For autoclaved aerated concrete (AAC), silica sand is not merely a filling aggregate. In sand-based formulations, quartz becomes a silica source that reacts under hydrothermal conditions after preparation and grinding. Qualification should therefore connect chemical and mineralogical composition with the actual fineness achieved, reaction kinetics and the producer’s autoclaving cycle [2–4].
In Morocco, IMANOR lists NM EN 771-4 for autoclaved aerated concrete masonry units [1]. This standard concerns the masonry product; it does not provide a universal specification for incoming silica sand. The raw-material specification must be built around the industrial process and finished-product performance.
1. What role does silica play in autoclaved aerated concrete?
AAC is generally produced from a silica source, calcareous materials, cement, water and an expanding agent, most commonly aluminium powder [3]. After expansion and initial setting, the material is hardened under saturated steam in an autoclave.
Under these conditions, the CaO–SiO₂–H₂O system evolves toward calcium silicate hydrates. The literature identifies 1.1-nm tobermorite and crystallized C-S-H among the important binding phases [2–4]. The silica raw material therefore participates directly in the reactions that build the product microstructure.
For the buyer, “sand rich in SiO₂” is useful information but not enough. The key questions are which mineralogical form carries the silica and how it reacts after grinding in the intended process.
2. Why is quartz generally ground?
Crystalline quartz has relatively low solubility under ordinary conditions. Grinding increases available surface and changes its reaction rate during hydrothermal treatment.
A classic AAC study compared ground quartz fractions of different sizes: finer fractions showed more advanced reaction at early autoclaving times, and tobermorite appeared rapidly in the studied system [2]. The same study also shows that at longer durations the relationship is no longer simply “finer = more reactive,” and other phases may appear.
Qualification point: quartz fineness is a process parameter. It should be defined together with time, temperature, steam pressure and mix composition - not copied from a publication.
3. Fineness: why one value is not enough
A specification may use sieve residue, D50, D90, specific surface or several of these indicators. They are not interchangeable: two powders can have a similar D50 while having different coarse tails or specific surfaces.
For AAC, fineness influences at least three practical questions:
- the surface available for hydrothermal reactions;
- the rate at which quartz is consumed during autoclaving;
- the balance between residual quartz and newly formed phases.
The literature confirms that quartz particle size is an important variable, but also shows that its effects interact with autoclaving duration and composition [2,4]. The selected sources therefore do not support one universal D50 or sieve residue for every producer.
4. Tobermorite: a reaction indicator, not an isolated purchasing target
Tobermorite is frequently discussed as an important phase in AAC microstructure [2–4]. Its formation nevertheless depends on constituent ratios, silica availability and the hydrothermal program.
The quartz-size study illustrates a useful trade-off: fine quartz can accelerate some early reactions, while coarser quartz produced more highly crystalline tobermorite under the reported experimental conditions [2]. After very prolonged treatment, other phases such as gyrolite were observed in some systems [2].
It would therefore be incorrect to turn tobermorite formation speed into an independent purchasing rule. The industrial objective is the final microstructure and performance, achieved with a controlled cycle.
5. Chemical composition: what should be checked beyond SiO₂?
SiO₂ content documents the chemical richness of the silica source. It does not describe mineralogical phase, fineness or hydrothermal reactivity.
A raw-material certificate can usefully document:
- SiO₂;
- Al₂O₃;
- Fe₂O₃;
- CaO and MgO;
- alkalis where relevant to the process;
- loss on ignition where relevant.
These parameters must be interpreted within the overall formulation balance. AAC literature shows strong interaction between silica–lime–cement composition and autoclaving conditions [4]. A chemical limit should therefore not be invented from one experimental formulation.
To distinguish chemistry from mineralogy, see our XRF/XRD silica-sand guide.
6. Mineralogy: total silica, quartz and secondary phases
XRD analysis identifies crystalline phases in the raw material. For silica sand intended for AAC, it helps verify whether silica is mainly carried by quartz and identify relevant secondary phases.
This complements XRF: a chemical SiO₂ content does not automatically mean that 100% of that silica is crystalline quartz, and two materials with similar chemistry can have different mineral assemblages.
The producer must then verify the reactivity of the prepared material in its own system. Incoming mineralogy does not replace examination of phases after autoclaving when such analysis forms part of process development or control.
