosulfonate is an anionic polymeric surfactant which primarily acts as a water reducer, or wetting agent. It is found active at the interfaces of the water/air and water/particle surface and decreases the surface tension within the system. The result is an increased ability of the flow and spread of water across all surfaces, thus reducing the need for water. Calcium lignosulfonate also works as a retarder which prolongs the hydration process of cement, ultimately increasing the compressive strength (Griersona et al. 2005). Triethanolamine is an organic chemical compound which serves two purposes within the admixture. Its primary purpose is as a pH balancer and aims to increase the pH of a mixture (although the pH of concrete water will already be high) but it also helps to keep the lignosulfonate in its charged (active) form. A second chemical admixture also used in the production of blockfill is an air entrainer, composed primarily of an alkylbenzene sulfonic acid surfactant in order to stabilise the entrained air. (BASF 2006a). There is a significant chemical difference between this surfactant and the calcium lignosulfonate found in the water reducer, but the practical difference is in the amount of foam stabilisation, the degree of substrate wetting (surface tension) and the strength of interaction between the surfactant and particles in the mixture. These factors have the net effect of influencing the fluidity/viscosity of the mixture, hydration rate, and final properties to some extent if the degree of interaction affects crystal growth during cement hydration. From an assessment of the active chemicals in paint, and the standard chemicals presently used within blockfill, conclusions can be drawn on the theoretical success of the chemical replacement. A primary ingredient in both chemical admixtures is surfactant, which covers a large range of chemicals. Water based paint is almost always alkaline and contains amines similar to triethanolamine, as well as various other types of surfactant that are suitable for dispersing and stabilising particles. Many of the paint chemicals discussed will be inactive as they are present in concentrations that ensure that they are only effective in serving the purpose for which they were added to paint, whilst others are free to interact with other ‘species’ such as cement. The chemicals active in the water reducer, which again are based on the amount of chemical that is actually available, can be found in waste paint in the form of a surfactant. The acrylic polymers found within waste paint have the potential to simulate the action of the calcium lignosulfonate as a hydration retarder (Wang et al. 2005). TESTING OF BASIC MATERIAL PROPERTIES Laboratory testing was completed to determine the optimal paint dosage for the target blockfill workability and strength properties. Testing was completed in two stages, initially with the industry recomm