Image: Collaborative Mix Design. Suppliers like Holcim offer high-performance concrete engineered for a wide range of conditions. Early collaboration between suppliers, contractors, and placers is key to clarifying specific requirements before the pour.
The transition to low-carbon concrete (LCC) is often framed as a burden or a compromise in quality. It is time to bin that narrative, says Tim Kleier from Concrete New Zealand.
This shift is not a hurdle; it is a necessary evolution in material science that international markets adopted decades ago.
There are no fundamental technical or climatic barriers to adoption here in New Zealand; there is only a requirement to sharpen our skills and employ site practices with more rigour.
This shift follows a global trajectory led by markets in Europe and the United Kingdom and is driven by a collective push from governments, clients, banks and the industry itself seeking to meet emission targets, improve long-term durability, bolster supply chain resilience, promote the circular use of industrial by-products and secure a distinct commercial edge through Green Star and Homestar certifications, improving tenant retention and yielding premium rates.
This is particularly relevant as the number of large projects targeting Green Star certification has grown significantly, rising from ~22 percent in 2018 to ~43 percent in 2025 (measured as a share of total building consents valued at over $10 million), and banks increasingly offer preferential rates for Green Star or Homestar-rated builds (e.g. ANZ Business Green Loan and Healthy Home Loan packages).
Mastering these mixes allows a contractor to move beyond standard placement and become a high-performance specialist, gaining a significant competitive advantage in a changing market.
Importantly, not all LCC behaves differently than conventional mixes. To understand why, it helps to know that concrete is labelled ‘LCC’ based on meeting specific emission targets, not by the specific ingredients used to get there.
Using lower-emission General Purpose (GP) cements can meet many emission-carbon reduction goals while maintaining the handling characteristics you’re used to. However, Supplementary Cementitious Materials (SCMs), like fly ash or slag, are often added for reasons beyond just lowering emissions. In fact, they’ve been used for decades to make concrete more durable and resistant to chemical attack.
By combining these SCMs with other efficiency levers, we can meet the industry’s most demanding environmental targets without sacrificing performance.
Crucially, while SCM-based concrete has the potential to develop a more refined microstructure, the mix delivered by the truck is only half the story. The concrete producer provides the potential; it is the contractor’s skill in placing, finishing, and rigorous curing that turns that potential into a high-performance reality.
The following advice focuses on the practical differences you might notice when working specifically with mixes containing SCMs.
Traditional mixes often exhibit visible ‘bleeding’, a process where water rises to the surface as solids settle. While many contractors have historically used this as a visual cue for timing their tools, low-carbon mixes with SCMs consume water more efficiently during hydration, resulting in significantly reduced bleed water.
This creates a more cohesive, denser mix that is far less likely to segregate under pressure, but it also alters the finishing window and requires specific adjustments to how we handle the material on-site.
Managing the flow of LCC starts with thorough site preparation. Rather than attempting to ‘fix’ a stiff mix by adding water at the truck, follow these preparatory steps to ensure the pump handles cohesive materials effectively.
- Prime properly: Use a professional priming agent or a rich cement slurry to lubricate the lines before the first load.
- Check your gear: Ensure your pump is rated for higher hydraulic pressures, as these more viscous, cohesive mixes may require more force to move through the pipe.
- Reduce friction: Use 125+ mm diameter pipes where possible; larger lines significantly lower the “pressure tax” on your equipment and reduce the risk of blockages.
- Trust the chemistry: Suppliers already add high-tech flow aids, such as superplasticisers or viscosity-modifying agents (VMAs) at the plant to keep the mix fluid without making it watery.
Placement
The reduced bleeding fundamentally alters the timing of the finishing window compared to traditional mixes. This requires a shift in how we prevent the surface from “closing” too early.
- Surface openness: Rather than a blanket prescription of one tool over another, professionals must focus on keeping the surface ‘open’ to allow any internal moisture or air to escape. However, because these mixes lack a significant bleed water layer, magnesium floats are the preferred choice for early passes to reduce friction and maintain an open surface without the aggressive sealing action of steel.
- Professional workability: If you require additional workability for finishing, use a dedicated finishing aid rather than water to maintain the integrity of the surface.
Whatever you do, resist the urge to ‘bless’ the surface with a hose. Spraying water to make finishing easier is a shortcut that destroys the concrete’s surface matrix. This practice leads to “measling” (spotted discolouration) and a soft, chalky finish that lacks long-term durability.
- Finishing: The reduced bleed water may also change the window for final finishing. While it is a common misconception that a lack of bleed water inherently causes delamination, these mixes are significantly less forgiving of mistimed finishing. The primary mechanism for failure is not the material itself, but the premature sealing of the surface, often because the absence of surface moisture is misread as the concrete being ready to close, which traps rising moisture or air beneath a finished top layer. To manage this risk, a specialist technician uses a tiered approach to verify the set.