7. Composition and autoclaving cycle must be studied together
A controlled AAC production study emphasizes that properties depend simultaneously on silica–lime–cement composition, water, expanding agent and autoclaving conditions [4]. The authors also note that literature conclusions can differ when these variables are not controlled in the same way.
This gives a useful specification sequence:
- characterize the silica raw material;
- define preparation and grinding;
- set a reference formulation;
- test the hydrothermal cycle;
- measure finished-product properties;
- only then lock supplier tolerances.
8. What does NM EN 771-4 mean for the sand buyer?
IMANOR lists NM EN 771-4:2016 - Specification for masonry units, Part 4: Autoclaved aerated concrete masonry units [1]. The same Moroccan catalogue also references methods covering AAC properties such as drying shrinkage and moisture content [1].
For the raw-material buyer, the distinction is essential: these references address the AAC product or its tests, not a universal grading for incoming quartz. Block conformity cannot be inferred from a single sand-analysis result.
9. Lot consistency: what should actually be monitored?
A raw material validated during development can become problematic if composition or grinding behavior drifts. Supplier control should therefore follow the parameters that matter to the process.
| Control | What it documents | Why connect it to the process |
|---|---|---|
| XRF | Overall chemical composition | Track oxides and lot drift |
| XRD | Crystalline phases | Distinguish quartz and secondary phases |
| Incoming-sand grading | Distribution before grinding | Monitor the material feeding the mill |
| Fineness after grinding | Distribution actually available to react | Connect preparation to autoclaving |
| Moisture | Dry mass and handling behavior | Stabilize dosing and preparation |
| Pilot trial | Reaction in the formulation | Confirm raw-material compatibility |
| Autoclaved product | Final phases and performance | Confirm the industrial result |
For sampling logic, see our guide to silica-sand sampling and lot control. For dry-mass dosing, see silica-sand moisture measurement.
10. Specification: separate incoming material, ground material and finished product
A robust specification distinguishes three levels.
Incoming sand or silica raw material
- product and lot traceability;
- chemical composition;
- mineralogy;
- moisture;
- particle-size distribution relevant to grinding;
- contaminants or secondary constituents considered critical by the process.
Quartz after preparation
- grinding method;
- selected fineness indicator(s);
- preparation repeatability;
- slurry or mixing behavior where relevant.
Finished AAC
- density and mechanical/thermal properties for the product range;
- dimensional stability and moisture under applicable methods;
- microstructure or phases when required for process control;
- conformity with product and project requirements.
11. Qualification mistakes to avoid
Avoid:
- requesting only an SiO₂ percentage;
- treating incoming sand and ground quartz as the same specification;
- copying a fineness from a scientific paper without reproducing its process;
- assuming that ever-finer quartz always improves the product;
- attributing block performance to the silica raw material alone;
- comparing particle-size analyses produced by different methods;
- approving a supplier from a non-representative sample or without lot history.
Conclusion: qualify silica as a hydrothermal-process reactant
In sand-based AAC, quartz is not selected merely to fill volume: after grinding, it becomes a silica source involved in hydrothermal reactions. Its fineness influences kinetics, but its effect depends on composition and the autoclaving cycle [2–4].
For a project in Morocco, robust qualification therefore connects chemistry, mineralogy, preparation, fineness after grinding, formulation trials and lot consistency. NM EN 771-4 provides the masonry-product framework; the sand specification must be built from the manufacturer’s actual process [1].
References
[1] IMANOR - NM EN 771-4 and Moroccan catalogue of standards relating to autoclaved aerated concrete.
[2] Isu, Ishida & Mitsuda - Influence of quartz particle size on the chemical and mechanical properties of autoclaved aerated concrete (I) tobermorite formation, Cement and Concrete Research, 1995.
[3] Previous and present investigations on the components, microstructure and main properties of autoclaved aerated concrete – A review, Construction and Building Materials, 2017.
[4] A comprehensive study on the production of autoclaved aerated concrete: Effects of silica-lime-cement composition and autoclaving conditions, Construction and Building Materials, 2017.
[5] Autoclaved aerated concrete in reinforced building applications: A systematic review of AAC/RAAC in the last 40+ years, Results in Engineering, 2024.