- Surface checks: The footprint or thumb test remains the baseline for checking relative stiffness. However, with our weather, wind and sun can cause the surface to ‘crust’ and appear dry while the underlying concrete remains plastic. Relying solely on surface sheen or a thumb test is a risky strategy that can lead to trapping moisture in the core.
- Objective testing: A pocket penetrometer provides a numerical value for the stiffness of the mortar fraction. This removes the subjectivity of ‘how it feels’ and provides a clear, defensible data point for when to begin power trowelling, reducing the risk of premature sealing. While this provides a numerical baseline, a specialist must ensure the probe penetrates past any wind-induced surface crusting to verify the underlying concrete is not still releasing moisture that would lead to delamination.
- Core monitoring: Ideally, LCC performance should be managed through maturity monitoring. Sacrificial sensors tied to the rebar transmit real-time temperature and strength data directly to a smartphone. This is the only way to know exactly what is happening in the centre of the slab, ensuring the core has set sufficiently before the surface is closed.
Regardless of the tool used, professionals must distinguish between finishing and curing. Curing will not fix a delamination that was baked in during the trowelling stage. Proper timing on the tools, informed by data rather than just guesswork, is the only way to prevent a peel.
- Curing: Another effect of the reduced bleed water is that it makes the material less forgiving of moisture loss during the early stages of placement. Plastic shrinkage cracking occurs when the surface dries faster than the concrete can set, a risk often heightened by New Zealand’s variable wind, sun, and humidity. In traditional mixes, bleed water often provides a “self-healing” buffer at the surface, but in modern LCC, this moisture is consumed more efficiently within the refined microstructure.
To manage these characteristics, it is suggested to prioritise a structured moisture retention strategy.
- Physical protection: Erect temporary windbreaks and sunshades to shield the slab from environmental drying caused by wind and sun.
- Chemical management: Apply evaporation retardants immediately after screeding and following each subsequent finishing operation to maintain a protective moisture film on the slab.
Where NZS 3109 is specified, as is standard for professional builds, these requirements should be treated as the minimum baseline for compliance rather than mere suggestions. Plastic shrinkage is not an inherent flaw of the concrete; it is a manageable environmental risk that requires professional discipline and technical rigour to control.
Every concrete supplier and region employs distinct strategies and utilises different raw materials, ranging from lower- emissions cement to imported slag and fly ash, to locally sourced natural pozzolans.
Because a mix in Auckland behaves differently from one in Invercargill, the rate of strength development is a material characteristic that requires specific operational planning.
To manage project schedules effectively while using high-performance mixes, specialist technicians employ the following strategies.
- The pre-pour mandate: It is a professional necessity to hold pre-pour meetings between contractors and suppliers to align on specific curing and strength requirements before the first truck arrives. This ensures all parties understand the performance curve of the specific regional mix being used.
- Data-driven stripping: Transition away from calendar-based schedules and toward maturity monitoring. Sacrificial sensors provide real-time strength data, allowing for precise decisions on when to strip formwork or load a slab, often saving days compared to conservative estimates.
- Propping strategy: In multi-storey construction, slower early-strength gain may require allowing for two sets of floor props or an extended support period to ensure structural integrity during the curing phase.
- Targeted admixtures: If the construction programme is non-negotiable, specify the use of hardening accelerators. While set accelerators primarily reduce the time until the concrete begins to harden, a true hardening admixture is required to increase the rate of early strength development.
Adopting these technical adjustments allows contractors to meet roadmap promises and improve environmental compliance without sacrificing the project timeline.
The narrative of a global SCM shortage is misleading when measured against actual demand. SCM-based LCC relies on SCMs working in combination with traditional Portland cement, not replacing it entirely.
To reach adoption levels seen in other developed nations, New Zealand would only require approximately 400,000 tonnes of SCMs annually, a tiny fraction of the hundreds of millions of tonnes produced globally every year.
Furthermore, the expertise gained from working with current SCMs like fly ash and slag provides the technical foundation for the next stage of the industry’s evolution. Domestic production of natural pozzolans, such as calcined clay and novel cements, is expected to establish a local supply chain well before industrial by-product availability becomes an issue. Contractors who master these materials now will be positioned to adopt new domestic alternatives even faster as they enter the market.
From a commercial perspective, pricing is an operational variable influenced by a complex range of factors.
Proximity to major import hubs plays a large role, but in rural settings, cost premiums are additionally tied to site constraints, specifically whether a plant has the bulk silo capacity to handle multiple cementitious streams or must rely on more expensive bagged products for specific mixes. As regional demand for low-carbon solutions grows, the general trend suggests that costs will begin to normalise through infrastructure upgrades and the wider adoption of pre-blended cements that allow plants to handle these materials more efficiently.
Handing over the baton